Responsive Gallium-Based Liquid Metal Droplets: Attributes, Fabrication, Response Behaviors, and Applications

Abstract

Gallium (Ga)-based liquid metals (LMs), as an emerging functional material, stand out among many candidates due to their combination of fluidic and metallic attributes, and they have extensively attracted the attention of academic researchers. When fabricated into droplet form, these metals are imbued with many fantastic characteristics, such as a high specific surface area and self-healing properties. Additionally, Ga-based liquid metal droplets (LMDs) achieve higher response accuracy to external stimuli, satisfying the demands of many applications requiring micro-size and precise stimulus-responsivity. Herein, we focus on reviewing the properties of Ga-based LMs and their droplets, the fabrication strategies of metal droplets, their stimulus-response motion under different external fields, and their applications in microfluidic systems, biomedical applications, and micromachines. To further advance the development of responsive Ga-based LMDs, the future outlooks with key challenges related to their further applications are also presented here.

1. Introduction

LMs are a unique family of metals and their alloys that exist as liquids near room temperature. The well-known LMs contain francium (Fr, 27 °C), cesium (Cs, 28.40 °C), rubidium (Rb, 38.89 °C), mercury (Hg, −39 °C), and gallium (Ga, 29.8 °C) [1]. They possess unique metallic and rheological properties, such as fluidity, low viscosity, and high electrical and thermal conductivity, providing unique application opportunities in reconfigurable or stretchable electronics and devices [2,3,4]. However, most of them have been significantly hindered in practical applications due to their own physical, chemical, and biological attributes. For example, francium is radioactive, and cesium and rubidium are unstable in air. Although mercury is not radioactive and is chemically stable compared to the other aforementioned elements, which has led to its use in many devices, including thermometers, barometers, batteries, mercury lamps, pumps, and switches, its use in medical and other fields has been strictly regulated due to its toxicity and high vapor pressure [5].

Recently, Ga-based LMs have attracted extensive attention, owing to their low toxicity, negligible vapor pressure (effectively zero at room temperature and only 1 kPa at 1037 °C) [6], and steady chemical property, allowing for safe and routine operation in common research environments. Additionally, Ga-based LMs possess exciting biocompatibility compared to other LMs and thus are much safer. Among them, galinstan (68.5 wt% Ga, 21.5 wt% In, 10 wt% Sn) and EGaIn (75 wt% Ga and 25 wt% In) are employed as highly desirable candidates in a variety of promising applications, including high-thermal-conductivity coolants, sensors, wearable electronics, energy harvesting, drug delivery, and even tumor therapy [7,8,9,10].

Ga-based liquid metal droplets (LMDs) not only retain the mechanical and rheological properties of bulk Ga-based LMs but also have several new, useful properties and capabilities, including a high surface-to-volume ratio, self-healing ability, more controllable behaviors, and so forth. All these novel properties of Ga-based LMDs have dramatically expanded their application horizons primarily. Ga-based LMDs have been employed in biosensors, owing to their advantageous high surface-to-volume ratio. When exposed to gases like NO₂ and NH₃, Ga-based LMDs detect them by undergoing a reduction or oxidation of their oxide, resulting in either positive or negative changes in resistance. The sensor exhibits a detection limit of 1 ppm for NO₂ and 20 ppm for NH₃ at 100 °C [11]. In terms of the self-healing ability, it enables the autonomous maintenance of electronic components, thereby enhancing the advantage of electronic packaging [12]. Significantly, Ga-based LMDs have gradually become remarkable functional materials, meeting the requirements of soft robotics and artificial intelligence components due to their controllable behavior in response to various external energy fields [13]. The controllable LMDs, once only seen in the sci-fi movies like Terminator, have now become a reality. Therefore, responsive Ga-based LMDs have recently drawn extensive attention and achieved significant achievements.

To better promote the advancement of responsive Ga-based LMDs, this review comprehensively focuses on systematically summarizing and analyzing their recent progress. We begin by briefly introducing the basic physicochemical characteristics of Ga-based LMDs to illustrate their feasibility and superiority, and then we move on to focus on recent advancements in the fabrication of Ga-based LMDs through different strategies. Furthermore, the responses induced via different stimulations are highlighted. Finally, we discuss recent applications of responsive Ga-based LMDs under external stimulation, including microfluidic systems, biomedical applications, and micro-robots. Particularly, we also outline and discuss the challenges and outlook for further developments in the application of Ga-based LMDs.

2. Attributes of Gallium-Based Liquid Metals and Their Droplets

Ga is a metal classified under Group 13 of the periodic table. Owing to its biocompatibility, low vapor pressure, low melting point, etc., Ga has been extensively employed in various fields as a substitute for mercury, such as dental filling materials [14], thermometers [15], and optical devices [16]. Based on the previous studies, this section overviews the important physicochemical characteristics of Ga-based LMs and their droplets. For easy reference, the basic physical properties of Ga-based LMs are summarized in

Table 1. Basic properties of Ga-based LMs, Hg, In, and water [17,18,19,20,21].

Materials	Melting Point (°C)	Boiling Point (°C)	Density (kg/m3)	Thermal Conductive (w/m·k)	Electrical Conductive (106 S/m)	Viscosity (10−7 m2/s)
Water	0	100	1000	0.55	10−10	11.2
Hg	−38.8	357	1353	8.5	1.0	13.5
Ga	29.8	2204	6093	29.28	3.7	3.24
In	156.6	-	-	32.8	-	-
GaIn24.5 [EGaIn]	15.5	2000	6280	25.9	3.4	2.7
Ga68.5In21.5Sn10	10	1300	6440	16.5	3.46	3.73
Ga68In20Sn12	10.8	-	-	23.67	-	-
Ga67In20.5Sn12.5	10.5	1300	6360	-	3.1	2.98
Ga61In25Sn13Zn1	8	1000	6320	23.2	2.8	1.95

.

2.1. Rheology

With a lower melting point, pure Ga and its eutectic alloys can keep a liquid state near room temperature. Compared with water, Ga and EGaIn kinematic viscosities in Table 1 (Ga: 3.24 × 10⁻⁷ m² s⁻¹; EGaIn: 2.7 × 10⁻⁷ m² s⁻¹; and water: 11.2 × 10⁻⁷ m² s⁻¹), Ga and EGaIn have a lower kinematic viscosity, presenting excellent fluidity. However, a thin oxide layer can rapidly form on the droplet surface, even when it is exposed to oxygen concentrations as low as a few ppm at room temperature (Figure 1a) [22]. Although the oxide thickness is a few nm (about 0.7 nm), it can dramatically vary the rheology of LMs [23]. For example, the liquid metals encased in the solid oxide skin are imbued with non-Newtonian rheological properties, unlike the Newtonian liquids with no surface oxides [24]. Hence, as long as the pressure is strong enough to break the oxide layer, the inner metal flows as a low-viscosity liquid. According to the different conditions, the required pressure is about 0.2–0.6 N/m. The attributes mentioned above imbue liquid metals enclosed in oxide films with yield stress behavior [25], allowing them to retain their fluidity properties above the critical stress. On the contrary, Ga-based LMs only undergo elastic deformation, while this behavior allows LMs to maintain their shape stability in pattern technology. For instance, Figure 1b shows the different rheological behaviors between separately injecting EGaIn and Hg into a microchannel. When the pressure reaches the critical point, EGaIn and Hg can easily be filled into the microchannel. In contrast, when the pressure is lower than the critical point, EGaIn LM can stabilize itself in the microchannel, owing to the mechanical property of the oxidation layer, whereas Hg will shrink due to the high surface tension. Subsequently, after the oxidation skin of EGaIn is eliminated with a HCl solution, EGaIn shows the same behavior as Hg, which demonstrates the effect of oxide skin on rheological behaviors [24]. Therefore, the rheological behavior of Ga and Ga-based LMs influences LM-patterning technology.

2.2. Electrical Properties

It is well known that metals exhibit better electrical conductivity properties than nonmetals. With an excellent electrical conductivity property, LMs can always be used to connect a circuit. As illustrated in Figure 1c, other common electrical flexible materials do not provide the combined characteristics of excellent softness and high conductivity afforded by LMs [18]. Among the five types of elemental room-temperature LMs, the electrical conductivity of pure metal Ga is approximately 3.7 × 10⁶ S/m, which is about three times higher than Hg. Remarkably, electrical conductivity can be regulated by alloying other metals or preparing alloys with different ratios of metals to meet various requirements. According to Matthiessen’s rule, the electrical conductivity of pure LMs (except for Hg) is typically lowered by alloying with most metals [28]. As shown in Table 1, compared with Ga, the electrical conductivity of EGaIn and Ga_68.5In_21.5Sn₁₀ (3.4 × 10⁶ S/m and 3.46 × 10⁶ S/m, respectively) is lower, which is different from Ga₆₇In_20.5Sn_12.5 (3.1 × 10⁶ S/m) due to the different proportions of the alloy composition. Furthermore, the electrical conductivity of Ga and Ga-based LMs declines with increasing temperature (Figure 1d). Compared with the conventional rigid metal conductors (such as Cu:5.96 × 10⁷ S m⁻¹), the electrical conductivity of LMs is an order of magnitude lower than that of Cu at room temperature. However, it is orders of magnitude larger than most commonly applied conductive fluids, such as acidic or alkaline electrolyte solutions, which only have conductivity in the range of a few S m⁻¹. Additionally, the conductivity of Ga and its alloys in a liquid state is better than in a solid state [32]. As mentioned above, the oxide film not only affects the rheological properties of LMs but also influences its electrical conductivity. The oxide skin of Ga and its alloys feature a semiconductor characteristic with a 4.6–4.9-eV bandgap, making the metal droplets switch between a conductor and an insulator with different oxide film thicknesses [33]. With excellent fluidity and electrical properties, Ga and its alloys provide huge potential for preparing flexible microelectronics. For instance, the manufacturing technology of submicrometer-scale EGaIn thin-film patterns is used to create all-soft microelectrode arrays with a high resolution and high density. These microelectrodes can endure mechanical deformation while maintaining electrical functionality [34]. This achievement can drive the development of wearable/implantable devices. As shown in Figure 1e, by using the microcontact-printing approach, it is possible to reproducibly fabricate LM-based soft and stretchable microelectronics with line widths as small as 15 μm [29]. Consequently, these soft microelectronics are expected to be mass-produced and applied in daily life.

2.3. Thermal Properties

As illustrated in Table 1, the melting point of Ga is 29.8 °C. Meanwhile, Ga has a high boiling point of 2204 °C, making it stable enough to remain in a liquid phase over a wide temperature range (29.8–2200 °C) [35]. It has been reported that Ga atoms form a uniquid crystal structure with a covalent character in the solid state, leading to weaker metallic bonds compared to other metals [36]. These weak metallic bonds are easily overcome via thermal energy at room temperature, causing Ga to melt into a liquid. Additionally, pure Ga can be mixed with certain metallic elements in a certain ratio to produce Ga-based alloys with an even lower melting point than that of their constituents. Ga-based LMs’ melting point is lower than their respective single elements. For instance, EGaIn (75.5 wt% Ga and 24.5 wt% In) has a melting point of 15.5 °C, which is lower than the melting point of Ga and In (29.8 °C and 156.6 °C, respectively) [37]. Currently, the lowest melting point of Ga-based LMs is attributed to Ga₆₁In₂₅Sn₁₃Zn₁ at about 8 °C [17]. The low-melting-point characteristic allows Ga-based LMs to undergo liquid–solid-phase transitions at room temperature. Based on the phase-transition mechanism of LMs, a novel concept of reversible and rapid-molding bone cement has been proposed for reinforcing and repairing damaged bones. During the liquid–solid-phase transition process, a phenomenon of a delayed phase transition, known as supercooling, can occur, which can be observed in both pure metals and alloys [38]. Ga and Ga-based LMs have higher supercooling temperatures in low-melting-point liquid metal, reaching up to 30–50 °C. By contrast, mercury and indium only have a low ability for supercooling at about 1−10 °C [28].

Thermal conductivity is always one of the main concerns for thermal properties. Pure Ga, Ga-In_24.5, and Ga₆₈In₂₀Sn₁₂ exhibit high thermal conductivity of around 28.7 W/mk, 25.9 W/mk, and 23.67 W/mk at room temperature, respectively, and these values are about 40 times higher than the thermal conductivity of water and 600 times higher than that of air [39,40]. With an excellent heat-transfer capability, Galinstan has been successfully applied in an integrated liquid cooling system (Figure 1f) [30]. Usually, owing to the limitation of phonon transport dynamics during heat transport, it is difficult to have both high thermal conductivity and excellent flexibility for a flexible material simultaneously [41]. Nevertheless, LMs can satisfy the requirements of wearable computing, soft robotics, and other emerging applications for materials with high thermal conductivity and low mechanical stiffness [42,43]. The thermal conductivity of Ga-based LMs can effectively increase by adding heat-conductive nanoparticles. For instance, adding Ag nanoparticles into a GaInSn matrix can increase the thermal conductivity from 27.0 W/mK to 46.1 W/Mk [19,44]. Nevertheless, the nanoparticle will affect the rheological properties of liquid metals.

2.4. Lubrication Properties

Based on its inherent physicochemical properties, Ga-based LMs were investigated as novel liquid lubricants and a lubricant additive working in some special situations where traditional lubricants are not suitable. For instance, due to its combination of excellent lubricity and good thermal properties, Ga-based LMs exhibit extreme-pressure lubrication [45] and can be used in an ultra-wide range of temperatures [46]. Moreover, as a metal, Ga-based LMs have better electrical conductivity than other liquid lubricants. Thus, Ga-based liquid metal can be used as a current-carrying lubricant [47]. Experimental results show that, compared to no current, applying an electrical current can significantly reduce wear, lowering it by 56% [48]. Trace-element doping to regulate frictional interfaces is an effective way to change the lubrication properties of Ga-based LMs [49]. After the oxidation of Ga-based LMs, a Ga₂O₃ oxide layer forms on the surface. At the interface between the liquid metal and the Ga₂O₃ layers, metallic Ga atoms coexist with covalent Ga atoms bonded to O²⁻ ions within the Ga₂O₃ structure [50]. At room temperature and 200 °C, a Ga-based LM alloy doped with Ag suppresses the oxidation of Ga-based LM, which can actively improve the lubricity of Ga-based LM and reduce the wear rate of a steel disk [51]. At room temperature, doping an Al element in a Ga-based LM achieves an Al-rich film at a frictional interface, which can improve lubrication performance [52]. Although LM lubricants demonstrate many advantages, unfortunately, they have certain corrosive properties. For example, at 800 °C, Ga-based LM lubricants can corrode T91 steel [53]. Therefore, when using LM as a lubricant, it is necessary to ensure appropriate working conditions and select matching materials for manufacturing parts to avoid safety issues caused by corrosion. Overall, with continuous in-depth and detailed research on the lubrication properties of LMs, LM lubricants will be widely applied in future engineering.

2.5. Self-Healing Features

The ability to merge Ga-based LMDs and their inherently high conductivity form the basis for the self-healing mechanism of Ga-based LMDs. This property of conducting materials facilitates the autonomous maintenance of electronic components and ensures the long-term stability and mechanical and functional robustness of soft circuits during repetitive deformation, including bending, folding, and twisting. Ga-based LMDs have demonstrated their capacity as self-healing agents in circuits [54] and antennas [26,55]. As illustrated in Figure 1g, Markvicka et al. reported an elastomer containing Ga-based LMDs [31]. These Ga-based LMDs are sintered into a patterned circuit using a pen plotter. Under sufficient pressure to break the solid oxide layer, such as during twisting or cutting, a small amount of LMs flows out of the Ga-based LMDs and merges with adjacent droplets to create new conductions and reroute conductive pathways without interruption. The ability to recover electric conductivity after cutting has attracted attention for applications in solar cells [56], batteries, and circuits [54].

3. Fabrication of Gallium-Based Liquid Metal Droplets

Micro-nano size gallium-based LMDs have a wide range of applications, such as sensors [57], conductive inks [26], and LM pumps [58]. Due to their high surface tension, they need to prevent the coalescence combination of adjacent droplets during the process of preparing metal droplets through the addition of a surfactant. The method for fabricating micrometer and nanometer gallium-based LMDs is an important research field for academia and industries. For almost a decade, fluidic jetting [59], molding [60], sonication [61], and microfluidics [62], etc., have been widely utilized to generate micro–nano-metal droplets. These approaches will be elaborated in the following sections.

3.1. Molding

Mohmmed G. et al. reported a molding technique for the preparation of Ga-based LMDs. Figure 2a depicts the process of molding for fabricating EGaIn spheres within a wide diameter range from hundreds of microns to several millimeters [60]. Firstly, when polydimethylsiloxane (PDMS) is poured on an acrylic mold, containers for EGaIn spheres are formed. It is noteworthy that an isopropanol coating on a PDMS container can prevent the spheres from adhering to the mold. Then, the bulk EGaIn is spread over the PDMS mold, and voids are filled in. With the oxide layer on the EGaIn surface, the metal can be stabilized in the voids, and an irregular shape is kept (Figure 2b) [63]. Finally, when the oxide film is removed via HCl vapor, the EGaIn in the voids shrinks to perfect sphericity due to high surface tension. The droplets’ diameter can be varied by adjusting the size of the voids. Although the device for molding is relatively simple, and the process of fabricating LM spheres does not require a surfactant, this approach is limited to generating large-sized spheres (≥100 μm), and the microsphere size cannot be tuned dynamically during droplet preparation.

3.2. Self-Shearing

The self-shearing technique provides an effective and simple way to fabricate microscale, Ga-based LMDs (tens to hundreds of microns). As illustrated in Figure 2c, Yu et al. proposed a channelless fabrication method that needs just a syringe and a petri dish to fabricate pure-metal Ga droplets [59]. Due to high surface tension, the liquid Ga stream is split into microdroplets when the bulk Ga is injected from a syringe with a needle into an aqueous solution in a petri dish. There exist five stages of shape changing during this process, including stream, neck, shuttle, irregular, and spherical (Figure 2d). Significantly, a surfactant was added to prevent the Ga droplets’ merging with each other and keep the droplets monodisperse. Actually, this droplet-forming technique can also work for other LMs, such as EGaIn and galinstan. Although this strategy can achieve the large-scale fabrication of Ga-based LMDs, it is not convenient enough for continuous preparation, owing to the mechanical operation mode. Therefore, Fang et al. reported a novel electrically controlled method for preparing the Ga and its alloys’ droplets, substituting the traditional mechanical ejection with an electro-hydrodynamic effect (Figure 2e) [65]. As the balance between the pressure and interfacial tension of a two-phase flow can be broken at a low-magnitude DC electric field, the Ga-based LM was split into a droplet when it was injected from a capillary nozzle into a NaOH electrolyte solution. As shown in Figure 2f, when the applied voltage exceeds the critical voltage of triggering the electro-hydrodynamic effect (2.5 V), the shooting velocity increases with the increasing voltage magnitude, while the size of the metal droplets is only related to the diameter of the capillary nozzle. This electro-hydrodynamic effect provides a rather convenient and easy-going means for continuously generating Ga-based LMDs with a large scale and a controllable size.

Admittedly, self-shearing strategy is comparatively straightforward. Nevertheless, there are many complicated factors restricting the Ga-based LMDs fabrication process, such as the jetting velocity, the diameter of the pinhead, and the concentration of the solution. A smaller syringe needle and a higher velocity are two valid ways to reduce droplet diameters. However, a key barrier to fabricating smaller droplet sizes is the diameter size limitation of the pinhead aperture.

3.3. Ultrasonication

An ultrasonic bath, a sonication probe, and nebulization are three effective types of sonochemical synthesis experimental equipment to fabricate Ga-based LMDs. Tang et al. developed a microfluidics-based system for efficiently preparing EGaIn droplets in a solution environment using acoustic wave-induced forces (Figure 2g). The droplet size can be easily regulated by adjusting the microchannel dimensions. With the exception of an expensive sonication probe, the acoustic wave-induced droplet formation technique is suitable for any ultrasonic bath equipment [61]. Compared with an ultrasonic bath, a sonication probe requires conventional bulky and expensive high-power probes, while it can deliver a larger power density than an ultrasonic bath (probe sonication < 100 W/L and bath sonication > 100 W/L). Therefore, a sonication probe achieves higher efficiency for generating nano-sized metal droplets (Figure 2h) [68]. Nevertheless, high-power density induces a temperature skyrocket, causing undesired oxidation and dealloying during the process of Ga-based LMDs generation. Meanwhile, probe sonication introduces acoustic cavitation, i.e., the formation, growth, and implosive collapse of bubbles, which can greatly shorten the service life of a device [68,69]. By contrast, nebulization can obtain a metal droplet diameter of no more than 500 nm, which is approximately two times smaller than the sonication probe and ultrasonic bath methods (Figure 2i) [70]. Tang et al. showed a vapor cavity-generating ultrasonic platform for nebulizing LM within aqueous media by inducing vapor cavities and collapsing them at the LM–medium interface. This strategy enables the production of stable and functional EGaIn droplets in just one step (Figure 2j) [67]. Although ultrasonic technology has many advantages in fabricating Ga-based LMDs, it cannot ensure droplet size uniformity, and the cost of ultrasonic occurrence equipment is relatively high.

3.4. Droplet Generation via Microfluidics Technology

With the vigorous development of microfluidic technology, Ga-based LMD generation via microfluidics has gained the widespread consideration of researchers, as it has the advantages of fast generation, high stability, and a controllable droplet size (ranging from several nanometers to dozens of micrometers). However, as we all know, the characteristic of a microfluidic flow is that the Reynolds number is very low, and viscous forces play a major role, which allows microscale flows with stable laminar flows. Thus, inducing fluid flow instability and nonlinearity is essential for fabricating droplets in a microchannel [71]. A microfluidic channel provides the boundary for microflow, and thus its geometry would impact droplet generation as well [72]. Generally, it can be categorized into three types with different microchannel geometries: (i) cross-flow, (ii) flow-focusing, (iii) co-flow. Herein, we separately discuss their performance in Ga-based LMDs’ fabrication process.

3.4.1. Cross-Flow

In cross-flow geometry, the dispersed-phase fluids enter the continuous-phase fluids at a certain angle, θ (from 0–180°). The shear force generated via the continuous-phase fluid is adopted to overcome the surface tension of the dispersed-phase fluid, inducing momentum instability and breaking the dispersed-phase fluid into droplets. Among them, the T-junction method (two-phase fluids meet at an angle of 90°) is one of the most used for generating microfluidic droplets [72]. As shown in Figure 3a, Thorsen et al. first proposed to fabricate droplets with a T-junction microfluidic device. This method was widely adopted due to its simple structure and high productivity [71]. In subsequent studies, the relationships between droplets’ size and the cross-junctions angle, flow velocity, and flow rate ratios of two-immiscible phase flow, and other factors were reported [73,74]. This strategy can also be used for LMDs’ formation. However, due to the high interfacial tension between LMs and neighboring liquids compared to that of a liquid-oil system, it will induce different sizes and generation rates of metal droplets under the same condition as liquid-oil systems [75]. Interestingly, the axiolitic Ga-based LMDs can be fabricated in a T-junction (Figure 3b) [76]. Jin and coworkers chose EGaIn as the dispersed phase to investigate the relation of axiolitic metal droplets’ sizes (length of long axis (LA) and short axis (SA)) and flow features (including the flow rate, Q_C, of a continuous phase and the flow rate ratio, R, of the continuous phase and dispersed phase) with a certain viscosity. When the R is fixed, the length of LA decreases linearly with the increment of Qc, while the SA is kept constant. However, when Qc is fixed, the LA decreases exponentially with R, and the SA decreases slightly with the increasing R. Additionally, the continuous-phase viscosity has a significant impact on Ga-based LMDs’ formation. Jin and coworkers proposed that smaller metal droplets would be obtained with a higher continuous-phase viscosity, and the greater shear force makes the metal droplets’ preparation more convenient.

3.4.2. Flow Focusing

Droplets’ fabrication via a flow-focusing device is realized through the interaction of two immiscible fluids. The shear force generated via two countering-phase fluid streams overcomes the viscous force of dispersed-phase fluid (Figure 3c) [77]. Anna and coworkers firstly introduced this geometry structure to explore liquid droplet fabrication in liquid-oil systems, achieving a minimum drop with a diameter of about hundreds of nanometers [81]. Flow-focusing microfluidics also shows excellent performance in LMDs’ formation; it can obtain metal droplets with approximately several tens to hundreds of micrometers in diameter (Figure 3d). The microchannel size, flow velocity, and continuous-phase viscosity are crucial factors in controlling the microdroplets’ diameter. In subsequent studies, Hutter et al. reported the effect of diverse flow rate ratios on microdroplets’ formation in flow-focusing devices (Figure 3e) [78]. The results show that the microdroplets’ size diminishes with an increasing flow rate ratio from the continuous phase to the dispersed phase. The droplet size obtained via the traditional flow-focusing microfluidic method is limited due to the microchannel dimension; particularly, the smallest diameter of the droplets is not less than that of the orifice. Furthermore, Tang and coworkers reported a novel and simple flow-focusing device in which the Ga-based LMDs’ diameter can be tuned by controlling the interfacial tension of metal droplets using both electrochemistry and electrocapillarity (Figure 3f) [79]. This method can arbitrarily control the size and frequency of microdroplets by altering the voltage, and the microdroplets’ size can be 25% less than the microchannel dimension (Figure 3g). Aiming at the influence of continuous-phase viscosity, Liu et al. studied the scaling laws of EGaIn droplets’ size, considering the continuous-phase viscosity as expressed in Equation (1) [75].

$${\displaystyle \frac{v}{h{w}^2}}=0.2{\lambda }^{-0.19}C{a}^{-0.2}\left(1+1.79{\lambda }^{0.065}q\right)$$

(1)

where v is the droplet volume; h and w are the height and width of the channel, respectively; λ is the viscosity ratio; Ca is the capillary number; and q is the flow rate. The relative error between the predicted values achieved via Equation (1) and experimental data is less than ±10%, which confirms the accuracy of the scaling laws. This scaling law provides referenceable technical guidance for related applications.

Although flow-focusing microfluidic technology is currently the preferred method for preparing Ga-based LMDs, this strategy exhibits undesirable hydrodynamic instability, owing to the high pressure drop and shear rate within the microchannel. The unstable droplets’ size distribution occurs due to the swelling of PDMS in most organic solvents during the metal droplets’ preparation process. To avoid the aforementioned limitations, a co-flow glass capillary microfluidic method was proposed to achieve highly monodisperse and uniform metal droplets [70].

3.4.3. Co-Flow

As shown in Figure 3h, the co-flow channel is composed of an outer channel and an internal channel (capillary tube). The continuous-phase flows in the outer channel with the dispersed phase flowing in the inner channel. When the dispersed phase is injected from an inner microchannel nozzle into the continuous-phase microchannel, the dispersed phase will break up into a single droplet under the shear force of the continuous-phase fluid [82]. Compared with cross-flow or flow-focusing methods for fabricating microdroplets, co-flow can achieve a higher precise control of the liquid metal size. However, due to the large surface tension of Ga-based LM (nearly nine times more than water), it poses a great challenge to preparing monodisperse microdroplets with conventional co-flow equipment. Therefore, as illustrated in Figure 3i, on the basis of the original co-flow equipment, Hu and coworkers inserted a stainless-steel micro-needle into an internal glass capillary tube to overcome the Plateau–Rayleigh instability of dispersed-phase flow in a microchannel, achieving an effective strategy for the large-scale fabrication of galinstan microdroplets with good monodispersity and a controllable size [83]. The effect of the flow rate ratio of two-phase flow on the microdroplets’ size was experimentally investigated, and the influence of wettability between materials to break up galinstan was also discussed by comparing the stainless-steel and glass micro-needle-induced approach. Subsequently, a two-phase flow axisymmetric numerical model was proposed to understand the Ga-based LMD formation mechanism in a co-flowing capillary microfluidics device based on a phase-field model [62]. The numerical simulation results obtained via the proposed numerical model were in good accordance with the experimental observation results, providing an effective prediction of droplet formation with high surface tension in a glass capillary microfluidic device (Figure 3j).

4. Response Behaviors of Gallium-Based Liquid Metal Droplets

In droplet form, compared to bulk, Ga-based LMs exhibit more precise stimulus-response behaviors. With researchers’ great efforts over the past decade, Ga-based LMDs have achieved controllable motion under different external energy fields, such as electric fields, magnetic fields, optical fields, chemical fields, and ultrasound fields. Ga-based LMDs are a promising emerging material, so their motion behaviors have been explored in depth. The predictable motion behaviors show potential applications in drug delivery, micro-robots, microfluidic analysis systems, and sensors.

4.1. Electrically Induced Response

Compared with other methods, Ga-based LMDs actuated via an electric field have special advantages, as the microdroplets’ behaviors can be conveniently controlled by regulating the magnitudes, phase, and frequencies of the electrical field. To the best of the authors’ knowledge, the droplet dynamic behavior is closely related to the electrode location, and it can be roughly divided into deformation and movement [84]. In this section, we elaborate on the metal droplets’ deformation or locomotion mechanism and introduce some cases of electrical actuation separately.

4.1.1. Deformation

Ga-based LMD deformation occurs when an LM is connected directly to the electrode in electrolyte solution (Figure 4a). As shown in Figure 4b, when a cathode was linked up to the LM film and the anode made contact with water, the metal film surprisingly transformed itself into a small sphere within a time of 10 s, and its surface area changed thousands of times [85]. Electrocapillarity and electrochemistry are two effective approaches for controlling metal droplets’ deformation by adjusting the surface tension. It is well known that electrocapillarity, first reported as an approach to regulating LM surface tension, was put forward by Lippmann [86].

As an amphoteric metal, Ga can be dissolved in an acidic or alkaline solution. Those processes are accompanied by the formation of Ga³⁺ or [Ga (OH)₄]⁻, respectively. When the Ga is placed in an alkaline solution, the Ga reacts with the solution, and the chemical formula can be expressed as

$$\mathrm{Ga} +{ \mathrm{OH}}^{-}\to {\left[\mathrm{Ga}{\left(\mathrm{OH}\right)}_4\right]}^{-}$$

(2)

Hence, the Ga droplet is negatively charged, and the positive ions will be aggregated at the metal droplet/electrolyte interface to form an electric double layer [87]. The double layer is equivalent to a capacitor. Electrocapillarity changes the charge density and distribution in the electrical double layer via an applied electric potential, inducing effective interfacial tension [88]. Such a relationship can be described by Lippmann’s equation.

$$\gamma ={\gamma }_0-1/2c{V}^2$$

(3)

where, γ represents the surface tension, γ₀ is the maximum surface tension when V = 0, c is the capacitance of an electric double layer per unit area, and V is the potential difference across the electric double layer [89]. In short, electrocapillarity is used to study the interfacial tension (stress) between LM and an electrolyte solution as a function of the electrode potential [90].

However, concerning the Faradaic reactions, the effective surface tension of Ga-based LMDs can only be modulated on a small scale via electrocapillarity. In 2014, both Khan et al. [91] and Zhang et al. [89] proposed to modulate the effective interfacial tension of Ga and EGaIn droplets (~500 mJ/m² to near zero) via an electrochemical oxidation strategy. The surface tension modulation achieved via electrochemical oxidation is reversible, and its adjustment range is larger than that of electrocapillarity. Figure 4c shows the working mechanism for the large-scale reversible deformation of a Ga droplet induced via the electrochemical process. This process is achieved via the combination of electrochemical oxidation and chemical dissolution processes of oxide Ga upon the application of the external electric field and an alkaline or acidic electrolyte solution [89]. Zhang et al. also discussed the effect of different factors on the deformation behavior, such as the concentration of electrolytes, the potential, and the distance between two electrodes, etc.

Actually, electrocapillarity and electrochemical oxidation are difficult to distinguish. Even before the electrochemical oxidation method was clearly defined, it was mistaken for electrocapillarity [80]. Song et al. discussed the discrepancy between classical electrocapillarity and electrochemical oxidation by analyzing the effective interfacial tension of LM versus the potential [80]. As shown in Figure 4d, at the range of potential E from −0.5 V–0.1 V, the two curves fit perfectly. However, when Figure 4d(iv) is combined with Figure 4d(vi), we can conclude that −0.5 V represents the lowest value of applied effective potential for electrocapillarity under this condition because the interfacial tension is not affected by the potential, and bubbles form on the liquid metal surface when the potential E is lower than −0.5 V. By contrast, we can conclude that 0.1 V represents the upper limit since there is a large deviation between two curves, and the surface tension decreases quickly as the potential grows when the potential E is higher than 0.1 V.

It can be easily found that the electric field was applied to the metal droplet directly for the presented droplet-shape-modulating method above. In fact, electrowetting on dielectric material can change the liquid metal shape as well by changing the wettability of metal droplets on substrates. The Ga-based LMDs’ deformation characteristic induced via electrowetting-on-dielectric (EWOD)has been exploited to manufacture many electrical devices, including RF applications [92] and thermal conductance switches [93], etc. However, since the metal droplet processes a huge surface tension, the wettability must be changed by applying a large voltage (more than 100 volts). According to the results of EWOD actuation on galinstan, the contact angle of galinstan changes from 155° to 90° when a voltage ranging from 0 V to 130 V is applied [94]. Hence, this should prevent the phenomenon through which EWOD may induce a dielectric breakdown when the voltage exceeds the breakdown voltage of the substrate.

Figure 4. Deformation of LMs caused by an electrical field. (a) Schematic illustration of the deformation induced via a potential directly applied to metal droplets. Reprinted with permission from [28]. Copyright 2021, American Chemical Society. (b) Transformation of LM from the flattened state i to intermediate states ii and iii and finally to a sphere iv under an electric field. Reprinted with permission from [84]. Copyright 2014, John Wiley and Sons. (c) The reversible deformation of a Ga droplet induced via an electrochemical process. Reprinted with permission from [89]. Copyright 2014, Springer Nature. (d) (i) Schematic diagram of interfacial tension of EGaIn droplets versus potential E and series of images (ii–vii) depicting the counterpart point shown in (i). The blue triangles represent the effective interfacial tension measured at different potentials, and the red dotted parabola denotes the interfacial tension curve obtained from the electrocapillary equation. Reprinted with permission from [80]. Copyright 2021, ADVANCED INTEELIGENT SYSTEMS.

Figure 4. Deformation of LMs caused by an electrical field. (a) Schematic illustration of the deformation induced via a potential directly applied to metal droplets. Reprinted with permission from [28]. Copyright 2021, American Chemical Society. (b) Transformation of LM from the flattened state i to intermediate states ii and iii and finally to a sphere iv under an electric field. Reprinted with permission from [84]. Copyright 2014, John Wiley and Sons. (c) The reversible deformation of a Ga droplet induced via an electrochemical process. Reprinted with permission from [89]. Copyright 2014, Springer Nature. (d) (i) Schematic diagram of interfacial tension of EGaIn droplets versus potential E and series of images (ii–vii) depicting the counterpart point shown in (i). The blue triangles represent the effective interfacial tension measured at different potentials, and the red dotted parabola denotes the interfacial tension curve obtained from the electrocapillary equation. Reprinted with permission from [80]. Copyright 2021, ADVANCED INTEELIGENT SYSTEMS.

4.1.2. Movement

The directional movement of Ga-based LMD was actuated via a set of electrodes placed on either side of the metal in an electrolyte solution. Many interesting phenomena have been discovered under different electrical signals, such as self-rotation [84] and oscillation [95], etc. The summary of the metal droplets’ response phenomena caused by different electrical signals can be seen in

Table 2. Summary of the response characteristics of LMS caused by different electrical signals.

Stimulus	Mechanism	Methods	Advantage	Disadvantage	Application
Electric field	Electrocapillary	Electrode contacted with LMD	No faradaic reaction	Electrolyte solution required, relatively small deformation	Microvalves
	Electrochemical		Large-scale reversible deformation	Causing unexpected bubbles	Soft robots
	EWOD	An insulation is added between the LMD and electrode	No electrolyte solution required	Dielectric breakdown	Light valve
	Continuous electrowetting (interfacial tension gradient and Marangoni effect)	Electrode not in contact with LMD	Fast response, remote control, and low consumption	It is necessary to further improve the control accuracy	Micro-robots
Magnetic field	Magnetic force	Mixing with soft/hard magnetic particles	No electrochemical reaction, non-contact operation, and a large actuation force	The rheological properties are changed	Direct patterning of LM
		Coating with magnetic particles	Mechanical robustness, corrosion resistance, and stability in air		Obstacle-cleaning motor
	Lorentz force	The relative move between a droplet and a magnetic field	No magnetic nanoparticles are added	Relatively complex and bulky equipment is required	MEMS actuation
Light	Photochemical reaction	Coating with light-sensitive material	Non-contact	It is difficult to control precisely	Propelling small objects
	Photothermal effect	Mading into LMGNs with different diameters at both ends	High biocompatibility and a fast response		Nano-swimmer robot
Chemical field	Surface-tension gradient	Adding surfactants	Self-energy supply	Only bring about modest changes in interfacial tension	Drug delivery
		PH imbalance on both sides	Only contain liquid components, highly controllable	Specific solution environment needed	Pump and switch
	Bubble propulsion	Adding a nanoparticle to react chemically with the solution	Showing the unconventional behaviors of autonomous convergence and divergence	Rapid depletion of fuel, not suitable for enclosed spaces due to a large number of bubbles	Soft self-assembling machines
		Adding enzymes to catalyze the solution	A long lifespan	Enzymes can be inactivated in the wrong environment	Soft robot
Ultrasonic field	Acoustic radiation force	Absorbing and reflecting the acoustic wave	Contactless actuation and a long lifespan without additional fuel	Low control accuracy, specific wavelength required for control	Photothermal cancer therapy

, and these are more specifically introduced in this section.

Continuous electrowetting (CEW) is a general technique to actuate planar movement by an applying electric field in an electrolyte solution [96]. The essence of metal droplets’ locomotion caused by such a method is the interfacial tension gradients induced via the electric field (i.e., the interfacial tension was different on two ends of the droplet) [97]. When the Ga-based LMD is immersed in an electrolyte solution without an applied potential, the electrical double layer is uniformly distributed on the droplet surface between the electrolyte solution and LM. However, the potential discrepancy occurs on both sides of the metal droplet when a potential is applied due to the low conductivity of the electrolyte and high conductivity of Ga and their alloys. Hence, an interfacial tension gradient is generated on the metal droplet surface, resulting in a metal droplet driving to the anode (Figure 5a). The CEW effect offers huge potential for Ga-based LMDs’ locomotion [98].

It was found that the metal droplet movement driven by an electric field needs to reach the critical voltage that can induce a sufficient surface tension difference. Tang and coworkers experimentally reported that the critical voltage depends on the size of metal spheres. The corresponding critical voltages for galinstan droplets with diameters of 0.2, 0.5, 1, 1.5, 2, 2.5, and 3 mm were 8, 5, 2.5, 2, 1.5, and 1.2 V, respectively [99]. Meanwhile, Wang et al. reported that the size also had an influence on the velocities of galinstan droplets, as shown in Figure 5b [100]. When the voltage is lower than 7 V, the velocity increases with the metal droplet size. By contrast, when the voltage is higher than 22 V, the velocity decreases as the droplet size increases. At a smaller radius (r = 2, 2.5, or 3 mm), as the voltage increases, the velocity increases first and subsequently remains stable. However, at a larger radius (r = 3.5 or 4 mm), the velocity increases first and then decreases. Interestingly, if the voltage is applied at a large enough level, the Ga-based LMD will change the original movement direction and travel towards the cathode [101]. This phenomenon is caused by the excessive voltage inducing the surface oxidation of a metal droplet, which can make the interfacial tension gradient switch to the opposite direction. Additionally, Wang and coworkers also found that a Ga micromachine can travel to a cathode under the propulsion of the generated hydrogen bubbles, which is opposite to Ga-based LM (i.e., galinstan) micromachine movement direction caused by an interfacial tension gradient [102]. Meanwhile, due to the enhanced electroosmosis effect, the velocity of a Ga droplet increases with the width of confining channels decreasing upon applying DC electric fields.

Generally, the movement phenomenon of Ga-based LMDs (e.g., galinstan and EGaIn etc.) driven by an electric field was conducted in an alkaline electrolyte, as shown in Figure 5c(i). Recently, Wang and coworkers found the galinstan can also be actuated in an acidified KI electrolyte (Figure 5c(ii)). It performs a higher average and maximum speed at a lower applied voltage and power consumption than in an alkaline solution (4 V compared to 9 V in NaOH) due to achieving a high surface charge density of the electric double layer in acidified KI [103]. However, it is worth noting that the movement of a metal droplet in an acidified environment moves toward the opposite direction relative to that of an alkaline environment because their electrical double layers (EDLs) have different polarities (Figure 5d). The excellent motion performance in acidified environments provides potential opportunities for increasing the responsiveness of metal droplets in applications. Additionally, in pure water, the microdroplets of Ga and their alloys can also exhibit complex movements, but a larger voltage is required to provide the driving force [84].

When an AC electric field was applied, the Ga-based LMD exhibited fascinating oscillating phenomena. Yang and coworkers studied the factors affecting the oscillatory behavior, including the size of the GaIn₁₀ droplets, the electrode voltage, the electrolyte solution concentration, and the AC signal frequency, etc. [95]. In this study, two phenomena particularly stand out. Firstly, the oscillation amplitude of the metal droplet’s back-and-forth movement decreased with an electrical field frequency increase. Interestingly, when the frequency reached up to 22 Hz, the metal droplet exhibited resonance behavior. At a resonance frequency, a surface flow similar to the Marangoni effect was exhibited on the surface of the droplet. Secondly, the other outstanding phenomenon was that the bubble generation induced via solution electrolysis gradually decreased with the increase in the AC signal frequency. When the signal frequency exceeded 4 Hz, the electrolysis phenomenon was almost negligible, and the bubbles stopped forming. Although bubble generation has been shown to have only a very slight impact on the actuation of a metal droplet in an open channel, it prevents the further movement of a metal droplet due to the bubble possibly isolating the electrode from the solution in a closed channel [99].

Interestingly, if a Ga-based LMD is trapped within a chamber, it will exhibit a pumping behavior (the details of the pumping behavior are illustrated in Section 5.1.1). However, a few seconds later, the pumping will stop due to the oxide layer forming on the side of the droplet near the anode. This phenomenon can be prevented by using an AC potential difference with a DC offset [58]. Furthermore, Hu and coworkers reported a versatile movement of a galinstan droplet driven by a signal with a DC bias voltage and AC voltage when a metal droplet was placed in a channel (Figure 5e) [104]. The impressive transportation and oscillating characteristic of Ga-based LMDs opens a possibility for developing flexible electronics and reconfigurable structures.

Figure 5. Movement of Ga-based LMDs caused by an electrical field. (a) A schematic diagram of the experimental device and the EDL distribution caused by the CEW effect. (b) The average velocity of metal droplets with different sizes under different voltages. Reprinted with permission from [100]. Copyright 2017, ROYAL SOCIETY OF CHEMISTRY. (c) A schematic diagram of galinstan actuation (i) in an alkaline solution and (ii) in acidified KI. (d) The images of galinstan droplet movement in (i) a 1 M NaOH solution at 6 V and (ii) an acidified 0.5 M KI solution at 3 V. Reprinted with permission from [103]. Copyright 2019, American Chemical Society. (e) Images of Galinstan droplet versatile movements. (a–j) Continuous captures of LMD oscillation between two driving electrodes. Reprinted with permission from [104]. Copyright 2020, MDPI.

Figure 5. Movement of Ga-based LMDs caused by an electrical field. (a) A schematic diagram of the experimental device and the EDL distribution caused by the CEW effect. (b) The average velocity of metal droplets with different sizes under different voltages. Reprinted with permission from [100]. Copyright 2017, ROYAL SOCIETY OF CHEMISTRY. (c) A schematic diagram of galinstan actuation (i) in an alkaline solution and (ii) in acidified KI. (d) The images of galinstan droplet movement in (i) a 1 M NaOH solution at 6 V and (ii) an acidified 0.5 M KI solution at 3 V. Reprinted with permission from [103]. Copyright 2019, American Chemical Society. (e) Images of Galinstan droplet versatile movements. (a–j) Continuous captures of LMD oscillation between two driving electrodes. Reprinted with permission from [104]. Copyright 2020, MDPI.

4.2. Magnetically Induced Response

A magnetic field, as a powerful means of control and drive, due to contactless control, can reduce the possibility of contamination; it has been widely applied to microfluidic analysis systems [105] and life sciences [106], etc. (Figure 6a,b). Although the Ga-based LMs do not contain magnetic metallic elements, i.e., they themselves are non-magnetic, it has been found that the magnetic drive of metal droplets can be realized by mixing magnetic particles into them, such as Wang and coworkers’ reported magnetic galinstan droplet composite [107,108]. Under an external magnetic field, the metal droplet mixed with carbonyl iron (iron pentacarbonyl) showed excellent athleticism motion characteristics, including reversible telescopic deformation, bending, and on-demand locomotion (Figure 6c). According to the performance that was obtained, the magnetic actuation method of a metal droplet has been applied to reconfigurable circuit welding and transient electronics, which can be controlled remotely (Figure 6d). Furthermore, Ma et al. showed a simple method for the direct patterning of an EGaIn droplet composite (mixing with magnetic microparticles, Ni) on substrates using a magnetic field [109]. As illustrated in Figure 6e, under the action of permanent magnets, Ni magnetic microparticles deform the EGaIn droplet composite from a sphere into a linear shape. The above magnetic liquid metal droplets (MLMDs) achieved by dispersing or suspending soft magnetic particles, such as iron and nickel into LM, are called soft magnetic liquid metal droplets (S-MLMDs), and they can be easily manipulated under a magnetic field due to favorable deformability and flexibility. On the contrary, hard magnetic particles such as neodymium iron boron (NdFeB) with high residual magnetization can be dispersed into LM as well, in which case the result is called hard magnetic liquid metal droplets (H-MLMDs). Compared with soft ones, H-MLMDs possess stronger surface tension and mechanical robustness, as well as better electrical conductivity and strength [105]. The coating of magnetic particles on a surface is also a common method for fabricating magnetically controllable LMDs. Chen and coworkers reported an LM marble, which was coated with the mixture of ferronickel and polyethylene microparticles on galinstan, as a controllable obstacle-cleaning motor controlled via a magnet (Figure 6f) [110]. The preparation process shown in Figure 6g indicates that the NaOH-treated galinstan droplet was placed in a vessel filled with a mixture of ferronickel and polyethylene microparticles for rolling, and these microparticles adhered to the surface of Ga-based LMDs. From the SEM image, it can be seen that the microparticles almost covered the entire surface of droplets (Figure 6h). Such a combined structure not only exhibits outstanding magnetic controllability and mechanical robustness but also presents excellent corrosion resistance and stability in air.

As non-magnetic metals, the Ga-based LMDs nearly do not respond to a permanent magnetic field. But, as conductive fluids, the Ga-based LMDs can be actuated via Lorentz force, which is derived from the relative movement of Ga-based alloy droplets and permanent magnetic fields (Figure 6i) [111]. Compared with the above methods that are magnetic field-driven, manipulating metal droplets via Lorentz force does not compromise the intrinsic properties of Ga-based LMDs. Shu et al. introduced an innovative method utilizing the Lorentz force as the driving force to control the self-rotation and circular movement of EGaIn droplets with a magnetic field without coating/mixing magnetic particles (Figure 6j) [112]. However, the experimental facility for this strategy is relatively complex and bulky, requiring rotating magnets and a motor, and it can be simplified using a programmed electromagnetic field. Overall, magnetic field-driven methods are highly suitable for remote control and closed microchannels due to their advantages of no electrochemical reaction and a non-contact operation [20].

Figure 6. The control mechanisms to modulate Ga-based LMDs via a magnetic field. (a) Microvalve controlled via a magnetic field. Reprinted with permission from [105]. Copyright 2021, Elsevier. (b) Magnetic heating property of liquid metal for breast cancer therapy. Reprinted with permission from [106]. Copyright 2019, John Wiley and Sons. (c) Schematic showing the magnetic field-controlled liquid metal (i) telescopic deformation and (ii) bending deformation. (d) Reconfigurable circuit welding and transient electronics. (a and c1–c8) Schematic diagrams of the circuit; (b1–b8) Optical images of the actual circuit. Reprinted with permission from [108]. Copyright 2022, ADVANCED INTELLIGENT SYSTEMS. (e) Schematic showing the magnetically controlled deformation of the liquid metal mixed with a Ni microparticle. (i) The liquid metal droplet adhered on the targe substrate; (ii) The Ni microparticles were aggregated at the droplet bottom; (iii) The liquid metal droplet deformed into a continuous line. Reprinted with permission from [109]. Copyright 2019, John Wiley and Sons. (f) An obstacle-cleaning motor controlled via a magnet. (g) Schematic diagram of the fabrication of the magnetically controllable liquid metal marble. (h) SEM image of the liquid metal marble. Scale bar = 500 μm. The locally magnified SEM image in the yellow rectangle. Scale bar = 50 μm. Reprinted with permission from [110]. Copyright 2019, John Wiley and Sons. (i) Schematic illustration of the equivalent force and torque exerted on a metallic sphere. (j) Image of the self-rotation and locomotion of an EGaIn droplet. Reprinted with permission from [112]. Copyright 2018, ROYAL SOCIETY OF CHEMISTRY.

Figure 6. The control mechanisms to modulate Ga-based LMDs via a magnetic field. (a) Microvalve controlled via a magnetic field. Reprinted with permission from [105]. Copyright 2021, Elsevier. (b) Magnetic heating property of liquid metal for breast cancer therapy. Reprinted with permission from [106]. Copyright 2019, John Wiley and Sons. (c) Schematic showing the magnetic field-controlled liquid metal (i) telescopic deformation and (ii) bending deformation. (d) Reconfigurable circuit welding and transient electronics. (a and c1–c8) Schematic diagrams of the circuit; (b1–b8) Optical images of the actual circuit. Reprinted with permission from [108]. Copyright 2022, ADVANCED INTELLIGENT SYSTEMS. (e) Schematic showing the magnetically controlled deformation of the liquid metal mixed with a Ni microparticle. (i) The liquid metal droplet adhered on the targe substrate; (ii) The Ni microparticles were aggregated at the droplet bottom; (iii) The liquid metal droplet deformed into a continuous line. Reprinted with permission from [109]. Copyright 2019, John Wiley and Sons. (f) An obstacle-cleaning motor controlled via a magnet. (g) Schematic diagram of the fabrication of the magnetically controllable liquid metal marble. (h) SEM image of the liquid metal marble. Scale bar = 500 μm. The locally magnified SEM image in the yellow rectangle. Scale bar = 50 μm. Reprinted with permission from [110]. Copyright 2019, John Wiley and Sons. (i) Schematic illustration of the equivalent force and torque exerted on a metallic sphere. (j) Image of the self-rotation and locomotion of an EGaIn droplet. Reprinted with permission from [112]. Copyright 2018, ROYAL SOCIETY OF CHEMISTRY.

4.3. Optically Induced Response

Light, as high-frequency electromagnetic radiation, can also be applied to actuate Ga-based LMDs remotely, and it has advantages in renewability and eco-friendliness. Ga-based LMDs with a core-shell structure, where the light-sensitive material is coated on the surface of metal droplets, are widely used in light field-based actuation. When light-sensitive materials are illuminated with specific light, it triggers a photochemical reaction that is accompanied by the formation of bubbles, causing the droplet to move to the bubble-free side. Tang et al. demonstrated that galinstan LM marbles coated with the light-sensitive material WO₃ in a H₂O₂ solution could be actuated in response to ultraviolet (UV) light (Figure 7a) [113]. The LM marbles were propelled to the opposite side of localized regions where the oxygen bubbles were produced, and their average speed was close to the movement speed of the light beam. Furthermore, the photothermal effect of Ga-based LMD is another effective technology for driving metal spheres. Wang and coworkers presented a needlelike liquid metal gallium nano-swimmer (LMGNS) moving along the radius of a decreasing orientation under the control of near-infrared light (Figure 7b) [114]. The LMGNS was prepared via Ga droplets through molding technology, and owing to the solid oxide Ga on the surface, the LMGNS maintained a needlelike shape. The driving force was mainly the thermophoresis force, which is caused by the temperature gradient along the longitudinal axis of the LMGNS due to the different diameters at each end. Although the method for driving metal droplets via light can easily adjust the excitation parameters (light intensity and frequency), the catalytic efficiency of the light-sensitive material and the angle of incidence of light also have a non-negligible impact on movement. Hence, it is difficult to achieve the accurate control of metal droplet movement through a light field, and those methods must be carried out in a solution environment.

4.4. Chemically Induced Response

Using surface-active agents is one of the most common methods to change the interfacial tension of Ga-based LMDs via a chemical reaction. But surfactants can only change the interfacial tension of metal droplets in a small range; for instance, the interfacial tension of Hg is only changed by approximately 10% when an alkythiol monolayer is added to Hg [120,121]. However, it is worth noting that the native oxidation skin, which acts as a natural surfactant, can significantly alter the interfacial tension from more than 600 mn/m⁻¹ to approximately 0 mm/m⁻¹. The oxide layer can be modulated by implementing chemical reactions with an acidic solution or basic solution due to the properties of amphoteric oxides [122]. In contrast, a NaOH solution affords faster oxide removal than HCl [123]. As shown in Figure 7c, Zavabeti et al. presented the mechanical actuation of a galinstan droplet through the modification of the liquid electrolyte surrounding it [115]. When the HCl and NaOH were separately filled into either side of galinstan, owing to the different electric double layers on both sides of the metal droplet, the balance of surface tension was broken (Figure 7d). Consequently, under the action of the driving force produced via the surface-tension gradient, galinstan droplets will move to one side spontaneously.

In addition to changing the interfacial tension by modulating the oxide layer of Ga-based LMDs, chemical and biochemical reactions by adding particles and biologically active enzymes can also realize metal droplet actuation. Tang et al. presented their findings that an EGaIn droplet achieved sub-millimeter intermittent jumping in a NaOH solution triggered by adding nickel particles [124]. This phenomenon is often referred to as self-actuation that converts energy from the environment into mechanical energy for directional motion. Al, Cu, and Ag have also been reported to be the fuels for metal droplet self-actuation [125,126]. For example, Zhang and coworkers showed a self-fueled EGaIn droplet robot with Al as its fuel, which could move spontaneously in an open channel (Figure 7e), and Yuan and coworkers displayed a violin-like horizontal oscillation behavior of a copper wire with a frequency of about 1.2 Hz in GaIn₁₀ droplets (Figure 7f) [116,117]. However, this self-actuation by adding metal particles has its own limitations, such as the fact that the fuels are quickly used up. By contrast, the self-driven motion of Ga-based LMDs driven by bubbles, which are produced by the catalytic action of enzymes on the solution, can last longer. (Figure 7g). But this strategy also has limitations, such as the fact that enzymes can be inactivated in the wrong environment [118].

4.5. Ultrasound-Induced Response

Using ultrasound technology to control and drive objects has been extensively explored over the past decade. The ultrasound field, as a safe and effective external driving force, has been used in combination with Ga-based LMDs for biomedicine, such as sonodynamic therapy (i.e., utilizing ultrasound for tumors’ treatment) [64,127]. In the ultrasound field, the acoustic radiation force obtained by absorbing and reflecting a wave drives metal droplets to move without direct connection. Recently, Wang and coworkers reported shape-transformable, fusible, rod-like, swimming, liquid metal gallium nanomachines (LGNMs) actuated via an ultrasound field, which can move autonomously in the levitation plane (Figure 7h) [119]. The motion performance of LGNMs is affected by many factors, such as the frequency and voltage of an ultrasound field and the length of LGNMs. And, when all other factors are unchanged, the LGNMs can achieve a maximum velocity of about 23 μm·s⁻¹ when the frequency of an ultrasound field reaches 420 kHz. Based on the photothermal effect, such ultrasound-driven nanomachines, combined with near-infrared light, can be used for photothermal cancer therapy (Figure 7i). Although ultrasound actuation shows the merits of a long lifespan without additional fuel, this strategy needs to produce a specific wavelength matching the size of a metal droplet to drive the object motion, so it entails certain requirements for the equipment. Hence, the motion driven by an ultrasound field still needs to be further studied, and it will provide a new method for the contactless actuation of LM nanodroplets and promote the development of next-generation precision theragnostics.

5. Applications of Gallium-Based Liquid Metal Droplets

In droplet form, compared to the bulk, Ga-based LMDs achieve higher response accuracy to various external stimuli, such as electric, magnetic, acoustic, light, thermal, and chemical fields. Among these methods, electric field control is common due to its convenience, non-contact nature, and pollution-free features. It is widely used in multi-functional devices that are small in size and highly integrated. Herein, we focus on discussing the representative applications of gallium-based LMDs in micro-robots, biomedical applications, and microfluidic systems, including pumps, microvalves, and mixers, which aim to show the excellent response characteristics and superiority of Ga-based LMDs used in these applications.

5.1. Microfluidics

5.1.1. Pumps

As the heart of a microfluid system, the micropump is the power source of microfluid transportation and an important symbol of the development level of microfluid systems. Generally, the detection platform with a microfluidic chip as the core often uses an external fluid pump device. Although the external pump can provide accurate and stable control for fluid transportation, and it can perform complex programmed actions, this method will undoubtedly greatly weaken the convenient performance of the microfluidic chips and cause a huge waste of test samples due to the dead volume of the extension pipeline. Therefore, a new type of micropump needs to be developed urgently. Fortunately, Ga-based alloys, as an emerging stimulus-response material, can be utilized to pump a sample solution in conventional and new ways.

The traditional pumping method is to discharge liquid by moving components, which is known as a mechanical pump, including diaphragm pumps (the moving element is a flexible membrane) and rotary pumps (the moving element is a microscale gear). Li et al. proposed the beating behavior of the heart generated by electronically controlling galinstan droplets, and they reported a diaphragm pump based on this phenomenon (Figure 8a) [128]. The galinstan droplet heartbeat motion can be accurately controlled by modulating the driving voltage amplitude and frequency, allowing precise control for the fluid flow of a diaphragm pump. At a 1.5 V_PP and 2 Hz square wave, this pump creates high flow rates of about 70 μL/min. The movement behavior of Ga-based LMD is limited to a specific liquid environment, which is undoubtedly the biggest obstacle to its further development in the fabrication of a liquid metal pump. Therefore, Xue et al. designed a driving module that can transmit rotational motion to fabricate a rotary pump for universality (Figure 8b) [129]. Subsequently, the rotary pump was used to achieve continuous/intermittent propulsion to simulate human venous/arteries and the flow rate can be modulated by adjusting the driving voltage.

Compared with mechanical pumps, due to the high integration of LM non-mechanical pumps, it can greatly improve the convenience of detection devices based on a microfluidic chip. Hence, LM non-mechanical pumps attract wide attention from scientists. As shown in Figure 8c, Tang and coworkers demonstrated a pump that replaced mechanical moving parts with a galinstan droplet, which is called an LM-enabled pump, for driving a range of liquids upon the application of a modest electric field [58]. This pumping behavior is achieved by applying an electric field to metal droplets to induce electrowetting/deelectrowetting effects on their surface. The proposed LM-enabled pump shows the unique advantages of high efficiency and simple manufacturing. It fundamentally promotes the development of highly integrated microfluidics system. However, the LM enable pump can only be employed in a limited solution, such as neutral sodium chloride and sodium hydroxide, etc. For this reason, Xue et al. further expanded the pumping scope of the solution sample of the LM-enabled pump, that is, the pumping of ionic liquids via LM-enabled electrocapillary under DC-biased AC forcing (Figure 8d) [130].

5.1.2. Microvalves

Fluid control is the core of the operation of a microfluidic lab-on-a-chip. All the processes involved in the lab-on-a-chip, such as sampling, mixing, separation, and reaction, etc., need to be completed in the controlled fluid. A microvalve is one of the main components of a microfluidic system, and it can control the liquid flowing in a specified direction in the microchannel. Ga-based LMD is a promising material for manufacturing microvalves due to its miniature size and stimulus-response behavior via an electrical field. Additionally, Ga-based LMDs are immiscible with an aqueous solution, which allows them to be applied as microvalves themselves. As illustrated in Figure 8e, Gong and coworkers reported a novel, on-chip, liquid-metal-enabled microvalve by electrically controlling the deformation of a metal droplet in a designed post array in a microchannel [131]. This microvalve avoids complex structures and saves much of the space occupied by the traditional mechanical structure of a valve core, making it easy to integrate with other functional parts on the same chip layer. Meanwhile, this microvalve also exhibits the advantage of a low leak rate, which is approximately less than 0.43 μL/min at 330 mbar, and it guarantees effectiveness within 145 switching operations.

5.1.3. Mixer

At the microscale, the flow state of microfluid is the laminar flow, rather than the turbulent flow, mainly because the Reynolds coefficient of the fluid in the microchannel is relatively low. In this laminar-flow state, the traditional turbulent mixing between two liquids cannot be produced, and fluid mixing can only occur through molecular diffusion in a laminar flow. Hence, the rapid mixing of fluids is an important requirement for microfluidic technology, especially in drug delivery, DNA analysis, and micro-reactors, etc. One method to achieve this is to design the microstructure (e.g., grooves or mechanical barriers) on the inner wall of the microchannel, which is a passive approach for inducing chaotic advection. Nevertheless, this strategy is complex, and it increases the cost of the equipment. The active method is to employ an external power supply source to increase the mixing quality, including electrical, magnetic, thermal, and ultrasonic, etc. [134,135,136,137]. By contrast, active mixers have a higher mixing efficiency than passive mixers [138,139]. As shown in Figure 8f, Tang et al. reported a galinstan LM actuator for inducing chaotic advection [132]. When a sinusoidal signal is applied, due to the continuous electrowetting effect, the harmonic Marangoni flow is generated at the surface of a galinstan droplet. This strategy has been applied in a microfluidic system, demonstrating both low power consumption (about 0.3 mW) and a high mixing efficiency even at a high flow rate (95, 84, 80, and 70% at the flow rates of 25, 50, 75, and 100 μL/min, respectively). Jafarpour et al. introduced an LM-enabled pump mechanism, developing a mini-scale Y-tape mixer. This mixer utilizes two liquid metal droplet pumps, separately inducing the chaotic advection flows in the two input branches to enhance the mixing rate (Figure 8g) [133]. Although the mixer based on a metal droplet can achieve a small structure, high efficiency, and high integration, it is only suitable for a very limited range of sample solutions.

5.2. Biomedical Applications

Based on their excellent biocompatibility and liquid state at human body temperature, gallium-based LMs have the potential to be applied in biomedicine and nanomedicine. For example, the surface functionalization of Ga-based nanoparticles is used in the treatment of infectious diseases, cancer, and so on [140,141,142,143]. In the medical field, micro-devices are among the most important medical tools of the future, capable of accomplishing tasks that were previously difficult or impossible to complete. Functionalized Ga-based LMDs can respond to external physical stimuli, presenting an unprecedented opportunity to serve as micro-robots within human cavities. These Ga-based LMD micro-robots can be used as drug carriers to accurately transport drugs to diseased sites, improving drug safety and utilization. Wang et al. studied rod-shaped LM micro-robots driven by sound energy that move to target cells and pierce and internalize into the cells [119]. Meanwhile, based on their self-propelling ability, LM micro-robots can also realize drug-delivery functions [144]. When the functionalized LM micro-robot reaches the inside of a cell, it can be triggered by light irradiation to achieve contactless, controlled drug release [145]. Additionally, the strong photothermal effect of LM can be utilized to target and kill cancer cells. For example, after specific modifications, LM micro-robots can be effectively heated by a laser and used in photothermal therapy for mice tumors, showing good curative effects [146].

5.3. Micro-Robot

With the rapid development of micro-nano technology, the requirements for micro-robots are becoming more and more acute. Due to the decrease in the robot size, the application of traditional robots is limited by complex construction and a traditional motor drive. Especially in the fields of medical experiments, cell culture, and lab-on-a-chip, etc., micro-robots are required to work in special circumstances such as liquid environments. Hence, many scientists are exploring new micro-robots. Ga-based LMD has excellent characteristics for metal and fluid, good biocompatibility, and mobility under an electric field, which provide new ideas for micro-robots. However, the extremely high fluidity of LMs poses a significant challenge to precisely controlling their movement in robotics. For this reason, Ge et al. designed a EGaIn-droplet micro-robot with rigid armor to make the movement more controllable (Figure 9a) [147]. This micro-robot can move along a predetermined trajectory, guided by a desktop-level small automatic guided vehicle (sAGV) system (Figure 9b). Meanwhile, it can load and transport goods by installing mechanical structures on the armor, such as mechanical arms and cargo platforms. As illustrated in Figure 9c, the same team showed a novel robot arm powered by two armored micro-robots, achieving a transfer of the motion of a metal droplet from an alkaline liquid environment to different environments [148]. The robot arm is capable of moving in complex trajectories and implementing different functions by changing various end effectors, such as pattern drawing and drug concentration detection, etc. A wheeled robot driven by a Ga-based LMD can realize directional movement as well, and it can move outside a liquid environment (Figure 9d) [149]. The EGaIn droplet inside the robot will climb toward to the anode direction along the wheel, which can alter the robot’s center of gravity. The rolling torque induced by the rising center of gravity as the driving force of the continuous forward movement of the wheeled robot generates a steady speed with about a maximum angular velocity of 2.5 rad s⁻¹. However, this single-wheel robot is unable to change directions (Figure 9e). Therefore, a tripodal, wheeled mobile robot was designed by Xue et al., and it enabled free movement in a two-dimensional plane (Figure 9f,g) [150].

The engine is the heart of the robot, and engines made of metal droplets have unique performance. As shown in Figure 9h, Li et al. reported a robot boat powered by Ga-based LMDs engines, and the driving force was induced via the Marangoni flow effect of metal droplets. These Ga-based LMD engines can precisely control the speed and direction of a robot boat without mechanical moving parts (Figure 9i) [151]. Although the LM engine provides less power than the electric motor, it has the advantage of a light weight, a low cost, and a simple structure. However, this LM engine needs to be kept always in a solution environment, which greatly limits its application. Wang and coworkers reported an LM motor that can create stable and continuous torque outside a liquid environment by encapsulating the electrolyte and multiple EGaIn droplets in an enclosed system (Figure 9j) [152]. As illustrated in Figure 9k, experiments showed that the engine can produce a sufficient driving force to drive the movement of a micro-robot.

6. Summary and Outlook

Ga-based LMs exhibit extensive potential in various fields, owing to their unique chemistry and physical properties, such as fluidic and mellitic attributes, low toxicity, low vapors, and excellent biosafety, making liquid metal a rapidly emerging research subject for functional materials. In the past decade, extensive research has been carried out on Ga-based LMDs. Remarkable achievements have been made in stimulus-response motion for external fields and applications, whereas there are still plenty of trouble that needs to be solved, which extremely hinders the further development in applications.

First of all, the properties of the material play an important role in its practical application. Among the extensive fantastic properties of LM, self-limiting oxidation is particularly prominent. However, due to its complexity, it has not been fully understood until now. A few-nm-thin oxide skin can change the rheology of LM, imbuing the LM with non-Newtonian fluid properties, which is beneficial to fluid patterning technology. Nevertheless, the surface oxide layer also increases the viscidity at the same time, and owing to the high viscosity, LM adheres to the substrate, which is not convenient enough for reconfiguring patterns. Additionally, the thickness of the oxide layer affects the electrical conductivity of LM materials, with variations ranging from insulating to conducting. Although this variation offers potential application opportunities, uncontrolled oxide film thickness results in unpredictable conductivity values, which can lead to inaccurate data for applications. Hence, there is still a long way to go in studying the properties of LM and applying them to practical products.

Secondly, according to the above-mentioned considerations, self-shear, microfluidics, molding technology, etc., are effective means for the preparation of Ga-based LMDs. Although the diameter of metal droplets has been achieved at as low as a few micrometers through current fabrication technology, the strategy for the large-scale production of metal droplets with stability and monodispersity is still hard. Generally, to prevent the metal droplets’ coalescence phenomena caused by high surface tension, surfactants need to be added to maintain the monodispersity of droplets. Nevertheless, this method also cannot stabilize metal droplets for a long time; meanwhile, the surfactant layer has adverse effects on their intrinsic properties, such as hindering electrical conductivity. For this reason, the surfactant layer needs to be removed before use, which greatly increases the complexity of operations. Therefore, the strategy for fabricating Ga-based LMDs should be speedily improved in future research, and it should be closely connected to practical applications.

Finally, the stimulus-response behavior of Ga-based LMDs to the external electrical field provides an opportunity for liquid metals as ideal smart materials. Currently, much research has shown the smart stimulus-responsive behavior of metal droplets, such as soft robots, microfluidic devices, and RF switches, etc. Nevertheless, it should be noted that the electrical response behavior must be carried out in the solution environment, which greatly limits the scope of its application. Moreover, the driven force is so weak that it cannot complete certain specified tasks, and since the metal droplets cannot precisely regulate the local surface tension, the control accuracy is relatively low. Overall, further research on the controlling method and theory is important, and it can establish a solid foundation for Ga-based LMDs in future applications.

Ga -based LMDs show clear and broad prospects in the field of micro-robots, medical treatment, and microfluidics, even showing potential for unexplored fields. Nevertheless, overall, the study of liquid metals is so cutting-edge that many application scenarios are still in the realm of imagination or fantasy, like the LM robot T1000 in the Terminator film, which is still a long way from practical application.