Lubrication and Drag Reduction for Polymer-Coated Interfaces

Abstract

Lubrication is a well-established strategy for reducing interfacial frictional energy dissipation and preventing surface wear. Various lubricants have been developed, including mineral oil materials, vegetable oil materials, polymer-based materials, and solid lubrication materials. Among these, polymer-based lubrication materials have gained significant interest due to their versatility, leading to the development of tailored strategies to meet diverse application demands. In load-bearing scenarios, polymer-based materials enhance interfacial hydration, exhibiting exceptional frictional properties, including extremely low friction coefficients, high load-bearing capacity, and superior wear resistance. In contrast, in non-load-bearing scenarios, polymer-based coatings improve interfacial hydrophobicity, promoting boundary slip and reducing frictional resistance at the solid–liquid interface (SLI), making them an important strategy for drag reduction. Despite substantial advancements in polymer-based lubrication and drag reduction (PBLDR), the underlying microscopic mechanisms remain incompletely understood. Therefore, this review aims to provide a comprehensive analysis of the fundamental principles governing PBLDR. The main topics covered will include the following: (1) the fundamentals of the surface forces and hydrodynamic force, (2) the mechanisms underlying hydration lubrication, (3) joint lubrication and polymer brush lubrication, (4) the friction tuning and interfacial drag reduction via polymer coating design, and (5) the potential and limitations of polymer-based materials. By summarizing recent advancements in PBLDR, this work will provide valuable contributions to future research and applications in related fields.

1. Introduction

Friction is ubiquitous in our daily lives and a major contributor to energy consumption. It has been observed to impede the relative movement between two objects in contact and is commonly described in macroscopic mechanics by Amontons’ law. At the microscopic level, the Prandtl–Tomlinson model provides a foundational understanding of the mechanisms underlying friction [1,2]. Tabor and colleagues have proposed that adhesion between sliding interfaces plays a pivotal part in the friction process, with surface junctions forming and subsequently fracturing during sliding [3,4]. This fracture process results in irreversible energy loss, primarily as heat, and is often accompanied by wear [5,6]. Consequently, the minimization of friction and wear has become a pivotal area of research in tribology, with lubrication emerging as a primary strategy to reduce interfacial resistance and prevent surface damage [7,8].

Various lubrication strategies have been developed to address these challenges, including oil-based lubrication, water-based lubrication [9,10,11,12], polymer-based lubrication [13,14,15,16,17], and solid lubrication [18,19,20]. However, growing environmental concerns have weakened the appeal of oil-based lubrication. Industrial manufacturing processes often involve excessive use and leakage of lubricating oil, which can lead to environmental contamination [21]. In contrast, water, as an abundant and environmentally friendly solvent, offers a promising alternative. Its low dynamic viscosity reduces shear resistance between sliding surfaces, making it an attractive lubricant. Approximately two decades ago, Klein introduced the concept of hydration lubrication, demonstrating that a hydrated layer can reach an exceptionally and notably diminished friction coefficient (down to 0.001) on account of the elevated fluidity of water molecules within layers [10,22,23]. In aqueous systems, the stable hydrated layer formed around charged interfaces or adsorbed charged groups can effectively minimize friction and prevent interlayer overlapping, thereby reducing wear. However, the water’s limited load-bearing capacity restricts its utilization as a standalone lubricant, as it is easily expelled from the contact area when subjected to high levels of load. To overcome this limitation, researchers have turned to polymer-based materials, inspired by natural friction scenarios observed in biological systems [24,25,26,27]. These materials combine the advantages of hydration lubrication with enhanced mechanical properties, offering a versatile solution for modern lubrication challenges.

In this context, a growing body of research has emerged to explore the mechanism of joint lubrication scenarios, which is characterized by a low coefficient of friction and long-lasting wear resistance. Joint boundary lubrication relies on key components including hyaluronic acid (HA) [28,29,30], lubricin [31,32], proteoglycan [33], and phospholipids [33,34], which adsorb onto the surfaces of cartilage and bone. These components, characterized by their hydrophilic groups, form a robust hydration layer that facilitates effective lubrication. This provides valuable insights into the field of polymer lubrication. Functionalized polymer-based lubricants with strong hydrophilicity have emerged as a very attractive option for lubricant coating design [35,36,37,38,39,40]. These materials typically exhibit three fundamental characteristics: (1) strong adsorption onto sliding interfaces, (2) hydrophilic functional groups that establish a stable hydration layer, and (3) high load-bearing capacity.

Among polymer materials, brush-like polymers, in particular, can adhere to interfaces through physical adsorption or chemical interactions and reversibly swell in suitable solvents [16,41,42,43]. This configuration is conducive to enhanced carrying capacity. The most typical example is a polyelectrolyte. Polyelectrolytes, which are composed of multiple ionizable functional groups, have emerged as particularly promising materials. In solution, polyelectrolytes exhibit diverse ionization properties, allowing them to be electrostatically adsorbed onto oppositely charged interfaces to form protective films. Its lubrication performance can be adjusted with the environmental pH, making it easy to meet the regulation of different lubrication. Additionally, many polyelectrolytes are amphiphilic, featuring both hydrophilic and hydrophobic groups within their molecular structure, which enhances their surface activity. The properties outlined above serve to demonstrate the suitability of polyelectrolytes for a multitude of applications, including the field of drug delivery [44,45,46], biomimetic lubrication [8,14,47,48,49], and various other scenarios [13,50,51,52].

In addition to hydration-based lubrication, another critical interfacial mechanism of PBLDR has gained attention for its distinct operational principles [53,54,55]. Unlike polymer-based coatings that prioritize load-bearing capacity, drag-reduction strategies aim to minimize friction at SLI by leveraging interfacial slip effects. Recent advancements in micro/nanoscale detection technology have revealed that the conventional no-slip boundary condition (NSBC) may be invalid in certain instances [56,57,58,59]. Experimental validation using surface forces apparatus (SFA) further confirmed that interfacial wettability plays a crucial role in regulating dynamic slip effects during surface approach [58]. It is noteworthy that this slip effect can reduce frictional resistance by two to four orders of magnitude compared to classical hydrodynamic predictions. These findings have driven the development of novel drag-reduction strategies that exploit interfacial slip rather than hydration layers. Moreover, studies have established a robust correlation between this slip-enhanced lubrication and surface wettability [60,61,62]. As wettability decreases, boundary slip increases, leading to reduced energy dissipation at the interface [63,64]. These findings have important implications for boundary drag reduction [65,66], bionic tribology [67], and fluid transport [68,69].

A comprehensive understanding of the fundamental mechanisms underlying PBLDR is essential for advancing lubrication strategies. The purpose of this article is to explore these mechanisms to provide direction for the ongoing design and advancement of applications in biomedicine, interface protection and decontamination, nanopore fluid transport, and associated domains. Therefore, this review will examine the surface forces governing polymer-based boundary lubrication, the hydration-based lubrication mechanism, strategies for the active control of polymer friction and the principles of polymer-based biomimetic lubrication, and the role of hydrodynamic slip in friction reduction. Additionally, some experimental techniques, interface modification methods, and challenges and limitations of polymer-based materials will also be discussed.

2. Experimental and Surface Modification Techniques

2.1. SFA

The SFA is an advanced force-probing technology extensively utilized to investigate surface forces, including van der Waals force (vdW), double layer (DL) force, hydration force, adhesion force, cation-π effect, and others [70,71,72,73,74,75]. This technique allows the user to characterize in real time the surface distances and surface film morphology while simultaneously measuring forces. It is particularly valuable for analyzing nanoscale molecular features such as adsorption film thickness and the refractive index of media. The instrument’s force measurement is based on Hooke’s law, while distance measurement is primarily achieved through multiple beam interferometry (MBI). The instrument achieves a spatial resolution of 0.1 nm and a force measurement accuracy of 10⁻⁸ N. The system setup is illustrated in Figure 1a. Furthermore, advancements in SFA technology have enabled its widespread application in assessing friction forces and dynamic effects. With regard to friction measurement, it is important to note that there are two main types of SFA devices: the SFA 2000, introduced by Israelachvili [76], and the surface force balance (SFB), developed by Klein [16]. In the SFA 2000, the lower surface disk is positioned on a bimorph slider, with shear motion at low shear velocity (ranging from 10⁻⁸ to 10⁻² m/s) actuated and controlled by a function generator. In contrast, the SFB utilizes a piezoelectric tube to control the movement of the upper surface. The frictional force between the two surfaces is recorded in real time by a strain monitoring system attached to the friction cantilever springs, which support the upper surface disk.

A typical SFA experiment measures the force–distance (F/R-D) profiles between two freshly cleaved muscovite mica surfaces with a thickness ranging from 1 to 5 μm. The mica is then affixed to the surface of a cylindrical disk with a radius (R) that is larger than the separation distance (D) throughout the experiment. Consequently, according to the Derjaguin approximation, the contact mode can be considered analogous to a sphere with a radius (R) gradually approaching a plane. The interaction energy, W(D), between the surfaces can be converted into a force, F(D), using the following equation [78]:

$${\displaystyle \frac{F\left(D\right)}{R}}=2\pi W\left(D\right)$$

(1)

The SFA measurements provide an extended contact area and enable precise quantification of interaction energy per unit area. Moreover, the application of SFA can be expanded by modifying the mica surface, thereby extending its measurement capabilities from pure electrolyte solutions to macromolecular systems, such as chitosan [79] and phospholipid bilayers [80,81].

2.2. AFM

Atomic force microscope (AFM) is a highly effective and precise technique for measuring nanoscopic forces, with a resolution of approximately 0.1 nm. The fundamental principle is illustrated in Figure 1b. As the microcantilever tip of the AFM approaches the sample surface, interactions between the tip and the surface atoms induce a deflection in the microcantilever. This slight deflection is detected by monitoring the positional shift of a laser beam reflected from the back of the microcantilever, which is captured by a photodiode array. The interaction force is subsequently calculated using Hooke’s law. Furthermore, AFM not only characterizes surface morphology by measuring tip-sample interactions but also facilitates the manipulation, modification, and processing of atoms and molecules. This capability aids in the design and fabrication of innovative structures and materials. In comparison to the SFA, AFM is easier to operate and requires less stringent environmental conditions, making it versatile for use in various settings, including atmospheric, ultra-high vacuum, and liquid environments. Additionally, AFM offers flexibility in measurement modes to accommodate specific applications, such as non-contact mode to prevent damage to the sample surface and lateral force mode for investigating the tribological properties of surfaces.

2.3. Interface Modification Methods

The application of polymer coatings is an effective and straightforward method for interface modification. Compared to individual molecules, polymers exhibit superior mechanical strength. The functional groups of polymers can be tailored to satisfy the multifarious requirements of an extensive range of applications. For instance, polymer-based materials have been extensively utilized in bionic joint lubrication [8,27,82], drug delivery [83,84,85], and antifouling coatings [86,87,88]. As illustrated in Figure 2a, the grafting of polymers with different functional groups can significantly modify the wettability of an interface.

The application of polymer coatings is a versatile and effective approach to interface modification. Polymer molecules can adsorb onto surfaces through two primary mechanisms: physical adsorption and chemical adsorption [90], as illustrated in Figure 2b. Physical adsorption is driven by non-chemical bonding interactions (e.g., electrostatic interactions, van der Waals attraction) and is a relatively simple method of adsorbing polymer molecules onto a substrate. However, coatings formed through this mechanism often exhibit limited long-term stability under certain environmental conditions. In contrast, chemical adsorption involves tethering the polymer to the target surface through chemical bonds, resulting in more durable and stable modified surfaces. A further classification of the chemisorption-based approach to polymer coating synthesis reveals two distinct strategies: “grafting-to” and “grafting-from” [91], as shown in Figure 2b. The “grafting-to” method involves attaching pre-synthesized polymers modified with functional groups to a target surface with complementary functional groups. This approach enables precise interfacial functionalization. However, this strategy could result in a diminished grafting density due to the steric hindrance caused by pre-adsorbed polymer chains. Conversely, the “grafting-from” method begins with the surface modification of the target substrate using a surface-free radical initiator, which facilitates the polymerization reaction and results in the formation of polymer brushes possessing a comparatively higher grafting density than the “grafting-to” strategy. Overall, these different surface modification strategies can be employed based on specific requirements and environmental conditions, ensuring optimal performance of polymer coatings.

3. Fundamentals of the Surface Forces and Hydrodynamic Force

In load-bearing environments, the properties of the lubricating film undergo modification, which in turn influences the tribological behaviors between the surfaces. The intrinsic mechanism of the influence can be attributed to the action of surface forces. Moreover, studies have shown that the apparent strength of chemical bonding varies with the stretching angle [92]. This is also an important consideration for the stability of polymer coating during shear. Therefore, before examining PBLDR mechanisms, it is essential to review existing literature on surface forces and fluid dynamics.

3.1. Hydration Force

The hydration force refers to a type of short-range repulsion interaction arising from the energy cost associated with disrupting the ordered arrangement of water molecules. This interaction is particularly pronounced at charged or highly hydrophilic interfaces, where surface-induced polarization leads to the formation of a structured solvent layer. The magnitude of this force decays exponentially with separation distance, reflecting the short-range correlation of molecular orientations within the hydration layers. As a fundamental phenomenon in aqueous systems, it plays an essential role in the stabilization of colloids, the preservation of protein structure and shape, the facilitation of lubrication in bones and joints, and the support of a multitude of physiological functions. This force arises from the uneven charge distribution of water molecules, which creates dipole moments that adsorb around charged particles or ions due to electrostatic interactions. Consequently, in an aqueous solution, charged particles or ions are typically closely surrounded by water molecules, as illustrated in Figure 3a.

In the early studies conducted by Israelachvili and colleagues on surface forces, it was identified that two mica surfaces in an electrolyte solution exhibit short-range exponential decay repulsion [70,93]. This phenomenon did not align with the DL theory and was instead attributed to the hydration shell surrounding the ions. Following this, Pashley conducted a more detailed experimental investigation and concluded that this force arises from the dehydration of hydrated cations as the surfaces approach one another [94,95]. They also classified the hydration force as a structural force [71]. Moreover, Attard hypothesized that the genesis of the hydration force is not exclusively attributable to electrostatic effects, but rather, it is also contingent on a diminution of configurational entropy within the hydrogen bond network of the surrounding water molecules [96]. These unique mechanical properties of the hydration shell enable it to withstand significant stress during the hydration lubrication shearing process.

Figure 3. (a) The illustration depicts the distribution of charge of the water molecule. The region of the molecule that is partially negatively charged contains the two unbonded “lone pairs” of electrons, which are held in a relatively dispersed manner in proximity to the oxygen atom. It forms a hydrated shell around the charged particles. Below: Schematic diagram of the principle of action of hydration repulsion. When the hydrated shells of adjacent charges overlap, they experience a repulsive interaction that originates from steric. (Reprinted with permission from Ref. [97]. Copyright 2013 Tsinghua University Press & Springer). (b) Results of a typical AFM experiment to measure hydration forces (non-DLVO force) and DLVO force by directly measuring the data results of the repulsive force F as a function of the separation D between the PA active layer of ESNA1-LF with a silica probe (R = 1.1 μm) in water (black) and in NaCl solution at different concentration in mol/L. (Reprinted with permission from Ref. [98]. Copyright 2017 American Chemical Society). (c) The experimental force–distance curves (dot plots) of PS-g-PEO with mica (asymmetric structure) and two PS-g-PEO surfaces (symmetric structure) were fitted using the Alexander–de Gennes theory in a 1 mM NaCl solution. (Reprinted with permission from Ref. [99]. Copyright 2012 American Chemical Society). (d) Schematic diagram of slip at a fluid-solid interface. The no boundary slip means that the velocity of the fluid at the wall is zero. This means that there is no relative sliding between the fluid and the solid wall. Conversely, the boundary slip means that the velocity of the fluid at the wall is not zero, but has some slip velocity v_wall. This means that there is relative sliding between the fluid and the solid wall. The slip length b is usually used to reflect the extent of this boundary slip. (e) The hydrodynamic forces between two crossed cylinders were measured using SFA. In the upper panel, the surfaces are wetted (mica; circles) or partially wetted, and the contact angle is about 44° (OTE; diamonds). The lower figure shows a schematic of the experiment. The data are compared with the hydrodynamic forces (dashed lines) expected from the NSBC (Equation (5)). (Reprinted with permission from Ref. [58]. Copyright 2001 American Physical Society).

Figure 3. (a) The illustration depicts the distribution of charge of the water molecule. The region of the molecule that is partially negatively charged contains the two unbonded “lone pairs” of electrons, which are held in a relatively dispersed manner in proximity to the oxygen atom. It forms a hydrated shell around the charged particles. Below: Schematic diagram of the principle of action of hydration repulsion. When the hydrated shells of adjacent charges overlap, they experience a repulsive interaction that originates from steric. (Reprinted with permission from Ref. [97]. Copyright 2013 Tsinghua University Press & Springer). (b) Results of a typical AFM experiment to measure hydration forces (non-DLVO force) and DLVO force by directly measuring the data results of the repulsive force F as a function of the separation D between the PA active layer of ESNA1-LF with a silica probe (R = 1.1 μm) in water (black) and in NaCl solution at different concentration in mol/L. (Reprinted with permission from Ref. [98]. Copyright 2017 American Chemical Society). (c) The experimental force–distance curves (dot plots) of PS-g-PEO with mica (asymmetric structure) and two PS-g-PEO surfaces (symmetric structure) were fitted using the Alexander–de Gennes theory in a 1 mM NaCl solution. (Reprinted with permission from Ref. [99]. Copyright 2012 American Chemical Society). (d) Schematic diagram of slip at a fluid-solid interface. The no boundary slip means that the velocity of the fluid at the wall is zero. This means that there is no relative sliding between the fluid and the solid wall. Conversely, the boundary slip means that the velocity of the fluid at the wall is not zero, but has some slip velocity v_wall. This means that there is relative sliding between the fluid and the solid wall. The slip length b is usually used to reflect the extent of this boundary slip. (e) The hydrodynamic forces between two crossed cylinders were measured using SFA. In the upper panel, the surfaces are wetted (mica; circles) or partially wetted, and the contact angle is about 44° (OTE; diamonds). The lower figure shows a schematic of the experiment. The data are compared with the hydrodynamic forces (dashed lines) expected from the NSBC (Equation (5)). (Reprinted with permission from Ref. [58]. Copyright 2001 American Physical Society).

3.2. DLVO Forces

The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, a classical model, has been extensively utilized to elucidate the interactions between charged colloidal particles in electrolyte solutions [100,101,102]. This theory is predominantly dictated by the equilibrium between electrostatic repulsion and vdW. In simple electrolyte systems, counterions accumulate near charged surfaces, thereby forming double-layer (DL) structures. In a system that is in a state of thermal equilibrium, the number density of these counterions adheres to the Boltzmann distribution, consequently engendering osmotic pressure within the DL. The phenomenon of repulsion is exhibited when two surfaces approach in proximity to each other, resulting in an overlap of their respective DL.

The normalized DLVO force, given by the following equation, can be expressed in terms of the distance between the two surfaces [98]:

$${\displaystyle \frac{F_{\mathrm{D}\mathrm{L}\mathrm{V}\mathrm{O}}}{R}}\left(D\right)=F_{\mathrm{v}\mathrm{d}\mathrm{W}}\left(D\right)+F_{\mathrm{E}\mathrm{D}\mathrm{L}}\left(D\right)=-{\displaystyle \frac{A_{\mathrm{H}}}{6D^2}}+{\displaystyle \frac{4\pi ({\psi }_{\mathrm{t}\mathrm{i}\mathrm{p}}{\sigma }_{\mathrm{P}}-0.5(\kappa \epsilon {\epsilon }_0{{\psi }_{\mathrm{t}\mathrm{i}\mathrm{p}}}^2-\frac{{{\sigma }_P}^2}{\kappa \epsilon {\epsilon }_0})\mathit{exp}(-\kappa D))}{\mathit{exp}\left(-\kappa D\right)+\mathit{exp}\left(\kappa D\right)}}$$

(2)

where ${\psi }_{\mathrm{t}\mathrm{i}\mathrm{p}}$ is the surface potential of the probe, ${\sigma }_P$ is the surface charge of the polymer active layer, A_H is the Hamaker constant, ${\kappa }^{-1}$ is the Debye length, ${\epsilon }_0$ is the vacuum permittivity, and $\epsilon$ is the relative dielectric constant of the solution. A typical DLVO and non-DLVO force profile is displayed as in Figure 3b.

Polyelectrolytes, a specialized category of polymers, exhibit an electrical charge in the context of their electrolyte group dissociation within aqueous solutions. This characteristic provides them with a distinct advantage in various interfacial modification applications. For instance, polyelectrolytes can be readily and effectively bound to target surfaces through electrostatic adsorption, and their conformation can be easily adjusted by exploiting the repulsive effects of the DL. The adsorption of these polyelectrolytes alters the charge properties of the original interface. In the case of certain polyelectrolytes, surface charges can be neutralized or even over-screened through polyelectrolyte adsorption, resulting in the formation of a new charged interface. Furthermore, a localized DL structure develops around charged groups that are not involved in shielding the charged sites at the interface [103]. As surfaces approach one another, counterions are drawn to these charged sites, generating significant osmotic pressure [42]. The repulsive force resulting from this osmotic pressure serves a dual function: it prevents the aggregation or entanglement of polymer chains, which is essential for maintaining the stability of the polyelectrolyte solution, and it simultaneously improves the compressive strength of the polymer film.

In polyelectrolyte systems, several forces are typically at play, including electrostatic force, steric repulsive force, vdW, hydration force, and osmotic forces. These forces collectively influence the behavior and properties of polyelectrolytes. The balance of these forces is crucial for the stability and functionality of charged interfaces in various applications, such as the formation of polyelectrolyte multilayers, where precise control over charge density and polyelectrolyte conformation is essential.

3.3. Repulsive Steric Force

Steric repulsion is a non-DLVO force that exerts a profound influence on the load-bearing capacity and biomimetic lubricating materials throughout the lubrication process or natural systems. It is imperative to acknowledge the significance of this force in ensuring the stabilization of colloidal particle distributions within these colloidal systems. The primary function of this force is to impede the aggregation of colloidal particles, thereby preserving the system’s steric configuration [104].

The steric repulsion of polymers is primarily determined by their chain length, conformation, pH, ionic strength, and adsorption density at the interface [41,98,105,106,107]. Furthermore, the flexibility of polymer chains is controlled by their persistent lengths, where shorter chain segments display elasticity, and longer chain segments exhibit more compliance and thermal fluctuations. These characteristics endow polymers with a certain load deformation reversibility in practical applications. In the majority of cases when the separation between contact interfaces is less than a few gyration radii (R_g), repulsive forces begin to arise between the interfaces. The steric repulsion exhibited by these polymers within the solution environment is predominantly attributed to the repulsive osmotic pressure that is generated by interfacial polymers. This pressure functions to counteract alterations in configurational entropy during compression, in conjunction with the elastic distortion of polymer chains.

The force expression exhibits a high correlation with the adsorption density at the interface [108]. At low surface densities, the repulsion energy per unit area between the two surfaces is expressed in an exponential form, approximated by [109]:

$$W\left(D\right)=36k_{\mathrm{B}}T{e}^{-D/R_g}$$

(3)

where k_B is Boltzmann’s constant, T is the absolute temperature.

At high surface densities, the force expression changes. When two polymer brush-covered surfaces come into close contact (for D < 2L₀), the strength of the contact force follows the Alexander–de Gennes equation [110,111]:

$$F\left(D\right)=k_{\mathrm{B}}T/s^3[{\left(2L_0/D\right)}^{4/9}-{\left(D/2L_0\right)}^{3/4}]$$

(4)

where s is the mean distance between anchors, L₀, is the stable polymer thickness, and D is the surface separation. This equation includes two terms, which represent the contributions from the osmotic pressure-induced repulsion and the polymer brush’s elastic deformation, respectively. When D/2L₀ falls between 0.2 and 0.9, the pressure can be approximately expressed in an exponential form analogous to Equation (1). A typical curve for the polymer steric force is shown in Figure 3c.

3.4. Hydrodynamic Force

Hydrodynamic force refers to the mechanical interaction between a fluid and a submerged surface, arising from either viscous shear stresses within the fluid or spatial variations in hydrodynamic pressure. In lubrication systems, this force manifests as a velocity-dependent shear stress within the fluid film separating two surfaces in relative motion, governed by the principles of hydrodynamic lubrication. In the process of lubrication, interactions between the SLI are often involved. The conventional boundary condition typically posits that there is no slip at the boundary and calculates shear stress at the SLI based on the classical law of internal friction. To illustrate this point, consider the case of Couette flow, where the shear frictional force is directly proportional to the velocity gradient perpendicular to the wall. This assumption holds well for most super-hydrophilic surfaces, mainly because water molecules tend to be trapped near hydrophilic interfaces and the relative velocity between them is zero. It is applicable to most macro-scale fluid flows. However, as attention to scale effects increases, it has become apparent that the NSBC is frequently invalid at the hydrophobic surface [59,112]. In these cases, the velocity of liquid molecules close to the solid wall does not decay to zero but instead has a certain value relative to the wall (Figure 3d). Preliminary simulation studies indicate that as the interaction between the solid surface and liquid molecules diminishes, the interfacial wettability declines, leading to intensified boundary slip [113,114]. Zhu et al. employed the SFA to ascertain the hydrodynamic force generated by a Newtonian fluid on a smooth surface (Figure 3e) [58]. The measurement principle was based on the assumption that, when a classical sphere approaches a plane, the first-order expression of the Navier–Stokes equation can be solved, yielding the following theoretical equation for the hydrodynamic force (F_H) under the NSBC:

$$F_{\mathrm{H}}=f^{\ast }6\pi R^2\eta \left(1/D\right)\left(dD/dt\right)$$

(5)

where R is the mean radius of curvature of the solid, η is the bulk viscosity. The parameter f* is an indicator: f* = 1 indicates no slip at the boundary; f* is not equal to 1 is indicative of the occurrence of boundary slip.

Furthermore, the interfaces of polymer coatings may involve additional surface forces in the lubrication and drag-reduction process, such as cation-π bonding, hydrogen bonding, π-π interactions, and so forth [115,116]. These provide new perspectives on lubrication and drag-reduction strategies at the molecular scale [117].

4. Mechanisms Underlying Hydration Lubrication

The properties of hydration lubrication are derived from the distinctive physical and chemical characteristics of water molecules. These molecules are electrically neutral in essence, yet they manifest a pronounced dipole moment, an outcome of the existence of residual charges on hydrogen and oxygen atoms. Consequently, these molecules are capable of establishing a hydration layer that envelops charged particles or polar groups through electrostatic interaction. This repulsion functions to prevent the interfaces from coming into contact with each other, preventing the process of contact, which was driven by vdW [94,95]. Furthermore, these water molecules of the hydrated layer are not completely bound but incessantly move and exchange with surrounding bulk water [104]. The fluidity makes the hydrated layer retain an excellent shear fluidity similar to the bulk water, effectively reducing the interfacial friction in an aqueous medium, even under high pressure.

In an early SFB experiment conducted over two decades ago [22], Klein observed that when two mica surfaces were gradually brought into contact with each other and were allowed to reach a certain distance within their aqueous solution ($D_{\mathrm{j}}=2.5~4.5 \mathrm{n}\mathrm{m}$), the vdW attraction between mica surfaces overcame the electrical DL repulsion, resulting in the two surfaces jumping into contact (thickness decreasing from $D_{\mathrm{j}}$ to $D_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{t}}$) [22]. The jump time, ${\tau }_{\mathrm{j}}$, was recorded and the corresponding kinetic equation was established to obtain a function of ${\tau }_{\mathrm{j}}({\eta }_{\mathrm{e}\mathrm{f}\mathrm{f}},D_{\mathrm{j}},D_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{t}})$, which is expressed as follows:

$${\tau }_{\mathrm{j}}\left({\eta }_{\mathrm{e}\mathrm{f}\mathrm{f}},D_{\mathrm{j}},D_{contact}\right)={\displaystyle \frac{18\pi R{\eta }_{\mathrm{e}\mathrm{f}\mathrm{f}}}{A}}({D_{\mathrm{j}}}^2-{D_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{t}}}^2)$$

(6)

As demonstrated in Figure 4a, the experimental findings suggest that the effective viscosity of the confined water film, even within a confined environment, remains at a level that is three times the viscosity observed in its bulk state. Subsequent shear tests on sodium chloride solutions at varying film thicknesses demonstrated that when the thickness of the liquid film was compressed to approximately 1 nm, the film retained the shear flow characteristics of bulk water [10], as illustrated in Figure 4b,c. This phenomenon was ascribed to the accelerated interchange of bulk water molecules with those in the surrounding hydration layer, thus giving rise to the notion of hydration lubrication [22,23]. Building on these findings, Bogdan et al. investigated the impact of monovalent metal salts (LiCl, NaCl, CsCl) on interfacial lubricity. They found that smaller ions with higher hydration energy enhanced lubricating performances [118]. Additionally, the lubrication effect was observed to increase with rising electrolyte concentration. These results can be explained by the different levels of mobility of water molecules within the hydration shells surrounding cations with different hydration energies. In pursuit of a more profound comprehension of the underlying mechanisms governing this phenomenon. The employment of MD research methodologies was a pivotal aspect. Subsequently, Leng and colleagues utilized MD simulations to explore the mobility of confined water film, with results shown in Figure 4d, and explained the molecular-scale mechanism of the phenomenon in Klein’s experiments [119]. When the liquid film thickness was 1.65 nm, the effective viscosity of the water film was still equivalent to the bulk value. The simulation results demonstrate that the structure factor is analogous to the bulk value even when the separation reaches 1.65 nm, which further indicates that bulk properties are maintained (Figure 4f). However, in a highly confined environment (D = 0.92 nm), the effective viscosity underwent a substantial increase and demonstrated a strong correlation with the shear rate. This phenomenon can be attributed to the more orderly orientation of water molecules and the disintegration of the hydrogen-bond network within the liquid film as shear rates increased.

Moreover, the nonlinear velocity-friction relationship in hydration lubrication deviates from the classical Newtonian law of internal friction, suggesting the possibility of an alternative energy dissipation mechanism distinct from the traditional macroscopic model. This discrepancy highlights the necessity of a rational kinetic physical theory model to facilitate the investigation into the intrinsic lubrication energy dissipation mechanism. To address this, Ma and colleagues developed a theoretical model of hydration lubrication that elucidates the mechanism by which friction energy is dissipated during lubrication [120]. Their model distinguishes two regimes based on interfacial separation. When the distance between two moving interfaces exceeds the hydration radius of the adsorbed ions, the energy dissipation is primarily due to viscous interaction in the liquid, with the friction force proportional to the moving speed. For the distance less than the hydration size, the mechanism resembles a thermal activation model, where the ion hydration shell in proximity to the interface acts as a potential barrier to friction. Upon contact between the two surfaces, the energy dissipation primarily occurs within the hydration shell, as depicted in Figure 4b,c. Similar results have been observed for high-valence electrolyte solutions, and the theory was also adopted [121,122]. These studies provide further evidence of the efficacy and practical applicability of hydration lubrication.

While Ma’s theoretical framework has provided a well-established energy dissipation mechanistic underpinning of hydration lubrication, its practical implementation in aqueous systems is inherently constrained by inherent limitations. The restricted load-bearing capacity of water molecules, along with the narrow operational pH/temperature ranges, poses challenges for broader industrial applications. Therefore, functionally modified water-based lubricants have emerged as a favored strategy to address these constraints. Modifying the interface functionality can significantly affect the surface wettability, as illustrated in Figure 2a, which in turn modulates the interfacial lubrication characteristics. The improvement of boundary lubrication durability can be achieved through chemically anchoring polymer networks. This molecular design strategy successfully integrates the three fundamental criteria for effective lubrication (strong adsorption, hydration maintenance, and high load capacity) within a single material system. As a result, friction coefficients below 0.01 can be achieved even under physiological pressure conditions. Polymers with distinctive properties, including excellent chemical stability, wear resistance, and low friction coefficients, have shown considerable potential in the domain of water-based lubrication.

Figure 4. (a) Evaluation of the effective viscosity of confined water based on jump data from the SFA experiment. The cartoon (upper right) shows the jump between interfaces driven by van der Waals attraction when van der Waals attraction is greater than hydration repulsion. The inset (lower right) shows the jump in time with respect to the jump distance. When the jump-in starts at Dj ≈ 3.5 nm according to Equation (5), the time ${\tau }_{\mathrm{j}}$ varies with the jump distance. The main figure shows the effective viscosity ${\eta }_{\mathrm{e}\mathrm{f}\mathrm{f}}$ of the liquid squeezed out of the gap corresponding to the contact jump time ${\tau }_{\mathrm{j}}$. The vertical shaded band is the experimentally determined range of ${\tau }_{\mathrm{j}}$, so the effective viscosity range of the confined water film in the thickness interval corresponding to the experiment is determined by the overlap of the fold line with the shaded band (horizontal dashed line). This possible range of values is about 3 times the bulk. (Reprinted with permission from Ref. [22]. Copyright 2001 Macmillan Magazines Ltd.); (b) The variation of shear force with the sliding velocity under different load conditions. The insets in the figure are schematic diagrams of the corresponding friction energy dissipation mechanisms. a, b, and c represent different moments in the process of hydration shells passing each other. a represents before passing over the hydration shells, b represents when the hydration shells overlap each other, and c represents after passing over the hydration shells. (c) corresponds to the relationship between speed and friction under low load. The insets in the figure are schematic diagrams of the corresponding friction energy dissipation mechanisms. The dashed line shows the trend line of speed and friction. ((b,c) reprinted with permission from Ref. [120]. Copyright 2015 Springer Nature Limited). (d) Statistical results of data simulating the SFA experimental environment using molecular dynamics. The shear viscosity versus shear rate for different thicknesses of confined water films and bulk water are plotted in a log-log coordinate system. (e) Water oxygen mean-square displacements (MSD) in the x-y plane as a function of time for different hydration layers and the bulk water; ((d,e) reprinted with permission from [119]. Copyright 2005 American Physical Society). (f) The variations of the structure factors for different hydration layers with the lateral motion of the upper mica surface at a driving velocity of 1 m/s. Shearing starts at 20 Å distance. (Reprinted with permission from Ref. [123]. Copyright 2006 American Institute of Physics).

Figure 4. (a) Evaluation of the effective viscosity of confined water based on jump data from the SFA experiment. The cartoon (upper right) shows the jump between interfaces driven by van der Waals attraction when van der Waals attraction is greater than hydration repulsion. The inset (lower right) shows the jump in time with respect to the jump distance. When the jump-in starts at Dj ≈ 3.5 nm according to Equation (5), the time ${\tau }_{\mathrm{j}}$ varies with the jump distance. The main figure shows the effective viscosity ${\eta }_{\mathrm{e}\mathrm{f}\mathrm{f}}$ of the liquid squeezed out of the gap corresponding to the contact jump time ${\tau }_{\mathrm{j}}$. The vertical shaded band is the experimentally determined range of ${\tau }_{\mathrm{j}}$, so the effective viscosity range of the confined water film in the thickness interval corresponding to the experiment is determined by the overlap of the fold line with the shaded band (horizontal dashed line). This possible range of values is about 3 times the bulk. (Reprinted with permission from Ref. [22]. Copyright 2001 Macmillan Magazines Ltd.); (b) The variation of shear force with the sliding velocity under different load conditions. The insets in the figure are schematic diagrams of the corresponding friction energy dissipation mechanisms. a, b, and c represent different moments in the process of hydration shells passing each other. a represents before passing over the hydration shells, b represents when the hydration shells overlap each other, and c represents after passing over the hydration shells. (c) corresponds to the relationship between speed and friction under low load. The insets in the figure are schematic diagrams of the corresponding friction energy dissipation mechanisms. The dashed line shows the trend line of speed and friction. ((b,c) reprinted with permission from Ref. [120]. Copyright 2015 Springer Nature Limited). (d) Statistical results of data simulating the SFA experimental environment using molecular dynamics. The shear viscosity versus shear rate for different thicknesses of confined water films and bulk water are plotted in a log-log coordinate system. (e) Water oxygen mean-square displacements (MSD) in the x-y plane as a function of time for different hydration layers and the bulk water; ((d,e) reprinted with permission from [119]. Copyright 2005 American Physical Society). (f) The variations of the structure factors for different hydration layers with the lateral motion of the upper mica surface at a driving velocity of 1 m/s. Shearing starts at 20 Å distance. (Reprinted with permission from Ref. [123]. Copyright 2006 American Institute of Physics).

5. Joint Lubrication and Polymer Brush Lubrication

5.1. Mechanisms of Polymers in Joint Lubrication

One of the most common tribological scenarios in organisms is human joint lubrication [48,124], which features an extremely lowered friction coefficient, high load-bearing capacity, and noteworthy wear endurance. This highly efficient water-based lubrication system provides durable protection for articular surfaces throughout an individual’s lifespan, the structural hierarchy of joint lubrication is illustrated in Figure 5a [17]. Joint lubrication is a composite phenomenon, primarily comprising joint bone fluid and cartilage lubrication layers, with major constituents including water, HA, lubricin, lipids, proteins, and glycosaminoglycans (GAGs). On the other hand, these biomolecules attached to the joint surfaces protect the joints from wear and tear during movement. In this complex structure, HA is the primary component providing necessary support, while lubricin and lipids are integral to the integrity of the structure. These components are interconnected through covalent bonds, forming a brush-mounted structure (Figure 5a) [17].

Research into joint lubrication mechanisms has identified several contributing factors [25,33,125,126,127,128]. Therefore, the focus on joint lubrication has progressed from single-component lubrication to multi-component synergistic lubrication [30,33,129,130]. This also renders the tribological behaviors of joint lubrication inherently complex and opaque with regard to the internal mechanism. Accordingly, the synergistic lubrication mechanism of joint lubrication components at the molecular level has become a subject of intensive investigation. Tannin and colleagues [33] conducted preliminary research on the lubricating properties of HA, proteoglycan, and surfactant phospholipids, both alone and in combination, and compared them with those of fresh bovine joint bone fluid (Figure 5b). Their experiment revealed that the combination of multiple components exhibited superior lubrication effects. However, the lubricating properties of a single component were significantly different from those of joint bone fluid [131]. These findings highlight the complexity of joint systems with considerable synergistic effects.

Figure 5. (a) Top is a schematic diagram of the composition and structure of lubricating fluid in a normal human joint. The bottom is a brush-mounted complex composed of HA, lubricin, and phospholipid. (Reprinted with permission from Ref. [17]. Copyright 2021 Springer Nature Limited). (b) To determine the lubrication mechanism of bone fluid lubrication components, whether it is dominated by a single component or multicomponent synergy. Statistical results of friction coefficients corresponding to different combination strategies of HA, proteoglycan 4 (PRG4), and surface-active phospholipids (SAPL). (Reprinted with permission from Ref. [33]. Copyright 2007 by the American College of Rheumatology). (c) Schematic representation of the HA trapping mechanism in cartilage during lubricated sliding under low-pressure (left) and high-pressure (right) conditions. (Reprinted with permission from Ref. [131]. Copyright 2011 National Academy of Sciences). (d) Shear forces compared to normal forces. The open symbols represent interactions between two surfaces coated with avidin-bHA in the presence of water, while the closed symbols denote interactions between two surfaces coated with avidin-bHA/Agg/LP also in water. The crossed symbols illustrate interactions between two avidin-bHA/Agg/LP-coated surfaces in phosphate-buffered saline (PBS). (* corresponds to p < 0.0001, This means that the probability of obtaining the observed results by random chance is less than 0.01%.) (Reprinted with permission from Ref. [125]. Copyright 2011 American Chemical Society). (e) Top panel: Schematic of the HA/PC complexes on top of the avidin layer. Down panel: lipid monolayers are adsorbed onto negatively charged cartilage surfaces by electrostatic forces, exposing their tail chains to the interface. The corresponding frictional energy dissipation mechanism is sliding friction in which the tail chains slide against each other (left). Schematic illustration of the principle of the frictional energy dissipation mechanism when phosphatidylcholine hydrophilic heads cross each other, and two hydrated layer planes cross each other. (right) (Reprinted with permission from Ref. [27]. Copyright 2021 Wiley-VCH GmbH) (f) Statistical results of the coefficient of friction of lubricin-mimicking diblock copolymers versus samples treated with PBS, binding block only, or lubricating block only. The simulated diblock copolymer significantly reduced the COF of articular cartilage. (Reprinted with permission from Ref. [132]. Copyright 2019 National Academy of Sciences). (g) The force curves observed in the interaction between two mica surfaces, both in their non-shearing and after undergoing shear motion across a human synovial fluid medium. (h) Variation of separation D between two mica surfaces, friction force, and normal force with time during shearing motion. Regime I represent elastohydrodynamic lubrication stage; Regime II signifies the formation stage of a thick gel layer; Regime III represents the formation and rolling stage of gel particles. a-f represent specific moments in the experiment when the sampled and analyzed the stripe morphologies. (i) Relationship between the normalized effective viscosity of synovial fluid films as a function of shear rate. ((g–i) Reprinted with permission from Ref. [126]. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Figure 5. (a) Top is a schematic diagram of the composition and structure of lubricating fluid in a normal human joint. The bottom is a brush-mounted complex composed of HA, lubricin, and phospholipid. (Reprinted with permission from Ref. [17]. Copyright 2021 Springer Nature Limited). (b) To determine the lubrication mechanism of bone fluid lubrication components, whether it is dominated by a single component or multicomponent synergy. Statistical results of friction coefficients corresponding to different combination strategies of HA, proteoglycan 4 (PRG4), and surface-active phospholipids (SAPL). (Reprinted with permission from Ref. [33]. Copyright 2007 by the American College of Rheumatology). (c) Schematic representation of the HA trapping mechanism in cartilage during lubricated sliding under low-pressure (left) and high-pressure (right) conditions. (Reprinted with permission from Ref. [131]. Copyright 2011 National Academy of Sciences). (d) Shear forces compared to normal forces. The open symbols represent interactions between two surfaces coated with avidin-bHA in the presence of water, while the closed symbols denote interactions between two surfaces coated with avidin-bHA/Agg/LP also in water. The crossed symbols illustrate interactions between two avidin-bHA/Agg/LP-coated surfaces in phosphate-buffered saline (PBS). (* corresponds to p < 0.0001, This means that the probability of obtaining the observed results by random chance is less than 0.01%.) (Reprinted with permission from Ref. [125]. Copyright 2011 American Chemical Society). (e) Top panel: Schematic of the HA/PC complexes on top of the avidin layer. Down panel: lipid monolayers are adsorbed onto negatively charged cartilage surfaces by electrostatic forces, exposing their tail chains to the interface. The corresponding frictional energy dissipation mechanism is sliding friction in which the tail chains slide against each other (left). Schematic illustration of the principle of the frictional energy dissipation mechanism when phosphatidylcholine hydrophilic heads cross each other, and two hydrated layer planes cross each other. (right) (Reprinted with permission from Ref. [27]. Copyright 2021 Wiley-VCH GmbH) (f) Statistical results of the coefficient of friction of lubricin-mimicking diblock copolymers versus samples treated with PBS, binding block only, or lubricating block only. The simulated diblock copolymer significantly reduced the COF of articular cartilage. (Reprinted with permission from Ref. [132]. Copyright 2019 National Academy of Sciences). (g) The force curves observed in the interaction between two mica surfaces, both in their non-shearing and after undergoing shear motion across a human synovial fluid medium. (h) Variation of separation D between two mica surfaces, friction force, and normal force with time during shearing motion. Regime I represent elastohydrodynamic lubrication stage; Regime II signifies the formation stage of a thick gel layer; Regime III represents the formation and rolling stage of gel particles. a-f represent specific moments in the experiment when the sampled and analyzed the stripe morphologies. (i) Relationship between the normalized effective viscosity of synovial fluid films as a function of shear rate. ((g–i) Reprinted with permission from Ref. [126]. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

HA, a high-molecular-weight polysaccharide, is a major constituent of cartilage lubricating fluid [133,134]. It has been widely accepted that this substance plays a significant and crucial function in the process of joint lubrication [33,127,131,135,136,137]. During the lubrication process, HA molecules can undergo dynamic structural alterations in response to mechanical strain exerted upon the joint, thereby optimizing their lubrication performance and safeguarding the cartilage surface against wear [131,135,137]. However, HA molecules alone are unable to achieve the minimal friction coefficients that have been observed in joint bone fluid [33,136,138]; instead, they collaborate with other molecules in the bone fluid to realize the formation of a grafted network at the cartilage surface, thus producing a lubricating effect. For instance, HA and aggrecan (AGG) have been shown to form a strong water layer on the cartilage surface due to the high charge points on AGG, which enhance interfacial hydration and lubrication [125]. HA can also form a complex with glycoprotein lubricant (LUB) [139], building a stable HA-LUB lubricating film under both low and high loads [131]. Under low loads, HA molecules reduce friction on a cartilage surface through physical adsorption and partial entanglement; under high loads, they are fixed on a cartilage surface through a mechanical trap mechanism that prevents wear of the surface (Figure 5c). In addition, research has shown that HA-phospholipid complexes play an important role in joint lubrication. A synergistic lubrication mechanism for hyaluronic acid-phospholipid complexes is also hypothesized [140]. The covalent bonds between the carboxyl group of HA and the amine group of phospholipids further stabilize the attachment of HA on the phospholipid layer [138]. The robust hydration layer surrounding the choline group of HA-phospholipid complexes facilitates the formation of a hydrated lubricating interface. Seror and colleagues employed the SFB to investigate the boundary lubrication mechanism of HA and HA–aggrecan facilitated by cartilage link protein (LP) [25,125]. The results indicated that the HA/AGG/LP composite lubricating layer has a superior performance compared to HA alone, as illustrated in Figure 5d. This is primarily due to the fact that the brush-like AGG segments with identical electrical properties at the contact interface prevent the entanglement and cross-linking of the segments, which diminishes the interfacial viscous dissipation. Meanwhile, AGG has more hydration sites than HA does, further promoting hydration lubrication between the interfaces. These studies have shown that the multilayer texture composite fluid lubricating film exhibits a robust load-bearing capacity, impedes direct contact between friction surfaces, and minimizes the probability of interface wear. The lubrication principle is illustrated in Figure 5e [26,27]. The presence of multiple interacting molecules between cartilage facilitates joint lubrication through a synergistic action, thereby offering a novel perspective for the development of polymer-based bionic cartilage materials. One example of a two-stage copolymer designed for glycoprotein lubrication is presented in Figure 5f [132].

As previously discussed, several molecular mechanisms contribute to joint lubrication. Nevertheless, the precise physical and structural mechanism by which these polymer macromolecules participate in boundary lubrication remains unclear. During the sliding process, friction energy is dissipated within the boundary layer [6,141,142]. It has been established that a proportion of the energy is dissipated in the form of irreversible heat, leading to bond breaking or disentanglement of the polymer membrane chains. Another proportion of the energy is stored in the elastic depression of the membrane, thus enabling the membrane to optimize its configurational entropy. Accordingly, understanding the conformational evolution in the lubricating membrane during bone joint lubrication is of considerable significance for the development of polymer-based bionic lubricants. Banquy and his colleagues found that the synovial fluid membrane structure under shear undergoes transitions to different lubrication states until it stabilizes [126]. A significant alteration in the force profiles was observed between the mica planes prior to and following the application of shear, with a notable augmentation in the spatial site-barrier repulsion distance (see Figure 5g). As shear motion occurred, the interfacial separation distance as well as the friction force underwent a significant transformation, and the results are detailed in Figure 5h. At supercritical shear rates, they also found a shear thinning behavior in the lubricating fluid, as evidenced by the observed dynamic equivalent viscosity (Figure 5i). This finding suggests that the polymer network within the lubricating fluid undergoes self-regulation under shear-induced conditions. Therefore, the bone fluid lubricating layer responds dynamically during shear motion; that is to say, the boundary lubricating properties of macromolecules in the joint fluid are capable of adapting to varying stresses and motion states through dynamic molecular conformational changes. These adaptive molecule changes effectively reduce the friction coefficient between joints, providing valuable guidance with regard to bionic lubrication and friction-control materials.

5.2. Polymer Brush Lubrication

Early work by Alexander identified the existence of distinct polymer brush structures [143]. Altering grafting density enables the modulation of the polymer configuration adsorbed at the surface, thereby augmenting interchain repulsion. At low grafting densities, the interactions between polymer chains are diminished as a consequence of large distances between grafting sites, and chain configurations are mainly influenced by the intramolecular interactions within chains, such as hydrogen bonding, vdW, and hydrophobic interactions. At high grafting densities, as the configurational entropy loss is increased, the rise in osmotic pressure between the chains intensifies the repulsive interaction. The enhanced interchain repulsions prompt the surface polymer chains to extend in the solvent, known as a benign solvent, and form a polymer brush structure [144,145,146].

Experimental analyses have confirmed the effects of grafting density on the interactions between polymers grafted on two closely spaced surfaces [42,109]. In an early study [108], Luckham and colleagues found that at low grafting densities, when two surfaces of adsorbed polymer molecules were close to each other, the interface exhibited gravitational behavior. This phenomenon can be attributed to the fact that polymers are more likely to overcome the hindrances posed by neighboring chains and adsorb onto void regions on the surface, and the bridging gravitational force between the chains is also greater than the osmotic pressure. In the case of substantial surface overlap, the interaction exhibits a long-range repulsive force, where the elastic contribution of the polymer chains predominates over the osmotic pressure. This work suggests that the adsorption density exerts a pronounced influence on the interfacial interactions, driving the reconstruction of polymer conformations on the adsorption surface [147,148]. Theoretically, when the separation among polymer chains is reduced to a scale smaller than their gyration radius $R_{\mathrm{g}}$, the interchain interactions become dominant, causing the stretching of the chain segments. The corresponding expression for surface grafting density ${\sigma }_0$ is as follows [149]:

$${\sigma }_0=N/\pi R_g^2$$

(7)

where N is the degree of polymerization.

A conformation change of the polymer chain from the mushroom to the brush can be observed when the grafting density (σ) is greater than a critical value (${\sigma }_0$). Wu and co-workers [147] demonstrated this structural transformation by grafting poly (acrylamide) (PAAM) onto a silicon substrate. When the grafting density was lower than ${\sigma }_0$, the film thickness increased as the grafting density decreased, as depicted in Figure 6a. A similar study was conducted by Lionel and colleagues, wherein they grafted poly(methyl methacrylate) (PMMA) brushes onto silicon substrates through the process of atom transfer radical polymerization. They employed an approach that enabled the modulation of grafting density by meticulously regulating the initiator concentration deposited from the solution [148]. These works showed that at high graft densities polymer brushes in good solvents exhibit two scaling laws for solubility, as shown in Figure 6b. This further substantiates the assertion that during the transition from sparse to dense grafting, the interactions between molecular chains gradually enhance, which in turn facilitates the stretching of polymer chains and overall influences the dynamic characteristics of the interfacial adsorption film. In a recent study, Tanoue and colleagues utilized quartz crystal microbalance (QCM) to examine the correlation between graft density and the viscoelastic characteristics of polymer membranes. Their findings indicated that upon attaining a specific threshold value for surface polymer graft density, the polymer brush layer transitions from a state dominated by viscosity to one governed by elasticity (Figure 6c) [150]. This transition not only inhibits adhesion between interfaces but also endows the interface with load-bearing capacity. A QCM study by Furusawa and colleagues [151] analyzed the behavior of polymer adsorption at interfaces with different graft densities. The degree of hydration of the polymer brushes was deduced through the comparison of shifts in frequency and energy dissipation between the dry air and aqueous phases. The viscoelastic characteristics of polymer brushes were determined by these same metrics. They found that as the surface polymer grafting density increased, the interchain solvation declined and the intra-membrane dehydration of the polymer membrane intensified. This phenomenon leads to a reduction in the hydrodynamic effect within the polymer film, as illustrated in Figure 6d. Consequently, this enhances the elasticity of the polymer. Saito and colleagues explored the adsorption behavior of block copolymers, which consist of hydrophilic PEG and hydrophobic polyisoprene (PI), with different chain lengths [152]. They observed that the short hydrophilic chain, I18E6, exhibited a minimal variation in interfacial hydration energy (Figure 6e). This can be attributed to its low branching density and a folded conformation, which collectively limit the alteration in interfacial energy. In comparison, the long hydrophilic chain I6E16 could provide a more substantial increase in entropy and solvation energy in water. This enhancement has been shown to drive the polymer brush stretching and enhance the membrane surface hydrophilicity, which ultimately leads to a decrease in interfacial energy, as illustrated in Figure 6f. The increased hydrophilicity benefits water permeability and antifouling, both critical for efficient and durable separation membranes.

In the act of lubricating polymer chains, the repulsive forces between monomers within the brush structure keep the chains in an extended and upright orientation [108,144]. This alignment ensures the formed surface film to withstand considerable loads [153]. When two symmetrically coated contact surfaces are delineated by the polymer films containing solvent molecules that are more concentrated than those in bulk [154] (see Figure 7a), the contact interfaces preserve the relative fluid characteristic [153,154,155,156]. This phenomenon arises from solvent molecular fulfillment of two critical roles: firstly, it creates a hindering effect that maintains a specific boundary lubricating film thickness; second, it enables the lubricating film to act as a bearing role, preventing alterations in configurational entropy during polymer conformational changes. This is achieved because the polymer film can elastically contract and store elastic energy [42,157,158]. Given these advantages, water-based polymer lubrication is expected to find extensive applications in the future. The unique structural feature of polymer brushes makes them highly adaptable, with potential uses in lubrication, antifouling [52,159,160], antibacterial [49], and other areas [13,87,161].

Preliminary trials by Klein and his team used the SFB to observe the lubricating properties of polymer brushes in non-abrasive solvents [153,155]. They found that at a contact pressure of 1 MPa, the effective friction coefficient was reduced to below the detection threshold of the measurement device ($\mu <0.001$). Later, they examined the flow behavior of the confined liquid films, finding that these films maintain robust flow characteristics even at sub-nanometer thicknesses. This helps explain the remarkably low friction coefficients achieved by hydrophilic polymer brushes at interfaces [22,23,168].

Polymer brushes can be classified into three types, based on their electrical charge in solution: neutral polymer brushes, polyelectrolyte brushes, and polyzwitterion brushes. In a preceding investigation, Raviv and colleagues utilized the SFB to examine the characteristics of neutral and polyelectrolyte brushes (Figure 7b) [39], revealing distinct shear behaviors between these two types. For neutral polymer brushes, the observed polymer chain extension is primarily driven by the volume effect and strong interchain repulsion, which arises from the accumulation of solvent molecules in the shear zone. For charged polymer brushes, their performances are influenced by variations in counterion concentration among the polymer chains. These variations cause osmotic pressure gradients across the membrane, thereby enhancing the excluded volume effect [42]. Moreover, the hydration layer surrounding the adsorbed counterions on the chain segments is highly mobile, which aids in lubrication within the shear contact area, as illustrated in the schematic diagram of Figure 7c. Saphiannikova and colleagues utilized dynamics simulation to explore the dynamic response mechanisms of polymer brushes [162,169], with the results displayed in Figure 7d. In the context of shear flow, the conformation and kinetic response of polymers adsorbed on the substrate surface are influenced by a number of factors. These include shear forces, the elastic force generated by the deformation of polymer chains, and the presence of osmotic pressure arising from the solvent among polymer chains. As the shear stress increases, the hydrodynamic response between polymer chains weakens, and the penetration depth of the flow decreases. At low shear stress, the response of polymer chains to fluid velocity is relatively strong. In contrast, at high shear stress, the mechanical response of polymer chains is enhanced. In this scenario, the tribological performance is dominated by the coupling between fluid flow and polymer dynamics. Florent and colleagues [163] further investigated the dependence of the friction coefficient on the interpenetration depth of polymer brush systems, as illustrated in Figure 7e. Some phenomena were observed in other experimental studies [153,156]. At low compression, the interpenetration of chains is minimized as a consequence of the excluded volume effect, which engenders a reduced shear force and comparable lubricating properties between charged and neutral polymer brushes. In the context of high compression, the polyelectrolyte brush exhibits superior performance over its neutral counterpart. This enhanced performance can be attributed to the adsorption of counterions, which results in the generation of osmotic pressure. This increased osmotic pressure serves to augment the load-bearing capacity and impedes the formation of chain entanglement and interpenetration [170]. Meanwhile, the counterion hydration lowers the viscosity in the brush gap, which also improves the lubricating efficiency. Moreover, structural reorganization occurs in the permeation zone during sliding, allowing for self-adaptation [169,171]. In summary, for neutral electrolyte brushes, the major factors influencing lubrication are the elastic energy of the entropy spring between chain segments and the excluded volume repulsion between these segments. For polyelectrolyte brushes, the friction lubrication is primarily governed by the permeability differences, resulting from the adsorption counterions on the charged monomers and their respective hydration capacities.

Zwitterionic polymers, which possess both cationic and anionic functional groups, exhibit a net electrical neutrality, yet remain highly sensitive to alterations in their surrounding environment [172]. An individual zwitterionic monomer group generates a dipole moment [173,174,175], and subsequently forms a strong hydration layer due to dipole-dipole interactions (Figure 7f) [164,176]. The presence of multiple zwitterionic groups along the polymer chain leads to extensive hydration, allowing binding with a greater number of water molecules [177]. The water molecules that form hydrogen bonds with the zwitterions act as a physical shield, significantly reducing the interactions between the electrolyte and other zwitterionic polymer brushes [176], preventing potential cross-linking among the polymer chains. Additionally, the zwitterionic polymer brushes are more biocompatible and can support higher loads, which is ideal for biomimetic lubrication applications. Chen and colleagues explored the lubricating characteristics of poly[2-(methacryloyloxy) ethyl phosphorylcholine] (PMPC) grafted onto mica surfaces [165]. They discovered that at pressures reaching up to 7.5 MPa, the friction coefficient of the PMPC-grafted mica surface could be as low as 0.0004 in aqueous environments, as depicted in Figure 7g. Unlike polyelectrolyte brushes, the zwitterionic polymer chains do not generate osmotic pressure through counterion adsorption, as they are electrically neutral. Therefore, the exceptionally low friction coefficient observed under high loads is likely due to the substantial steric hindrance and pronounced hydration of the zwitterionic monomers [81,97]. At higher shear velocities, the shear force is governed by the viscous dissipation in the shear penetration zone. The polymer chain structure adjusts itself in response to velocity changes, achieving a relaxation rate of the polymer chains in the interpenetrating zone that matches the shear rate of the sliding brush [171], $\left[1/\tau \left(\delta \right)\right]\approx \left(v_s/\delta \right)$. Consequently, the relaxation rate is weakly influenced by shear velocity. In a recent study, Feng and his colleagues utilized a technique known as surface-initiated atom transfer radical polymerization (SI-ATRP) to graft PMPC onto a silicon substrate [166]. After a certain running-in process, they innovatively achieved a superlubricity state on the PMPC brush-modified surface. This observation was interpreted as a consequence of the partial dissolution and subsequent reabsorption of PMPC molecules during the sliding process, resulting in the formation of a more uniform lubrication layer. The running-in mechanism is illustrated in Figure 7h. In the initial stage, the friction was high due to polymer chain entanglement. As the sliding continued, a process known as “desorption–adsorption” occurred, whereby certain PMPC molecules detached from the probe and other polymers re-adsorbed on the probe due to electrostatic attractions. Upon reaching equilibrium, this dynamic process resulted in a stable friction force and the formation of a uniform hydrated layer, while the friction plane transitioned from a silicon/PMPC interface to a PMPC/PMPC interface, thereby achieving ultra-low friction. In the state of superlubricity, the friction force was measured to be within the range of 0.25 to 0.35 nN with no velocity dependence. This finding suggests that the PMPC brush operates under a boundary lubrication mechanism in this state, as evidenced by other experimental results [9].

Numerous investigations into the lubricating capabilities of polymer brushes attribute their exceptionally low friction coefficients or superior lubricating properties to the osmotic pressure exerted by counterions and the robust hydration of monomer segments. However, it is important to notice that the lubricating characteristics of these materials can vary under different lubrication conditions [126,158,178,179], meaning it would be overly simple to attribute the lubrication state solely to these two factors. Tsujii and colleagues proposed a mechanism for the lubrication of concentrated polymer brushes [167] (Figure 7i), noting that at low speeds the friction coefficient is weakly correlated with the shearing velocity. This behavior is primarily attributed to the entropy effect of highly stretched polymer brushes, which inhibits cross-penetration between chains. Conforming to Amonton’s law, the sliding friction force in this regime remains largely independent of velocity [180]. As the shear rate increases, the dominant shear resistance between interfaces transitions gradually from solid–liquid interactions to liquid–liquid interactions, at this point, shifting boundary lubrication to hydrodynamic lubrication.

6. Friction Tuning and Interfacial Drag Reduction via Polymer Coating Design

6.1. Friction Control of Polymer Brushes

The lubrication effectiveness using polymer brushes is closely linked to their structural attributes and inherent properties, both of which are highly sensitive to the solution environmental factors, including ionic strength [181,182,183], the type of ions present [107,157,184,185,186,187], temperature [188], and pH levels [41,189,190,191,192]. Recently, Yu and colleagues reported the response of the polyelectrolyte brush polystyrene sulfonate (PSS) to specific counterions [38,107,182,185]. This research revealed that the addition of counterions led to a structural collapse in the polymer brush, with the most evident change being observed in the thickness of the interfacial adsorption film. For monovalent counterions below the critical concentration threshold, the film thickness correlated slightly with the ion concentration, showing a moderate increase, which is mainly attributed to the osmotic brush mechanism. However, as the ion concentration further increased, the film thickness began to decrease, mainly due to excessive electrostatic shielding between chain segments, when the polyelectrolyte brushes transitioned to a “salt brush” state [182,193]. Notably, this structural collapse was particularly pronounced with multivalent counterions, of which the adsorption can cause charge inversion on the surface of polyelectrolyte brush and the formation of electrostatic bridges between the chains, impacting the lubricating properties of the polymer and its interaction with the surrounding environment [182,194]. Later, they examined the influences of different electrolyte solutions on the polyelectrolyte brushes, discovering that even a minimal concentration of 0.01 mM Y³⁺ ions would significantly degrade the lubricating properties of the polymer brushes through structural collapse; for NaCl solution with the same ionic strength, it always maintains excellent lubrication performance [38,107]. In contrast, Banquy and colleagues recently observed that the surfaces of zwitterionic brushes maintained a friction coefficient as low as 0.001 even with the introduction of high-valence ions, provided the surface films remained intact [195]. Song et al. further explored the effect of different valence cations, finding that the introduction of monovalent sodium ions reduced the number of salt bridges through adsorption competition. This resulted in weakened protein-polymer brush interactions and reduced adhesion. However, the presence of divalent calcium ions, while also reducing the number of salt bridges, complemented the more potent Ca²⁺ bridging interaction, thereby enhancing protein adhesion [196]. Moreover, other research has indicated that the polymer’s surface morphology influences its surface energy, as well as the interfacial hydrophilicity [197,198,199]. These findings offer valuable insights for controlling polymer brush friction.

Active control over friction has long been an aspiration in the realm of tribology, aiming at tailoring responses to diverse tribological situations. Given the pronounced sensitivity of lubricating characteristics to the surrounding solution environment, strategies involving external stimuli have been employed to modulate the lubrication performance of these brushes, enabling dynamic adjustment of frictional forces to meet specific needs or conditions [197,200,201]. Wei and colleagues described a method for managing the friction and lubrication characteristics of polymer brushes through certain solution parameters like ionic species and pH levels [197]. As depicted in Figure 8a, their experiments compared the friction coefficient of cationic poly [2-(methacryloyloxy)-ethyltrimethylammonium chloride] brush (PMETAC), which were regulated by different anions (Cl⁻, ClO₄⁻, PF₆⁻, TFSI⁻), with polyanion brushes regulated by cations or cationic surfactants (K⁺, TBAB, DTAB, and CTAB). For the polycationic brush, the friction coefficient in response to different anions underwent substantial variation with values ranging from 0.006 to 0.828 (Figure 8b). Concurrently, the surface wettability exhibited a transition towards a hydrophobic state, as evidenced by changes in contact angle from 10° to 75°, indicating a direct relationship between surface wettability and frictional behavior of the polymer brush. Throughout the experiment, the swelling rate of the polyelectrolyte membrane experienced a decrease from 3.4 to 1.2, as shown in Figure 8c, suggesting that the counterions in the membrane intensified the hydrophobic collapse. Among the tested counterions, hydrophilic Cl⁻ ion possesses a stronger hydration capability to form a robust barrier against partial collapse, thereby preserving the lubricating properties of the brush. This underscores again the importance of ion-specific effects in lubrication applications. For polyanionic brushes, the friction control effects arose from a combination of hydrophobic and electrostatic collapse mechanisms. Moreover, this study manipulated the friction coefficient by examining how the strong polyelectrolyte PSPMA and the weak polyelectrolyte PMAA responded to the pH levels of the solution, as depicted in Figure 8d. The disparity in the sensitivity of their friction forces is primarily because the ionization level of weak electrolytes like PMAA is pH-dependent. Adjusting the pH level triggers protonation reactions along the polymer chain, which makes the polymer brush structure expand or contract reversibly and, therefore, enables the reversible modulation of the friction coefficient. In contrast, the strong polyelectrolyte brushes that are fully ionized in solution have minimal sensitivity to pH changes, showing a largely unaffected friction coefficient across different pH conditions.

Iuster and colleagues conducted an experimental investigation into the tribological properties of cross-linked zwitterionic polymer brushes, with a particular focus on the performance of poly[2-(methacryloyloxy) ethylphosphorylcholine] (PMPC) with their non-cross-linked counterparts [200]. Their experiments revealed that the friction force exerted by the cross-linked zwitterionic brushes exhibited an exceedingly minimal correlation to speed variations, with the friction coefficient stabilizing within the range of 10⁻³ to 10⁻⁴, as delineated in Figure 8e. This stability is predominantly attributed to the structural impediment to inter-membrane penetration imposed by the cross-linked polymer brush network, as elucidated in Figure 8f. The findings of this study propose a novel concept for friction modulation strategies. The strong hydration capacity of polymers can be effectively mitigated by incorporating a cross-linked structural network, which efficiently restricts the permeation behavior between polymer molecules at the interfaces. As a result, this results in the attainment of an ultra-stable coefficient of friction across a broad range of velocities, which is of significant value in guiding the engineering applications of precision lubrication systems.

Drummond and colleagues demonstrate the feasibility of actively controlling global friction by adjusting local molecular configurations [197]. This was achieved by regulating the reverse osmotic pressure of the counterions and the conformation of polyelectrolyte coatings using alternating electric fields, which in turn allows for precise control of friction. As the applied electric field strength dictates the extent of penetration between adjacent polymer brushes in contact, it can directly affect the degree of chain stretching during sliding, and thus the overall friction. Experimental observations revealed that the use of an alternating electric field on a mica surface resulted in a substantial reduction in friction force by several orders of magnitude (Figure 8g). Additionally, an examination of the friction force indicated that it exhibited a non-linear relationship with the frequency of the alternating voltage. This observation was made at frequencies below 1000 Hz, as illustrated in Figure 8h, where the friction reduction effect was pronounced at lower frequencies. Interestingly, a frequency range near 500 Hz was identified to attenuate the friction reduction effect, which can be ascribed to the dynamic response of the polyelectrolyte brush to the external electric field. At lower frequencies, the field-induced oscillation between negative and positive charging states influenced the length and conformation of brush chains. In contrast, at higher frequencies, the rapid fluctuations of the electric field occurred too fast for the chains to respond effectively, diminishing their impact on friction. The underlying mechanism is depicted in Figure 8h, revealing possibilities for managing friction and lubrication of interfacial polymer films that rely on the relaxation behavior of molecular structures.

6.2. Polymer-Based Materials for Drag Reduction

Traditionally, frictional energy dissipation at the SLI has been regarded as a process of viscous dissipation, often neglecting the role of the solid surface. However, pioneering studies have observed deviations in the hydrodynamic forces exerted by fluids on certain solid surfaces from the theoretical predictions under the assumption of the NSBC. This deviation arises due to the phenomenon of boundary slip, characterized by a velocity disparity between liquid molecules in proximity to the wall and those in contact with the surface [56,57,58,113]. Recognizing boundary slip fundamentally challenges the classical no-slip boundary assumption, but has profound implications for the understanding of fluid dynamics at the microscale. This line of inquiry is crucial for advancing the knowledge of tribology, especially in the context of microfluidics, where surface effects become increasingly prominent [202,203].

Generally, wettability is important for characterizing the tendency of a liquid to adhere to a solid surface, with the contact angle commonly used as a measure of this property. The interactions between liquid molecules and solid surfaces with different wettability and spreading not only reflect the diffusion ability of the adsorbed liquid but also highly influence the surface tension and surface free energy. A high surface free energy is associated with good surface wettability, whereas a low surface free energy is indicative of high surface hydrophobicity, as depicted in Figure 9a [204]. The correlation between slip length and contact angle has been established by Huang and co-workers, who proposed a universal relationship that indicates the extent of boundary slip is contingent upon the wettability of the solid surface [61]. Wu and co-researchers synthesized extant experiments to delineate the impact of the contact angle on the behavior of fluid transport [62], as represented in Figure 9b, revealing that an increment in contact angle is associated with an enhancement in fluid transport efficacy. Regarding the frictional behavior, Zhu and colleagues found that the hydrodynamic force of a partially wetted surface (contact angle ≈ 44°) is 2~4 orders of magnitude smaller than the value calculated under the NSBC. This finding indicates that the presence of boundary slip significantly weakens the frictional forces at the SLI [58]. Furthermore, Majumder and colleagues have reported that the presence of boundary slip in fluid transport phenomena can result in a fluid flow rate within nanopores that is 4~5 orders of magnitude higher than predicted by the Hagen-Poiseuille equation [63], a model based on the assumption of NSBC. These studies collectively highlight the substantial influence of a solid surface’s contact angle on the tribological behavior at the SLI, suggesting a positive correlation between surface hydrophobicity and the magnitude of interfacial slip, which leads to a reduction in frictional forces.

Preliminary research employing MD simulations has indicated that the interaction strength between solid and liquid molecules is a determining factor in the behavior of liquid molecules near interfaces: weaker interactions enhance boundary slip, wherein liquid molecules exhibit motion relative to the solid surface [62,113,207,208]. Song and co-researchers investigated the relationship between surface hydrophobicity and interactions with surface liquid molecules by immobilizing polymers with varying wettability onto surfaces. They discovered that such interactions diminish with increasing interface hydrophobicity [89]. Zhou and colleagues used MD simulations to investigate the flow rate in nanopores functionalized with grafted polymers of different wettability [205]. The simulation results demonstrated that increased hydrophobicity of the pore wall facilitates the diffusion of water molecules along the wall, enhances boundary slip, reduces fluid resistance, and consequently elevates the flow rate within the nanopore (Figure 9c). These studies contribute to a burgeoning understanding of the role of surface properties in microscale fluid dynamics, offering valuable insights for optimizing fluid transport in practical applications and engineering surfaces with tailored wettability to improve tribological performance.

In a recent study, Lepikko and colleagues manipulated the wettability of hydroxylated SiO₂ surfaces by adjusting the coverage of self-assembled monolayers (SAMs) [206]. The schematic diagram illustrating the experimental principle is depicted in Figure 9d. The measured friction forces exerted by water droplets on these modified SiO₂ surfaces were found to be minimal at both low and high SAM coverages, while an intermediate SAM coverage corresponded to increased friction (Figure 9e,f). The authors proposed that on hydrophilic surfaces with low SAM coverage, the friction remains low due to the formation of a stable interfacial water layer, mediated by hydrogen bonding between water molecules and surface hydroxyl groups, which endows the layer with lubricating capacity. As the coverage density increases, the ability of water molecules to establish a stable film between the monolayer branches diminishes, while the increased interchain obstruction leads to a consequent rise in friction. At higher SAM densities, the strong hydrophobic effect of the surface restricts polar water molecules to flow only at the top of the SAM, enhancing boundary slip and reducing surface friction.

7. Summary and Prospects

The application of PBLDR technologies plays a crucial role in modern industrial and engineering practices. This article mainly reviews two aspects: polymer-based hydrophilic hydration lubrication and hydrophobic boundary lubrication drag reduction. In the context of lubrication, studies have revealed that hydration layers in highly confined environments exhibit exceptional load-bearing properties and mobility. These characteristics enable hydration lubrication to effectively respond to fluid dynamics, achieving ultra-low friction coefficients. On the other hand, polymer membrane provides excellent load-bearing capacity due to its viscoelastic skeleton structure and the osmotic pressure difference, which overcomes the limitations of aqueous solution with weak load-bearing capabilities. These advantages have prompted extensive research into polymer-based materials that utilize these mechanisms. Furthermore, the rich customizability also makes them have application prospects in various scenarios. This article discusses the advances in polymer film lubrication mechanisms, polymer intermolecular interactions, friction-active modulation strategies, and bionic lubrication. The literature highlights how the characteristics of polymer adsorption at interfaces directly impact the conformation, mechanical behavior, dynamics, and interfacial characteristics of polymer membranes, offering valuable insights for the practical application and advancement of polymer membrane technology. Moreover, this paper reviews strategies for polymer interfacial modification, intrinsic lubrication drag-reduction mechanisms, and applications. The discovery of the boundary slip phenomenon has demonstrated the significance of the wetting characteristics of the solid surface in determining the solid–liquid friction, thereby offering a potential avenue for optimizing transport drag. As the interfacial wetting state changes from hydrophilic to hydrophobic, the boundary slip gradually increases, significantly weakening the friction at the SLI and greatly enhancing fluid transport efficiency.

Despite substantial progress, several challenges in polymer lubrication remain that necessitate resolution and thorough investigation. Firstly, a deeper understanding of the relationship between interfacial hydration in polymer-based materials and lubricating properties is required, along with the underlying control mechanisms that improve lubrication effectiveness. And the research on bionic lubrication should be explored toward multicomponent synergistic lubrication and multifactor coupling mechanisms. Secondly, while current strategies mostly concentrate on improving the hydration capacity of polymer films within controlled laboratory settings, they often neglect crucial aspects such as the materials’ biocompatibility and potential biotoxicity. Future research endeavors can benefit from prioritizing the biosafety issues of these materials in realistic scenarios. This would ensure their safety when integrated into the systems, particularly in the context of biological applications, probably with the inclusion of in vivo organisms. Moreover, many existing control strategies manipulate the lubrication environment surrounding the polymer film, which could present challenges pertaining to feasibility, reversibility, and durability when transposed to real-world applications. Future research could focus on the development of active control strategies that do not require environmental modifications, thus enabling adaptability across diverse lubrication scenarios. In the field of fluid drag reduction, designing polymer coating materials featuring wettability gradients could prove beneficial for enabling the active modulation of interfacial transport behaviors. Such materials could provide a versatile means of controlling fluid dynamics in a variety of applications, including improving the efficiency of fluid transport systems and enhancing the performance exposed to flowing medium, making this a promising area for further investigation.