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• W3097933819 abstract "Liquid metals are a class of advanced materials with many wonderful and esoteric properties. The study of liquid metals has a long history and has been gaining renewed attention recently. However, there are still many gaps regarding basic physical and chemical properties of liquid metals. The surface of liquid metals provides a new perspective on understanding these materials, which in turn propels their developments. In this regard, many efforts have been devoted to exploring their surface properties. In this review, we provide a comprehensive summary of these surface properties of liquid metals and try to search out corresponding mechanisms underlying these characteristics. We summarize and envision some of the applications based on liquid metal surfaces. The findings will lay the foundation for the basic property exploration and practical application development of liquid metals. The surface of liquid metals plays a critical role in establishing thermodynamic equilibria (both mass and energy) at the interface between liquid metal and air. The exploration on the surface structure and electronic property of liquid metals triggers a new field of the liquid metal surface chemistry system, which is indispensable for researching scientific phenomena and developing practical technologies. In this review, we mainly focus on the low-toxicity, biocompatible, and room-temperature molding gallium-based (Ga-based) liquid metals. We firstly introduce these surface characteristics of Ga-based liquid metals such as wetting, oxidizing, layering, and freezing, and offer an in-depth discussion to unearth the underlying mechanisms. On this basis, these related various applications are presented. Finally, we conclude the review with insights into the challenges and prospects in this rapidly emerging research field. The surface of liquid metals plays a critical role in establishing thermodynamic equilibria (both mass and energy) at the interface between liquid metal and air. The exploration on the surface structure and electronic property of liquid metals triggers a new field of the liquid metal surface chemistry system, which is indispensable for researching scientific phenomena and developing practical technologies. In this review, we mainly focus on the low-toxicity, biocompatible, and room-temperature molding gallium-based (Ga-based) liquid metals. We firstly introduce these surface characteristics of Ga-based liquid metals such as wetting, oxidizing, layering, and freezing, and offer an in-depth discussion to unearth the underlying mechanisms. On this basis, these related various applications are presented. Finally, we conclude the review with insights into the challenges and prospects in this rapidly emerging research field. Due to its excellent conductivity and ductility, metal is a classic material in basic sciences and practical applications. In a broad sense, each metal can be known as the liquid metal when it exists in a liquid form. In this review, we focus on metals that are liquid at room temperature. Taking gallium (Ga) as an example, the existence of Ga dimer gives the metal certain covalent properties and decreases the melting point. These liquid metals have a metallic core (abundant electrons) and soft flexible shape, which lay the foundation of wearable electrical devices, soft robots, and liquid electrodes.1Daeneke T. Khoshmanesh K. Mahmood N. de Castro I.A. Esrafilzadeh D. Barrow S.J. Dickey M.D. Kalantar-Zadeh K. Liquid metals: fundamentals and applications in chemistry.Chem. Soc. Rev. 2018; 47: 4073-4111Crossref PubMed Google Scholar The electron-rich liquids also provide a unique reaction environment, ensuring the unique catalysis for energy transformation and material synthesis.2Zavabeti A. Ou J.Z. Carey B.J. Syed N. Orrell-Trigg R. Mayes E.L.H. Xu C. Kavehei O. O'Mullane A.P. Kaner R.B. et al.A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides.Science. 2017; 358: 332-335Crossref PubMed Scopus (205) Google Scholar Furthermore, some biocompatible liquid metals have been widely used in biological imaging and tumor therapy.3Yan J. Lu Y. Chen G. Yang M. Gu Z. Advances in liquid metals for biomedical applications.Chem. Soc. Rev. 2018; 47: 2518-2533Crossref PubMed Google Scholar These characteristics and applications have led to a boom in liquid metal research. The surface of liquid metals provides a new perspective on understanding these materials, which in turn propels their developments. Coulombic interactions of liquid metals consisting of ions are screened by the Fermi sea comprising conduction electrons, making the interatomic interaction strongly related to the electron density.4Yu S. Kaviany M. Electrical, thermal, and species transport properties of liquid eutectic Ga-In and Ga-In-Sn from first principles.J. Chem. Phys. 2014; 140: 064303Crossref PubMed Scopus (37) Google Scholar The electron density of liquid metals oscillates strongly at the surface due to the interaction change from intense metallic interaction to van der Waals interaction across the surface.5D'Evelyn M.P. Rice S.A. Structure in the density profile at the liquid-metal-vapor interface.Phys. Rev. Lett. 1981; 47: 1844Crossref Scopus (75) Google Scholar,6D’Evelyn M.P. Rice S.A. A pseudoatom theory for the liquid-vapor interface of simple metals: computer simulation studies of sodium and cesium.J. Chem. Phys. 1983; 78: 5225Crossref Scopus (96) Google Scholar Thus, the reason that makes liquid metals different from these nonmetallic liquids can be comprehended as the pairwise interaction model, in which the electron density of nonmetallic liquids smoothly changes along the normal line from bulk to surface, whereas in the case of liquid metals the change process is oscillating.7Regan M.J. Pershan P.S. Magnussen O.M. Ocko B.M. Deutsch M. Berman L.E. X-ray reflectivity studies of liquid metal and alloy surfaces.Phys. Rev. B. 1997; 55: 15874Crossref Scopus (110) Google Scholar,8Beysens D. Robert M. Thickness of fluid interfaces near the critical point from optical reflectivity measurements.J. Chem. Phys. 1987; 87: 3056Crossref Scopus (69) Google Scholar Understanding the relationship between surface order and electron density is fundamental in this aspect of surface chemistry, which is important for comprehending liquid metals. Surface chemistry dominates the behaviors of liquid metals and determines their application scope. These applications are illustrated in Scheme 1. The rheological behavior of liquid metals can be changed by means of designing a surface tension gradient, which is promising in microfluidics.9Dezellus O. Eustathopoulos N. Fundamental issues of reactive wetting by liquid metals.J. Mater. Sci. 2010; 45: 4256-4264Crossref Scopus (113) Google Scholar These flowing liquid metal droplets can be utilized to deliver drugs.10Kim D. Hwang J. Choi Y. Kwon Y. Jang J. Yoon S. Choi J. Effective delivery of anti-cancer drug molecules with shape transforming liquid metal particles.Cancers. 2019; 11: 1666-1677Crossref Scopus (4) Google Scholar However, the surface tension will be influenced by surface oxidation.11Dickey M.D. Emerging applications of liquid metals featuring surface oxides.ACS Appl. Mater. Interface. 2014; 6: 18369-18379Crossref PubMed Scopus (0) Google Scholar The surface oxidation layer gives liquid metals a certain rigidity, which stabilizes them in an unbalanced shape, ensuring the patterning of liquid metals.12Dickey M.D. Stretchable and soft electronics using liquid metals.Adv. Mater. 2017; 29: 1606425Crossref Scopus (435) Google Scholar These patterned liquid metals lay the foundation of flexible electrical devices. The surface oxidation layer is a natural two-dimensional (2D) material,13Carey B.J. Ou J.Z. Clark R.M. Berean K.J. Zavabeti A. Chesman A.S. Russo S.P. Lau D.W. Xu Z.Q. Bao Q. et al.Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals.Nat. Commun. 2017; 8: 14482Crossref PubMed Scopus (120) Google Scholar and the surface layering property and the ability of catalysis of liquid metals also open a new way to fabricate 2D materials.14Wang J. Zeng M.Q. Tan L.F. Dai B. Deng Y. Rümmeli M. Xu H. Li Z. Wang S. Peng L. et al.High-mobility graphene on liquid p-block elements by ultra-low-loss CVD growth.Sci. Rep. 2013; 3: 2670Crossref PubMed Scopus (41) Google Scholar Furthermore, liquid metal catalysts are surface-renewable, which can effectively avoid coking.15Taccardi N. Grabau M. Debuschewitz J. Distaso M. Brandl M. Hock R. Maier F. Papp C. Erhard J. Neiss C. et al.Gallium-rich Pd-Ga phases as supported liquid metal catalysts.Nat. Chem. 2017; 9: 862-867Crossref PubMed Google Scholar Generally, the surface dominates the properties of this increasingly important class of materials.16Lee J.G. Mori H. Solid/liquid two-phase structures in isolated nanometer-sized alloy particles.Phys. Rev. B. 2004; 70: 144105Crossref Scopus (24) Google Scholar In this review, we focus on the molding of Ga-based liquid metals to introduce their surface characteristics and the related applications (Scheme 1). For convenience, the “liquid metals” in this article refer to Ga-based liquid metals. We firstly introduce these characteristics of the liquid metal surface, such as surface tension, surface wetting, surface oxidation, and surface layering, focusing on the origin and the effect factors of these characteristics. We then introduce some related emerging applications based on these surface sciences. Finally, we also discuss further research trends and challenges in this area. Ultrahigh surface tension is one of the most intuitive fundamental properties of liquid metals (Figure 1A).17Kim D. Thissen P. Viner G. Lee D.W. Choi W. Chabal Y.J. Lee J.B. Recovery of nonwetting characteristics by surface modification of gallium-based liquid metal droplets using hydrochloric acid vapor.ACS Appl. Mater. Interfaces. 2013; 5: 179-185Crossref PubMed Scopus (101) Google Scholar Surface tension dominates many phenomena, such as the wettability of the substrate, low-friction rolling, and spontaneous fuse when splitting.18Wu J. Tang S.Y. Fang T. Li W. Li X. Zhang S. A wheeled robot driven by a liquid-metal droplet.Adv. Mater. 2018; 30: 1805039Crossref Scopus (33) Google Scholar,19Guo R. Sun X. Yuan B. Wang H. Liu J. Magnetic liquid metal (Fe-EGaIn) based multifunctional electronics for remote self-healing materials, degradable electronics, and thermal transfer printing.Adv. Sci. 2019; 6: 1901478Crossref Scopus (28) Google Scholar There are many endeavors for understanding the surface tension of liquid metals, including the hard sphere theory and the free electron model.20Reiss H. Frisch H.L. Lebowitz J.L. Statistical mechanics of rigid spheres.J. Chem. Phys. 1959; 31: 369Crossref Google Scholar,21Taylor J.W. XCVIII. The surface energies of the alkali metals.Lond. Edinb. Dublin Philos. Mag. J. Sci. 1955; 46: 867-876Crossref Google Scholar The surface tension value can be estimated using the following formula:γ=fm1dm≅N2|U(dm)|dm1dm=N2|U(dm)|dm2,(Equation 1) where fm is the force needed to bring an atom from the inner core to the surface, approximating to N2|U(dm)|dm. dm is the diameter of metal atoms, U(dm) is the interaction function of near atoms, and N is the number of atoms. It is necessary to multiply it by a coefficient of one-half because these surface atoms miss 50% of atom interaction at the air side. The underlying reason for the high surface tension of liquid metal according to the equation is that it is difficult to bring metal atoms to the surface from the inner core phase due to the intense interaction between metal atoms. This intense interaction results in the high fm and the asymmetrical force at the surface. Regulating the interaction between metallic atoms provides a route to control the surface tension, which is significant for engineering practice, for example, the processing of Ga compound semiconductors.22Tan S.C. Yang X.H. Gui H. Ding Y.J. Wang L. Yuan B. Liu J. Galvanic corrosion couple-induced Marangoni flow of liquid metal.Soft Matter. 2017; 13: 2309-2314Crossref PubMed Google Scholar At higher temperature, the aggravation in atomic thermal motion leads to an increase in kinetic energy and a decrease in the interatomic force. Meanwhile, the phase density difference at the surface will decrease when the temperature increases. Therefore, the surface tension of liquid metals decreases with the increasing temperature. There is some previous experimental research of liquid Ga surface tension over a wide range of temperature.23König U. Keck W. Measurement of the surface tension of gallium and indium in a hydrogen atmosphere by the sessile drop method.J. Less Common Met. 1983; 90: 299-303Crossref Scopus (22) Google Scholar,24Hardy S.C. The surface tension of liquid gallium.J. Crys. Growth. 1985; 71: 602-606Crossref Scopus (94) Google Scholar Some subtle differences among these results are associated with surface impurities. Generally, the surface tension reduces linearly as the temperature goes up (Figure 1B), which is described by Alchagirov and Mozgovoi:25Alchagirov B.B. Mozgovoi A.G. The surface tension of molten gallium at high temperatures.High Temp. 2005; 43: 791-792Crossref Scopus (17) Google Scholarσ=(715.3±1.0)–(0.090±0.002)(T–Tmelt).(Equation 2) The adsorbed solute molecules are polarized under the ionic potential of liquid metals and cover the surface of liquid metals. These molecules effectively reduce the interface energy. Therefore, the surfactants are always used to manipulate the surface properties of liquid metals. According to the Gibbs adsorption equation,τS=–dγRTd(lnas),(Equation 3) where τS is the excess adsorption capacity, which can be seen as the concentration of solute per unit interfacial area, and as is the activity of these solute molecules. Coupled with the Langmuir equationθs1−θs=Kas,(Equation 4) where θs is the cover degree of solutes and K is the cover coefficient, the surface tension variation can be described as follows:γ0–γ=RTτS0ln(1+Kas),(Equation 5) where γ0 is the surface tension of pure liquid metals and τS0 is the degree of saturated coverage. Thus, the absorbed solute molecules will decrease the surface tension of liquid metals. Alloying Ga with indium (In) and tin (Sn) will reduce the surface tension compared with pure Ga (Figure 1C). The predicted structure factor S(k) and the predicted pair correlation function (Figures 1D and 1E) indicate that the average atom distance in eutectic GaIn (EGaIn) and eutectic galinstan (EGaInSn) are broadened, which means a disordered structure in the alloy.4Yu S. Kaviany M. Electrical, thermal, and species transport properties of liquid eutectic Ga-In and Ga-In-Sn from first principles.J. Chem. Phys. 2014; 140: 064303Crossref PubMed Scopus (37) Google Scholar Coupled with the electron density change (Figure 1E),4Yu S. Kaviany M. Electrical, thermal, and species transport properties of liquid eutectic Ga-In and Ga-In-Sn from first principles.J. Chem. Phys. 2014; 140: 064303Crossref PubMed Scopus (37) Google Scholar the surface tension thus decreases due to the relatively weak atomic interaction. The influences of pressure are more complicated, since increasing the pressure requires the introduction of a second component, for example, inertia gas. On one hand, the pressure will increase the surface tension intrinsically according to the Gibbs interface model,∂γ∂PT,A=∂V∂AT,P,(Equation 6) where P is the pressure, T is the temperature, V is the volume, and A is Helmholtz free energy. However, on the other hand the surface tension always decreases with the increase of the pressure in many experiments. In this situation, the phase density difference between the two phases is decreasing and the adsorption of the gas on the liquid metal surface is also intensified, which will reduce the surface tension. There are other factors that influence the surface tension of liquid metals, among which the size effect is especially worth noting. At the macro scale, size has little effect on surface tension.26Tolman R.C. The effect of droplet size on surface tension.J. Chem. Phys. 1949; 17: 333Crossref Google Scholar However, when the size of liquid metal droplets reduces to the nanoscale, the surface tension is closely related to size. The surface tension of liquid metal in the nanoscale is difficult to measure and its value is often obtained by theoretical calculation. In 1949, Tolman proposed a new physical quantity, Tolman length, δ, defined as the difference between the radius of the equimolar surface and the radius of the Gibbs tension surface.26Tolman R.C. The effect of droplet size on surface tension.J. Chem. Phys. 1949; 17: 333Crossref Google Scholar According to the monolayer model and the Gibbs-Tolman-Kening-Buff equation, the surface tension of nanostructured liquid metals can be calculated as follows:σσ∞=11+2δ∞Rs,(Equation 7) where σ is the surface tension of liquid metal, σ∞ is the planar interface surface tension, Rs is the radius of tension surface, and δ∞ is a function associated with Tolman length, which depends on the size of droplets, atomic radius of liquid metal, and the atomic space utilization.27Xue Y.Q. Yang X.C. Cui Z.X. Lai W.P. The effect of microdroplet size on the surface tension and Tolman length.J. Phys. Chem. B. 2011; 115: 109-112Crossref PubMed Scopus (58) Google Scholar This physical quantity only works for the nanodroplets. Thus, we can draw a conclusion that the surface tension of nanoscaled liquid metals will decrease as the size reduces. Accompanied by adjustment in local surface areas, the generation of a surface tension gradient will propel liquid metal droplets into motion, which can be applied in many fields, such as drug delivery, mechanical motors, and bionics.28Zhang J. Guo R. Liu J. Self-propelled liquid metal motors steered by a magnetic or electrical field for drug delivery.J. Mater. Chem. B. 2016; 4: 5349-5357Crossref PubMed Google Scholar,29Chen S. Yang X. Cui Y. Liu J. Self-growing and serpentine locomotion of liquid metal induced by copper ions.ACS Appl. Mater. Interfaces. 2018; 10: 22889-22895Crossref PubMed Scopus (9) Google Scholar One important consequence of the ultrahigh surface tension is the poor wettability of liquid metals on most substrates. The wettability of liquid metals is a key issue involved in many applications, for example, circuit patterning.30Jeon J. Lee J.-B. Chung S.K. Kim D. Magnetic liquid metal marble: characterization of lyophobicity and magnetic manipulation for switching applications.J. Microelectromech. Syst. 2016; 25: 1050-1057Crossref Scopus (15) Google Scholar Therefore, it is necessary to make full sense of the wettability of liquid metals. According to Young's equation,31Young T. III. An essay on the cohesion of fluids.Phil. Trans. R. Soc. 1805; 95: 65-87Crossref Google Scholar,32Yuan B. He Z.Z. Liu J. Effect of electric field on the wetting behavior of eutectic gallium-indium alloys in aqueous environment.J. Electron. Mater. 2018; 47: 2782-2790Crossref Scopus (4) Google Scholarcosθ=σSV−σSLσLV,(Equation 8) cosθ=waσLV–1,(Equation 9) where wa is the adhesion work of liquid metal on the substrate and σ is the surface energy. The wetting is the joint effect of wa (presenting the interaction between liquid metal and substrate) and σ (presenting the cohesion of liquid metal). Manipulating wa or σ will change the wetting behavior, which is meaningful for liquid metal patterning and molding. Applying an electric field will form an electrically charged surface, which facilitates liquid metals to wet any substrate due to the electrostatic energy.33Beni G. Hackwood S. Jackel J.L. Continuous electrowetting effect.Appl. Phys. Lett. 1982; 40: 912Crossref Scopus (90) Google Scholar,34Lee J. Kim C.J. Surface-tension-driven microactuation based on continuous electrowetting.J. Microelectromech. Syst. 2000; 9: 171-180Crossref Scopus (0) Google Scholar Figure 2A shows the interface charge distribution between liquid metal and electrode under an electric field, producing an extra force to reduce the surface energy, which is termed Lippmann's effective energy:35Watson A.M. Cook A.B. Tabor C.E. Electrowetting-assisted selective printing of liquid metal.Adv. Eng. Mater. 2019; 21: 1900397Crossref Scopus (1) Google Scholar,36Mugele F. Duits M. van den Ende D. Electrowetting: a versatile tool for drop manipulation, generation, and characterization.Adv. Colloid Interf. Sci. 2010; 161: 115-123Crossref PubMed Scopus (50) Google Scholarσsleff=σSL–CV22,(Equation 10) where C=ϵ0ϵd, the capacitance per unit area of the interface. V is the applied voltage. The electrowetting contact angle is rewritten ascosθ=cosθ0+CV22σLV,(Equation 11) where θ0 is the intrinsic contact angle without the voltage and σLV is the interfacial energy of the liquid and vapor phase. This Lippmann-Young contact angle equation presents the excellent wettability of liquid metal on any substrates when it is applied with a large voltage. This concept can be applied to a liquid metal extrusion printing process (Figure 2B). The electrowetting-assisted selective printing is executed to improve the adhesion of liquid metal on substrates, whereby the pattern can be toggled and modulated by the magnitude of voltage.35Watson A.M. Cook A.B. Tabor C.E. Electrowetting-assisted selective printing of liquid metal.Adv. Eng. Mater. 2019; 21: 1900397Crossref Scopus (1) Google Scholar Electrowetting is a flexible method to generate and manipulate liquid metal droplets, which is widely applied to the fabrication of lab-on-chips.32Yuan B. He Z.Z. Liu J. Effect of electric field on the wetting behavior of eutectic gallium-indium alloys in aqueous environment.J. Electron. Mater. 2018; 47: 2782-2790Crossref Scopus (4) Google Scholar (A) Illustration of electrowetting device. The electrostatic energy between liquid metal and electrode improves the wettability.35Watson A.M. Cook A.B. Tabor C.E. Electrowetting-assisted selective printing of liquid metal.Adv. Eng. Mater. 2019; 21: 1900397Crossref Scopus (1) Google Scholar (B) Process of electrowetting printing. Scale bar, 250 μm. (A) and (B) reprinted (adapted) with permission from Watson et al.35Watson A.M. Cook A.B. Tabor C.E. Electrowetting-assisted selective printing of liquid metal.Adv. Eng. Mater. 2019; 21: 1900397Crossref Scopus (1) Google Scholar Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Reactive wetting for fabricating magnetic functional materials. Reprinted (adapted) with permission from Ren et al.37Ren L. Sun S. Casillas-Garcia G. Nancarrow M. Peleckis G. Turdy M. Du K. Xu X. Li W. Jiang L. et al.A liquid-metal-based magnetoactive slurry for stimuli-responsive mechanically adaptive electrodes.Adv. Mater. 2018; 30: 1802595Crossref Scopus (21) Google Scholar Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (D) Illustration showing the reactive wetting of EGaIn on the smooth In foil.38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.Langmuir. 2014; 30: 533-539Crossref PubMed Scopus (76) Google Scholar (E) SEM image of smooth In foil surface. Scale bar, 20 μm.38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.Langmuir. 2014; 30: 533-539Crossref PubMed Scopus (76) Google Scholar (F) Illustration showing the repelling of the textured surface sputtered by In to the EGaIn droplet.38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.Langmuir. 2014; 30: 533-539Crossref PubMed Scopus (76) Google Scholar (G) SEM image of the coarse In foil surface. Scale bar, 20 μm.38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.Langmuir. 2014; 30: 533-539Crossref PubMed Scopus (76) Google Scholar (H) Illustration of the textured surface of substrate. (D) to (H) Reprinted (adapted) with permission from Kramer et al.38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.Langmuir. 2014; 30: 533-539Crossref PubMed Scopus (76) Google Scholar Copyright 2014, American Chemical Society. On the contrary, the magnetic field will reduce the wettability of liquid metals.39Li C. Cao Y. Lippmann S. Ren Z. Rettenmayr M. Reduced wettability of solids by a liquid Ga-In-Sn alloy in a steady magnetic Field.J. Phys. Chem. C. 2018; 122: 27451-27455Crossref Scopus (1) Google Scholar The underlying mechanism remains to be clarified. One hypothesis posits that the poor wettability originates from the change of two-particle distribution function of liquid metals.40Iino M. Fujimura Y. Surface tension of heavy water under high magnetic fields.Appl. Phys. Lett. 2009; 94: 261902Crossref Scopus (17) Google Scholar Another opinion is that the magnetic dipole interaction will increase the surface tension of liquid metals.41Sun Z.H.I. Guo X. Guo M. Li C. Vleugels J. Ren Z. Van der Biest O. Blanpain B. Strong magnetic field effect on surface tension associated with an interfacial magnetic pressure.J. Phys. Chem. C. 2012; 116: 17676-17681Crossref Scopus (24) Google Scholar Recently, researchers discovered that the wettability of liquid metals may be related to the changes in viscosity under the magnetic field.39Li C. Cao Y. Lippmann S. Ren Z. Rettenmayr M. Reduced wettability of solids by a liquid Ga-In-Sn alloy in a steady magnetic Field.J. Phys. Chem. C. 2018; 122: 27451-27455Crossref Scopus (1) Google Scholar Generally, the contact angle equation under the magnetic field can be written as1+cosθ1+cosθm=1+13BLσϑ,(Equation 12) where θm is the contact angle between liquid metals and substrates in the magnetic field, B is the intensity of the magnetic field, L is a characteristic length, and ϑ is the viscosity of liquid metals. Thus, the contact angle will increase when liquid metals are exposed to an intense magnetic field. Besides electrowetting, there is another way to improve the wettability by improving the wa, namely reactive wetting, whereby these liquid metals chemically bond with substrates.42Zhang J.Q. Zhu X.H. Zeng M.Q. Fu L. Magnetically controlled on-demand switching of batteries.Adv. Sci. 2020; 7: 2000184Crossref Scopus (0) Google Scholar This is why Ga is highly corrosive and why we employ a dielectric layer in the electrowetting process.43Ludwig W. Bellet D. Penetration of liquid gallium into the grain boundaries of aluminium: a synchrotron radiation microtomographic investigation.Mater. Sci. Eng. A. 2000; 281: 198-203Crossref Google Scholar The reactive wetting strategy allows access to fabricating functional liquid metal particle mixtures, which are applied to magnetically controlled switches, implantable bioelectronics devices, and controllable obstacle cleaners (Figure 2C).37Ren L. Sun S. Casillas-Garcia G. Nancarrow M. Peleckis G. Turdy M. Du K. Xu X. Li W. Jiang L. et al.A liquid-metal-based magnetoactive slurry for stimuli-responsive mechanically adaptive electrodes.Adv. Mater. 2018; 30: 1802595Crossref Scopus (21) Google Scholar The reactive wetting process is irreversible, during which the wetting interface goes through complicated componential and morphological transformation and robust chemical bond forming. The chemical bonds stabilize these mixture systems. The chemical bond forming can be facilitated by modifying the components of liquid metals and/or substrates. For example, in Figure 2D, the EGaIn droplets readily wet the smooth In foil.38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.Langmuir. 2014; 30: 533-539Crossref PubMed Scopus (76) Google Scholar The existence of In in the liquid metals plays a catalytic role in accelerating the reactive wetting. The wetting process can be described as the power law,R=αtβ,(Equation 13) where R is the length of the baseline and t is time. At the initial time, when β = 1/2, the wetting behavior is similar to the complete wetting.44Kim S.J. Moon M.-W. Lee K.-R. Lee D.-Y. Chang Y.S. Kim H.-Y. Liquid spreading on superhydrophilic micropillar arrays.J. Fluid Mech. 2011; 680: 477-487Crossref Scopus (49) Google Scholar The β value then becomes 1/10, which means the behavior of EGaIn on the In foil is similar to that of a droplet spreading on a flat surface.45Courbin L. Bird J.C. Reyssat M. Stone H.A. Dynamics of wetting: from inertial spreading to viscous imbibition.J. Phys. Condens. Matter. 2009; 21: 464127Crossref PubMed Scopus (87) Google Scholar Generally, liquid metals can wet a substrate readily as long as there is intensive bonding in the surface. During the wetting process, whether the electrowetting or the reactive wetting process, the final contact angle is related to the roughness of interfaces. Compared with the smooth In foil mentioned above, a totally distinct phenomenon for EGaIn droplets adhering a coarse In foil sputtered by In was found, whereby the surface texture repels the EGaIn droplet (Figures 2D–2G).38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.Langmuir. 2014; 30: 533-539Crossref PubMed Scopus (76) Google Scholar Wenzel modified the Young's equation of liquid droplets on rough surfaces as follows:cosθ′=r(γs−γSL)γL=rcosθ,(Equation 14) where r is the roughness factor, which is defined as the ratio of actual surface area of a rough surface to the geometric projected area (Figure 2H).38Kramer R.K. Boley J.W. Stone H.A. Weaver J.C. Wood R.J. Effe" @default.
• W3097933819 created "2020-11-09" @default.
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• W3097933819 date "2020-11-01" @default.
• W3097933819 modified "2023-10-16" @default.
• W3097933819 title "Surface Chemistry of Gallium-Based Liquid Metals" @default.
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