Search Results (23)

We propose an integrated electrowetting-on-dielectric (EWOD) device incorporating a mesh-patterned heater to restore damaged hydrophobic coatings and evaluate its recovery performance. Hydrophobic degradation was induced under submersion and falling droplet conditions, and the damage and recovery mechanisms of the coating were examined. A damaged Cytop (CTL-809M) coating was thermally treated using the embedded heater at 200 °C for 24 h, successfully restoring its hydrophobicity. Coating properties before and after recovery were characterized by contact angle (CA) and contact angle hysteresis (CAH) measurements, evaluating EWOD performance and surface analyses using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). After treatment, the reduced CA and increased CAH were recovered, and wetting/dewetting performance in EWOD operation also recovered to pre-damage levels. AFM and XPS analyses confirmed the simultaneous restoration of the surface morphology and chemical composition. These results demonstrate a practical approach for restoring hydrophobic coatings within EWOD devices and offering a promising solution for improving device reliability and lifetime in applications related to EWOD. Full article

Precise manipulation of two-phase flow in micro-confined electrowetting pixels is limited by contact angle hysteresis (CAH). To elucidate this non-equilibrium process, we establish a high-fidelity electrohydrodynamic (EHD) phase-field simulation framework. The model rigorously couples Navier–Stokes equations with molecular kinetic theory (MKT) to characterize energy dissipation at the three-phase contact line (TCL) and further integrates charge transport kinetics. Numerical results reveal CAH is driven by physical pinning and interfacial charge trapping, with the latter dominating interfacial retreat and causing significant residual displacement. Furthermore, analysis shows alternating current (AC) waveforms mitigate charge accumulation and promote depinning via micro-oscillations, minimizing the hysteresis loop compared to direct current (DC) waveforms. Additionally, an overdrive strategy utilizing a suprathreshold Maxwell stress pulse rapidly overcomes static friction. This strategy significantly improves transient dynamics, substantially reducing the time to reach 90% of the steady-state target from 19.6 ms (under standard DC waveform driving) to 7.4 ms. This work provides a comprehensive theoretical basis and design criteria for optimizing active driving strategies in optofluidic and digital microfluidic systems. Full article

This study concerns the evaluation of adhesive and wettability energetic signatures of a model orthodontic wire exposed to commercial mouthrinses. The surface wetting properties were evaluated from the contact angle hysteresis (CAH) approach applied to dynamic contact angle data derived from the original drop on a vertical filament method. Young, advancing, receding CA apart from adhesive film pressure, surface energy, work of adhesion, etc. were chosen as interfacial interaction indicators, allowing for the optimal concentration and placement of the key component(s) accumulation to be predicted for effective antibacterial activity to eliminate plaque formation on the prosthetic materials. Surfactant compounds when adsorb at interfaces confer rheological properties to the surfaces, leading to surface relaxation, which depends on the timescale of the deformation. The surface dilatational complex modulus E, with compression elasticity E_d and viscosity E_i parts, determined in the stress–relaxation Langmuir trough measurements, exhibited the viscoelastic surface film behavior with the relaxation times (0.41–3.13 s), pointing to the vertically segregated film structure as distinct, stratified layers with the most insoluble compound on the system top (as indicated with the 2D polymer film scaling theory exponent y = 12.9–15.5). Kinetic rheology parameters could affect the wettability, adhesion, and spreading characteristics of mouthrinse liquids. Full article

As the most promising new reflective display technology, electrowetting displays (EWDs) have the advantages of a simple structure, fast response, high contrast, and rich colors. However, due to the hysteresis effect, the grayscales of EWDs cannot be accurately controlled, which seriously restricts the industrialization process of this technology. In this paper, the oil movement process in an EWD pixel cell was simulated, and the influence of oil viscosity on the hysteresis effect was studied based on the proposed simulation model. Firstly, the cause of the hysteresis effect was analyzed through the hysteresis curve of an EWD. Then, based on the COMSOL Multiphysics simulation environment, the oil movement process in an EWD pixel cell was simulated by coupling the phase field of laminar two-phase flow and electrostatic field. Finally, based on the simulation model, the influence of oil viscosity on the hysteresis effect in an EWD pixel cell was studied. We observed that the maximum hysteresis difference in the hysteresis effect increased with the increase in oil viscosity and decreased with the decrease in oil viscosity. The oil viscosity had little effect on the maximum aperture ratio of EWD. The pixel-on response time and pixel-off response time increased with the increase in oil viscosity. Full article

Glass fiber-reinforced polymer (GFRP) composites are widely utilized across industries, particularly in structural components exposed to hygrothermal environments characterized by elevated temperature and moisture. Such conditions can significantly degrade the mechanical properties and structural integrity of GFRP composites. Therefore, it is essential to utilize effective methods for assessing their hygrothermal aging. Traditional approaches to hygrothermal aging evaluation are hindered by several limitations, including time intensity, high costs, labor demands, and constraints on specimen size due to laboratory space. This study addresses these challenges by introducing a facile and efficient alternative that evaluates GFRP degradation under hygrothermal conditions through surface wettability analysis. Herein, a glass fiber-reinforced benzoxazine (BZ) composite was fabricated using the vacuum-assisted resin transfer molding (VARTM) method and was aged in a controlled humidity and temperature chamber for up to 5 weeks. When analyzing the wettability characteristics of the composite, notable changes in the contact angle (CA) and contact angle hysteresis (CAH) were 21.77% and 90.90%, respectively. Impact droplet dynamics further demonstrated reduced wetting length and faster droplet equilibrium times with the prolonged aging duration, indicating a progressive decline in surface characteristics. These changes correlated with reductions in flexural strength, highlighting the surface’s heightened sensitivity to environmental degradation compared with internal structural integrity. This study emphasizes the critical role of surface characterization in predicting the overall integrity of GFRP composites. Full article

Superhydrophobic coatings are beneficial for applications like self-cleaning, anti-corrosion, and drag reduction. In this study, we investigated the impact of surface geometry on the static, dynamic, and sliding contact angles in the Cassie–Baxter state. We used fluoro-silane-treated silicon micro-post patterns fabricated via lithography as model surfaces. By varying the solid fraction (ϕ_s), edge-to-edge spacing (L), and the shape and arrangement of the micro-posts, we examined how these geometric factors influence wetting behavior. Our results show that the solid fraction is the key factor affecting both dynamic and sliding angles, while changes in shape and arrangement had minimal impact. The Cassie–Baxter model accurately predicted receding angles but struggled to predict advancing angles. These insights can guide the development of coatings with enhanced superhydrophobic properties, tailored to achieve higher contact angles and customized for different environmental conditions. Full article

Glyceryl monostearate (GMS) was used as an organic friction modifier (OFM) and added to the base oil (PAO10, polyα-olefin) in this study. The film thickness and friction coefficient of the base oil added with GMS (PAO10G) under different slider inclinations and loads were investigated experimentally by using a slider-on-disc contact lubricant film measurement system, and the effect of the adsorption of GMS on the friction behavior of lubricant was studied. Contact angle hysteresis (CAH) was used to evaluate the wettability of the solid–liquid interface, and its correlation with the coefficient of friction was analyzed. The results show that CAH is in good agreement with the wettability of the solid–liquid interface. Compared with the base oil, the wettability of POA10G is weak, which can effectively reduce the coefficient of friction. However, different from the classical lubrication theory, the film thickness of PAO10G is higher than that of PAO10; this unusual phenomenon is preliminarily explained by the interface slippage in this paper. Full article

The study concerns the evaluation of the physicochemical and thermo-adsorptive surface properties of six commercially available mouthrinses, particularly surface tension, surface activity, partitioning coefficient, critical micellar concentration, Gibbs excesses at interfaces, surface entropy, and enthalpy. The aim was to quantify their effect on the adhesion and wettability of a model poly(methyl methacrylate) (PMMA) polymer. The adsorptive and thermal surface characteristics were derived from surface tension (γ_LV) vs. concentration and temperature dependences. Polymer surface wettability was characterized by the contact angle hysteresis (CAH) formalism, using the measurable advancing Θ_A and receding Θ_R dynamic contact angles and γ_LV as the input data. Further, wettability parameters: Young static angle (Θ), film pressure (Π), surface free energy (γ_SV) with its dispersive and polar components, work of adhesion (W_A), and adhesional tension (γ_LV cosΘ_A) were considered as interfacial interaction indicators. The mouthrinse effect demonstrated the parameter’s evolution in reference to the PMMA/pure water case: Θ, Θ_A and Θ_R↓, CAH↑, Π↓, W_A↓, γ_SV↓, and γ_LVcosΘ_A↑. Furthermore, the variations of the surface excess ratio pointed to the formation of multilayered structures of surfactants composing the mouthrinse mixtures considered. The contact angle data allowed for the penetration coefficient and the Marangoni temperature gradient-driven liquid flow speed to be estimated. Full article

In internal combustion piston engines, the formation of an oil film is completely different from that seen in industrial machines. The molecular adhesion force at the interface between the surface coating of engine parts and the lubricating oil determines the load-carrying capacity and the ability to form a lubricated film. The geometry of the lubricating wedge between the surfaces of the piston rings and the cylinder wall is created by the thickness of the oil film and the height of the ring’s coverage with lubricating oil. This condition is affected by many of the parameters that characterize the engine’s operation and the physical and chemical parameters of the coatings used for the cooperating pairs. For lubricant particles that reach energies that are higher than the potential energy barrier regarding adhesive attraction at the interface, slippage occurs. Therefore, the value of the contact angle of the liquid on the surface of the coating depends on the value of the intermolecular force of attraction. According to the current author, there is a strong relationship between the contact angle and the lubrication effect. The paper shows that the surface potential energy barrier is a function of the contact angle and contact angle hysteresis (CAH). The innovation of the current work consists in examining the contact angle and CAH under the conditions of thin layers of lubricating oil, in cooperation with hydrophilic and hydrophobic coatings. The thickness of the lubricant film was measured under various speed and load conditions, using optical interferometry. The study shows that CAH is a better interfacial parameter for correlation with the effect of hydrodynamic lubrication. This paper presents the mathematical relationships relating to a piston engine, various coatings, and lubricants. Full article

The development of a durable and green icephobic coating plays a vital role in the aviation industry due to the adverse impact of ice formation on aircraft performance. The lack of study into how temperature and surface roughness impact icephobicity is the main problem with present icephobic coatings. This study aims to qualitatively evaluate the icephobicity performance of a polytetrafluoroethylene (PTFE) solid lubricant film, as an environmentally friendly solution, with a custom-built push-off test device in different icing conditions utilizing a wind tunnel. The ice-adhesion reduction factor (ARF) of the film has been assessed in comparison to a bare aluminium substrate (Al 6061). The impact of surface energy was investigated by comparing the water contact angle (WCA), the contact angle hysteresis (CAH), and the pull-off force of the PTFE solid lubricant and Al with an atomic force microscope (AFM). The results of ice shear adhesion on the PTFE solid lubricant film showed a significant reduction in the ice adhesion force at various substrate temperatures and surface roughness compared to the bare aluminium substrate. The difference in the ice adhesion between the solid lubricant and aluminium alloy was attributed to the differences in the detachment mechanism. For the PTFE-based solid lubricant, the interfacial detachment mechanism was based on the formation of interfacial blisters towards the centre of the ice. Consequently, upon continued application of the shear force, most of the energy injected would be distributed throughout the blisters, ultimately causing detachment. In the comparison of ice adhesion on PTFE solid lubricant and bare aluminium, the film showed minimal ice adhesion at −6 °C with an adhesion force of 40 N (ARF 3.41). For temperature ranges between −2 °C and −10 °C, the ice adhesion for bare aluminium was measured at roughly 150 N. Full article

A superhydrophobic composite coating consisting of polytetrafluoroethylene (PTFE) and poly(acrylic acid)+ β-cyclodextrin (PAA + β-CD) was prepared on an aluminum alloy AA 6061T6 substrate by a three-step process of electrospinnig, spin coating, and electrospraying. The electrospinning technique is used for the fabrication of a polymeric binder layer synthesized from PAA + β-CD. The superhydrophilic characteristic of the electrospun PAA + β-CD layer makes it suitable for the absorption of an aqueous suspension with PTFE particles in a spin-coating process, obtaining a hydrophobic behavior. Then, the electrospraying of a modified PTFE dispersion forms a layer of distributed PTFE particles, in which a strong bonding of the particles with each other and with the PTFE particles fixed in the PAA + β-CD fiber matrix results in a remarkable improvement of the particles adhesion to the substrate by different heat treatments. The experimental results corroborate the important role of obtaining hierarchical micro/nano multilevel structures for the optimization of superhydrophobic surfaces, leading to water contact angles above 170°, very low contact angle of hysteresis (CAH = 2°) and roll-off angle (${\mathsf{\alpha }}_{\mathrm{roll}-\mathrm{off}}$< 5°). In addition, a superior corrosion resistance is obtained, generating a barrier to retain the electrolyte infiltration. This study may provide useful insights for a wide range of applications. Full article

Within these studies, the effect of surface topography for hydrophobic coatings was studied both numerically and experimentally. Chemically modified polyurethane coating was patterned by application of a laser beam. A set of patterns with variously distant linear peaks and grooves was obtained. The cross section of the pattern showed that the edges of the peaks and grooves were not sharp, instead forming a rounded, rectangle-like shape. For such surfaces, experimental studies were performed, and in particular the static contact angle (SCA), contact angle hysteresis (CAH), and roll-off angle (ROA) were measured. Profilometry was used to create a numerical representation of the surface. Finite volume method was then applied to simulate the behavior of the water droplets. The model developed herewith enabled us to reproduce the experimental results with good accuracy. Based on the verified model, the calculation was extended to study the behavior of the water droplet on the simulated patterns, both spiked and rectangular. These two cases, despite a similar SCA of the water droplet, have shown extremely different ROA. Thus, more detailed studies were dedicated to other geometrical features of such topography, such as the size and distance of the surface elements. Based on the results obtained herewith, the future design of superhydrophobic and/or icephobic topography is discussed. Full article

The aim of the work was to quantify the surface wettability of metallic (Fe, Al, Cu, brass) surfaces covered with sprayed paints. Wettability was determined using the contact angle hysteresis approach, where dynamic contact angles (advancing Θ_A and receding Θ_R) were identified with the inclined plate method. The equilibrium, Θ_Y, contact angle hysteresis, CAH = Θ_A − Θ_R, film pressure, Π, surface free energy, γ_SV, works of adhesion, W_A, and spreading, W_S, were considered. Hydrophobic water/solid interactions were exhibited for the treated surfaces with the dispersive term contribution to γ_SV equal to (0.66–0.69). The registered 3D surface roughness profiles allowed the surface roughness and surface heterogeneity effect on wettability to be discussed. The clean metallic surfaces turned out to be of a hydrophilic nature (Θ_Y < 90°) with high γ_SV, heterogeneous, and rough with a large CAH. The surface covering demonstrated the parameters’ evolution, Θ_A↑, Θ_R↑, γ_SV↓, W_A↓, and W_S↓, corresponding to the surface hydrophobization and exhibiting base substratum-specific signatures. The dimensionless roughness fluctuation coefficient, η, was linearly correlated to CAH. The CAH methodology based on the three measurable quantities, Θ_A, Θ_R, and liquid surface tension, γ_LV, can be a useful tool in surface-mediated process studies, such as lubrication, liquid coating, and thermoflow. Full article

The apparent surface free energy is one of the most important quantities in determining the surface properties of solids. So far, no method of measuring this energy has been found. The essence of contact angle measurements is problematic. Contact angles should be measured as proposed by Young, i.e., in equilibrium with the liquid vapors. This type of measurement is not possible because within a short time, the droplet in the closed chamber reaches equilibrium not only with vapors but also with the liquid film adsorbed on the tested surface. In this study, the surface free energy was determined for the plasma-activated polyoxymethylene (POM) polymer. Activation of the polymer with plasma leads to an increase in the value of the total apparent surface free energy. When using the energy calculations from the hysteresis based approach (CAH), it should be noted that the energy changes significantly when it is calculated from the contact angles of a polar liquid, whereas being calculated from the angles of a non-polar liquid, the surface activation with plasma changes its value slightly. Full article

The wettability of poly (dimethylsiloxane) (PDMS) coating on plasma-treated glass was studied at room temperature using polar and non-polar liquids. The wettability was investigated regarding the liquids’ surface tensions (STs), dielectric constants (DCs) and solubility parameters (SPs). For polar liquids, the contact angle (CA) and contact angle hysteresis (CAH) are controlled by the DCs and non-polar liquids by the liquids’ STs. Solubility parameter difference between the PDMS and the liquids demonstrated that non-polar liquids possessed lower CAH. An empirical model that integrates the interfacial properties of liquid/PDMS has been composed. Accordingly, the difference between the SPs of PDMS and the liquid is the decisive factor affecting CAH, followed by the differences in DCs and STs. Moreover, the interaction between the DCs and the SPs is of importance to minimize CAH. It has been concluded that CAH, and not CA, is the decisive attribute for liquid repellency of PDMS coating. Full article