Controlled Electrochemical Deformation of Liquid-Phase Gallium

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Controlled electrochemical deformation of liquid-phase gallium Adam F Chrimes, Kyle J Berean, Arnan Mitchell, Gary Rosengarten, and Kourosh Kalantar-zadeh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10625 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 2, 2016

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ACS Applied Materials & Interfaces

Controlled electrochemical deformation of liquidphase gallium Adam F. Chrimes,1,2,* Kyle J. Berean2 Arnan Mitchell,2 Gary Rosengarten,2 Kourosh Kalanterzadeh2 1. Institute of chemical and bioengineering, department of chemistry and applied biosciences, ETH Zürich, Switzerland. 2. School of engineering, RMIT University, Melbourne, Australia. KEYWORDS: Gallium, electrochemical, mirror, lens, freezing.

ABSTRACT: Pure gallium is a soft metal with a low temperature melting point of 29.8 °C. This low melting temperature can potentially be employed for creating optical components with changeable configurations on demand by manipulating gallium in its liquid state. Gallium is a smooth and highly reflective metal that can be readily manoeuvred using electric fields. These features allow gallium to be used as a reconfigurable optical reflector. This work demonstrates the use of gallium for creating reconfigurable optical reflectors manipulated through the use of electric fields when gallium is in a liquid state. The use of gallium allows the formed structures to be frozen and preserved as long as the temperature of the metal remains below its melting

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temperature. The lens can be readily re-shaped by raising the temperature above the melting point and re-applying an electric field to produce a different curvature of the gallium reflector.

INTRODUCTION The manipulation of liquid metals has great potential applications for creating components in microelectromechanical systems (MEMS) such as switches, pumps, valves, sensors, electrodes and reflectors.1-9 Mercury and eutectic gallium alloys are commonly used for such applications as they are in a liquid state at near room temperatures. Gallium alloys are now quickly taking over mercury for use in MEMS due to their low toxicity, small viscosity and low vapour pressure.8, 10-12 Pure gallium can be a beneficial material for establishing MEMS as well. One advantage of using pure gallium compared to mercury and other eutectic alloys, is that pure gallium can be readily switched between liquid and solid forms with minimal input of energy, as it has a melting temperature slightly above room temperature at 29.8 °C.13 This allows gallium to be readily deformed while in its liquid state, and then solidified into permanent structures. Gallium develops a thin (1-3 nm) oxide layer when exposed to air,14-18 preventing it from flowing freely in liquid form and giving it the ability to be shaped by electric fields.19-21 Additionally, this oxide layer increases the wetting effect by liquid gallium on glass surfaces. Nevertheless, the oxide layer can have a negative impact on the surface reflectivity and electrical conductivity. Contact with a strong base or acid solution can readily remove the oxide,20,

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creating a highly reflective surface; however this means that the gallium will not be able to hold


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its configuration. Nonetheless, this oxide layer can also present opportunities to mechanically stabilise gallium into non-equilibrium shapes.20, 22 Gallium and Galinstan can be liquefied and formed into distinct shapes at temperatures above their melting points by using a variety of applied forces.7, 20, 23 Electric fields have been shown to be effectively used for deforming liquid gallium, or its eutectic alloys like Galinstan, into specific physical configurations.6-7, 9, 23-24 The advantage of using gallium over Galinstan is its potential ability to be frozen into a non-equilibrium shape at near room temperatures (23°C), after the shaping process is over. This opens the door for many applications, as the process is convenient and requires minimal energy.25-26 Here, we show the use of gallium as an optical reflective element both in liquid and solid phases. Gallium is melted and frozen solid, while an electric field is incorporated to tune its physical configuration. We also investigate a method to generate periodic surface ripples during the gallium freezing process, which could be used for complex reflective element.

EXPERIMENTAL SECTION Gallium metal of 99.99 % purity was purchase from Sigma Aldrich. As the surface of molten gallium is easily oxidised in normal air, to prevent oxidisation of the surface the gallium was melted while it was submerged in a solution of 0.5 M sodium hydroxide (NaOH, Sigma Aldrich) in deionised water.23 This concentration of NaOH was chosen as it provides a good balance between two competing effects; (1) the concentration is high enough that gallium maintains a stable metal surface while submerged and (2) the concentration is low enough that bubble generation at the counter electrode has minimal impact on the droplet deformation performance.


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Initially, a singular copper probe was positioned inside of the NaOH solution and energised with an AC voltage (see supporting information Figure S1). When the probe was located beside a droplet of gallium, the gallium was attracted to the copper probe. This observation highlighted the need for a uniform electric field to surround the gallium droplet, with the expectation that the gallium will be attracted outwards in each direction equally, stretching and flattening the droplet in the process.9, 27-28 In order to provide a uniform electric field surrounding the liquefied gallium droplet, a copper ring with an internal diameter of 20 mm is placed around a 3 mm diameter gallium droplet (Figure 1). The copper ring is sealed against the copper substrate using a thin (< 1 mm) polyurethane film. The copper ring is 8 mm deep, and is filled with 1.7 mL of NaOH solution. The gallium droplet sits in the centre of the copper ring, and is in direct contact with the base copper substrate. This provides a conduction path for the gallium, and also serves to anchor the gallium droplet into a specific position. Surrounding the gallium droplet is a ring of PDMS with an inside diameter of 6 mm and a height of 1 mm. This PDMS ring was utilised to help keep the deformation of gallium uniform and spherical. According to our experiments, the ideal quantity of gallium for this configuration was 400 mg, as this produced and equilibrium shape small enough to be fully submerged in the NaOH solution. The positive terminal was connected to the substrate, and the negative terminal to the copper ring and a voltage was applied between the copper ring and the copper substrate, as shown in Figure 1. A signal generator was used to apply a square wave AC voltage to the system (Tabor Electronics 8200), and a CH InstrumentsCH700E electrochemical station was used for monitoring current flow. A thermoelectric module (TEC) was used as a heating element to bring the temperature of the system to above (40°C) and below the melting temperature of gallium, for meting and freezing the metal, respectively. The TEC temperature was stabilised with a precision of ±0.07 °C using a


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calibrated digital sensor on the device side, and a large (2.2 °C/W) heat sink on the opposing side (Figure 1).

Figure 1. Schematics of the system: (A) The entire system cross section showing (1) the copper ring (2) the gallium droplet (3) the NaOH solution (4) the insulating polymer layer (5) the copper substrate (6) the thermoelectric module and (7) the heat sink. (B) The entire system cross section when a voltage is applied between the copper ring and the copper substrate. Thickness for each layer is shown on the schematic in mm and the arrows represent the force vectors exerted on the liquefied gallium. RESULTS AND DISCUSSION In the first set of experiments an electric field was applied to a stationary droplet of liquid gallium anchored to the substrate by an exposed copper pad. The electric force applied was


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sufficient to alter the physical shape of the gallium, with the aim of creating a concave shape element for the focusing of light. Applied electric field was uniform around the droplet so as to stretch it on all sides with an equal force. When a DC voltage was applied to the system bubbles were generated at the negative copper electrode. It was observed that the bubbles form, particularly when more than 15 mA is applied across the electrodes, regardless of the voltage magnitude. Deformation of the liquid gallium droplet did not occur when the current was less than 15 mA; therefore bubbling at the negative electrode (the copper ring) could not be avoided if the gallium is to be deformed. When a DC voltage was applied, the deformation of the droplet did not reach a suitable equilibrium. The DC current caused gallium oxide to form on the surface of the liquid metal during the deforming process, and also promoted uneven deformations of the gallium droplet. To remove the oxide layer, bursts of negative voltage/current at regular intervals (AC voltage) were used. It was observed that the ideal frequency for the voltage depended on the desired deformation behaviour of the gallium droplet, and was also affected by the choice of the DC offset of the AC voltage. AC voltage waveforms including square, sinusoidal and triangular waves were applied, however only square waveforms showed significant deformations of the gallium. This is due to the fact that sinusoidal and triangular waveforms would require at least 1.4 times higher AC voltages, which causes a significant increase in the generation of hydrogen bubbles. The square wave signals were chosen as they were the most effective and also the analysis of their effect was more facile. The effect of the frequency of the applied AC signal was carefully studied. Ideally, the frequency of the AC voltage would be selected to reduce the oxide layer formation while having


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minimal effect on the deformation of the gallium droplet. It was observed that the largest deformation occurred with DC-only voltages, but with only DC-voltage there was a significant oxide layer which forms on the surface and violent bubbling occurs at the copper counter electrode. When the frequency of the voltage was increased above 200 Hz the droplet deformations were observed to reduce due to the mechanical damping of the system (Figure 2). When an AC voltage was applied without a DC offset, the gallium droplet oscillated rapidly. This oscillating effect has been effectively used by Tang et al.23 to create liquid metal based mixing systems. To avoid the strong oscillations of the gallium droplet, and to ensure a smooth force was applied to the gallium droplet, the voltage must have a DC offset applied. Figure 2 shows the effect of the DC offset being varied between +0.5 and + 2.5 V at different frequencies. The deformation of the droplet was assessed using the diameter of the droplet as seen from above using the microscope (see supporting information Table S1 – S6 for microscope images). The gallium diameter was measured before the voltage was applied, and then again after the application of voltage once the droplet was observed to be stable. Depending on the intensity of the DC offset, it took up to 30 seconds for the gallium to stabilise once a voltage was applied. The AC voltage magnitude was kept constant at 4V pk-pk as increasing the magnitude higher than this caused excessive DC voltage offsets in order to keep the droplet stable. Also, the duty cycle was set at 50% for all the following experiments, as any change in duty cycle altered the required DC offset voltage. It can be seen in Figure 2E that at 25 Hz the peak deformation was observed at an offset voltage of 1.5 V. Then, as the frequency was increased the peak deformation offset voltage reduced to ~1.3 V at 75 Hz. At frequencies above 100 Hz the deformation intensity was further reduced, and there was no longer a distinct peak observed for the offset voltage.


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Figure 2 - Images of the gallium droplet being deformed by a 200 Hz AC waveform with DC offsets of (A) no voltage, (B) 0.5 V, (C) 1.1 V and (D) 1.5 V. (E) The deformation % and DC offset/frequency plot and (F) the AC current waveform when 200 Hz 0.95 V offset voltage is applied. When high (+ 1.5 to + 2.5 V) DC offset voltages were applied across the system, the surface of the gallium became quickly oxidised and developed a rough texture (Figure 2D). If a smooth surface is needed for optical applications then this surface roughness negatively interferes with the gallium’s ability to reflect light, as light is scattered from the surface deformities. When the voltage was not applied (Figure 2A) and when the voltage with an offset of 0.5 V (Figure 2B) was applied the droplet remains spherical, having zero deformation on the horizontal plane. In this state, the droplet was being held together by the surface tension of liquid gallium.29 At the peak deformation of 1.5 V (from Figure 2D) the surface of the droplet developed textures due to


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the formation of an oxide layer. Instead, the most ideal voltage was 1.1 V (Figure 2C), which was high enough to cause significant deformations, but not so high as to form surface textures. It is clear from the microscope images in Figure 2 A–D that the electric field stretches the gallium droplet in the horizontal (equatorial) plane. The deformed gallium shape can be estimated as an oblate spheroid, where the flattening value for the spheroid can be calculated to show the extent of the vertical (polar) deformations. The equation relating the equatorial గ

diameter to the polar diameter is: ܸ = ቀ ଺ ቁ ‫ܣ‬ଶ ‫ܥ‬, where A is the equatorial diameter, C is the polar diameter and V is the volume of gallium. Since the volume of gallium is kept constant, the reduction in polar diameter can be calculated as ‫ ∝ ܥ‬1ൗ‫ܣ‬ଶ . When there is maximum deformation in horizontal plane of 170%, the height of the droplet in the vertical plane reduces to 34.6% of its original size (Supporting information Figure S2). This calculation assumes that the liquid gallium retains its oblate spheroid shape. An electrochemical workstation was used to monitor the current through the system in order to observe the current flow through the gallium during both positive and negative voltage cycles. Samples were taken at 50 µs intervals using a 200 Hz voltage with a DC offset of 0.95 V. Figure 2F shows the current during the positive voltage averages 45 mA, and for the negative voltage – 1 mA. The charge transferred to the system during the positive cycle is 903 µC (2.66 mJ), and charge removed in each negative cycle is 40 µC (–0.042 mJ). Over each cycle there is a net increase of 863 µC in the system. This net increase in energy is expected to be released though the generation of hydrogen gas at the negative electrode, with a small amount of energy being used to deform the gallium. To test the optical properties of the deformed gallium droplet as a reconfigurable reflector a 532 nm laser was set to illuminate the gallium at an angle of ‒45°. A laser pointer module


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(Jaycar, Australia) with a power output of 10 mW, and was positioned 70 mm from the gallium to illuminate the entire gallium surface. A beam profiling charge-coupled device (CCD) camera (Gentec Beamage CCD12 camera) was positioned at 45° at a distance of 70 mm (Figure 3A).

Figure 3 - (A) Schematic showing the positioning of a laser source and CCD detector, (B) a close up intensity image of the reflected laser light 23 seconds after the voltage was applied showing a focal spot ~ 200 µm in diameter and (C) a sequence of images showing the reflection of the laser light from the deforming gallium taken from the CCD output measured at 1 frame per second with each image scale bar equal to 1 mm.


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The CCD camera was set to capture an image every second and the aqusition time was 100 ms in all cases with a CCD detection area of 6.3 mm by 4.8 mm. The image sequence in Figure 3C was recorded from the Gentec beamage software, where the application of voltage is synchronised to occur at 0 seconds. The image sequence shows the formation of the gallium reflector using a voltage of 200 Hz with 1.1 V offset. There was a delay of ~15 seconds before any beam reflections were measured, after which three bright spots were observed on the CCD at 18 seconds (Figure 3B). After 5 more seconds the three spots merge into one tightly focussed spot with diameter of approximately 200 µm. However, this only lasted for one second, after which the focussing properties of the liquid gallium were significantly reduced. The focussing properties slowly decayed over time, and after ~60 seconds there was no longer a visible focal spot. This was due to the fact that after 60 seconds the gallium has most certainly reached its maximum deformed position, and the generation of oxygen at the surface of the gallium due to the applied voltage begins to oxidise the surface of the gallium. The oxide surface is no longer smooth, causing more of the incident light to be scattered. Freezing the optical element From the image sequence in Figure 3C it is clear that a stable focussing element cannot be created from molten gallium. The maximum measured intensity and focal spot diameter are measured between 19 and 23 seconds after the electric field is applied to the system. Using simple freezing it should be possible to physically lock the gallium into a shape at any specific time during the lens formation (i.e., between 19-23 seconds). The advantage of using gallium is that it can be readily frozen at temperatures below 29.77°C, which can be achieved using the


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TEC cooling/heating module. Up to this point the TEC module has only been used to keep the temperature stable at 40 °C and cooling down on demand. An additional advantage to freezing the gallium droplet is that once the gallium is frozen solid the deforming electric field can be removed, which stops the bubbles of gas forming at the electrodes. The formation of these bubbles will eventually lead to the evaporation of the NaOH solution, so the deforming voltage cannot be applied indefinitely without replenishing the solution. In order for the system to accurately freeze the gallium at a specific time, the cooling rates for the TEC system must be optimised. To cool the gallium droplet within the required time frame of 19 – 23 seconds, including cooling of the entire 1.5 mL of NaOH solution and the copper electrodes, the thermoelectric cooler needs to remove 94.5 J of thermal energy. A 40 Watt TEC was selected for this as it is be capable of transferring the 94.5 J from the liquid gallium system within the desired time frame. Based on the heatsink size, ambient temperature and thermal properties of the system, it was found that running the TEC at 3 A produced the highest temperature cooling rates, with a maximum rate of -11.8 °C.min-1. It is important to note that there is a noticeable delay of approximately 20 seconds from when the current is first applied to the TEC until the temperature of the gallium system begins to drop. Therefore the voltage applied to the gallium system must be applied after the current to the TEC is applied. A microcontroller is used to monitor the temperature of the NaOH solution, which is set to automatically apply the voltage to deform the gallium droplet when the temperature reaches a certain threshold (Figure 4A). Once the temperature of the system is below the freezing temperature of gallium the voltage is disconnected, and the NaOH solution removed to enable the study of the surface of frozen gallium.


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Figure 4 - (A) Temperature vs time, showing when the voltage will be applied to the gallium in relation to the gallium freezing temperature, (B) reflectivity measurements from the rapidly solidified gallium droplet, (C) photo of a solidified gallium droplet using a slow temperature ramp rate (-5 °C.min-1) and (D) photo of a solidified gallium droplet using a fast temperature ramp rate (-11.8 °C.min-1).


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Figure 4 C and D were stitched together from 36 different microscope images with a ×10 microscope objective using the ImageJ function developed by Preibisch et al..30 The images depict the gallium surface after the freezing process, and clearly show that the surface is covered in wrinkles and other surface deformities. This can be due to the fact that the gallium is cooled unevenly from the bottom substrate, and due to the expansion of gallium when freezing (3.1%).31 The gallium was frozen using two different freezing rates to see if it was possible to reduce these effect, however it appeared to make little difference to the quality of the surface textures (Figure 4 C-D). It is probable that the formation of a thin oxide layer on the surface is also contributing to the surface wrinkles. Gallium oxide’s thermal expansion coefficient is far less than that of the pure gallium metal (7×10-9 K-1 for the oxide32 and 7×10-9 K-1 for the metal33). Removal of the oxide layer through the use of highly concentrated acids (HCl) and bases (NaOH) has been shown to improve the reflectivity of a gallium based telescope mirror.34 Due to the uneven nature of the solid gallium surface, acquiring a consistent reflectivity measurement was difficult. A CRAIC microspectrophotmeter was used to obtain UV-vis and NIR specular reflectance readings from a 50 x 50 micron area. The CRIAC measurements were taken from a number of locations to gain an overall understanding of the reflectivity of the surface. The Black line in Figure 4B represents the average reflectivity of the surface (with reference to aluminium). The red represents the maximum reflectivity measure from the gallium, and the green represents the minimum. The average reflectivity is ~12 %, and some areas were found to be more reflective with reading up to 37 %; these low readings of specular reflectance and not surprising given the observed roughness of the gallium’s surface. Ripple formation


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One interesting artefact that was observed during the freezing of gallium occurred when the gallium droplet was frozen while a low frequency (25 Hz) and the slow freezing rate (~20 seconds) was used. Periodic ripples were observed on the surface of the gallium for the slow cooling/low frequency case. This effect was observed using the Bruker optical interferometry profiler on the solidified gallium droplets (Figure 5). Small ripples with a periodicity of ~100 µm can be observed on top of the larger wrinkles caused by the freezing. Alternatively, when the gallium was frozen quickly while being deformed by a higher frequency voltage (200 Hz) there was no observable smaller ripples. In both high and low frequency cases wrinkles can be observed in the frozen gallium, but only for the low frequency case can additional smaller ripples be observed.


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Figure 5 - (A) Surface profile of the solidified gallium when cooled rapidly with 200 Hz deformation voltage, (B) surface profile of the solidified gallium when cooled rapidly with 25 Hz deformation voltage and (C) 3D profile of the surface of slowly frozen gallium droplet with 25 Hz deformation voltage. It is expected that the periodicity of the ripples can be controlled by altering the freezing rate and the frequency of the applied voltage. Lower frequencies would produce ripples with wider spacing and higher frequencies would produce closely spaced ripples. The advantage of using the gallium is that the periodicity of the ripples can be changed relatively easily by melting the gallium and re-freezing it using the desired frequency settings. This type of reconfigurable structures can have potential applications in Fresnel type reflectors. The pitching of the peaks of the ripples can be adjusted by sweeping the frequency during the freezing process, this can be used to make the outer most ripples with close spacing which the inner ripples, close to the centre, would have larger spacing’s. Again, due to the use of gallium the Fresnel reflector could be reformed to have a different focal distance, or tuned to work at a different wavelength.

CONCLUSION Gallium has been shown to have similar properties to other eutectic liquid metals, including its ability to be deformed in the presence of electric fields. This ability was demonstrated in this work for use as a configurable reflecting element. Gallium deformation was characterised for different voltages and frequencies, with the optimum settings analysed for optical characterisations. The gallium was initially molten, and once exposed to a uniform electric field it was stretched form a concave lens-like shape. However, the system was not stable for very


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long, as the gallium continued to deform beyond the optimum shape. Therefore the gallium was physically frozen in the ideal shape, which also allowed the resulting metal mass to be analysed by reflectometry and surface profiling to determine its characteristics. Although the resulting gallium surface was not as reflective as anticipated, due to the presence of surface wrinkles and the poor reflectivity of gallium oxide, a reconfigurable gallium mass can be implemented for applications that do not require smoothed surfaces, such as mechanical valves, microwave frequency reflector or tuned RF absorption. Finally, we demonstrate the application of an AC electric field has a secondary side effect of producing periodic surface ripples that appear to be controlled by the electric field frequency and the freezing rate of the gallium. These surface ripples could potentially be used to create textured surfaces for optical elements, including Fresnel type reflection elements.

ASSOCIATED CONTENT Supporting Information. The supporting information contains more detailed microscope images of the droplet deformations, as well as simulations of height of the droplet with respect to deformation percentage. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding Authors: Adam Chrimes ([email protected]) and Kourosh Kalantar-zadeh ([email protected]) Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Funding Sources This work was supported by ARENA funding for the Micro Urban Solar Integrated Concentrator project. ACKNOWLEDGMENT Dr. Adam Chrimes would like to acknowledge the support of the Victorian Government through the 2015 Victorian Postdoctoral Research Fellowship program. REFERENCES (1) (2) (3) (4)

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Mohammed, M.; Sundaresan, R.; Dickey, M. D. Self-Running Liquid Metal Drops That Delaminate Metal Films at Record Velocities. ACS Appl. Mater. Interfaces 2015, 7, 23163-23171. Zhang, J.; Yao, Y.; Sheng, L.; Liu, J. Self-Fueled Biomimetic Liquid Metal Mollusk. Advanced Materials 2015, 27, 2648-2655. Mangum, B. W.; Thornton, D. D.; Division, I. f. B. S. H. The Gallium Melting-Point Standard. U.S. Dept. of Commerce, National Bureau of Standards: The University of Michigan, 1977. Dumke, M.; Tombrello, T.; Weller, R.; Housley, R.; Cirlin, E. Sputtering of the GalliumIndium Eutectic Alloy in the Liquid-Phase. Surf. Sci. 1983, 124, 407-422. Regan, M. J.; Pershan, P. S.; Magnussen, O. M.; Ocko, B. M.; Deutsch, M.; Berman, L. E. X-Ray Reflective Studies of Liquid Metal and Alloy Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55, 15874:1-11. Regan, M. J.; Tostmann, H.; Pershan, P. S.; Magnussen, O. M.; DiMasi, E.; Ocko, B. M.; Deutsch, M. X-Ray Study of the Oxidisation of Liquid-Gallium Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55, 10786:1-5. Scharmann, F.; Cherkashinin, G.; Breternitz, V.; Knedlik, C.; Hartung, G.; Weber, T.; Schaefer, J. A. Viscosity Effect on Gainsn Studied by Xps. Surf. Interface Anal. 2004, 36, 981-985. Dickey, M. D. Emerging Applications of Liquid Metals Featuring Surface Oxides. ACS Appl. Mater. Interfaces 2014, 6, 18369-18379. Hayes, G. J.; Ju-Hee, S.; Qusba, A.; Dickey, M. D.; Lazzi, G. Flexible Liquid Metal Alloy (Egain) Microstrip Patch Antenna. IEEE Trans. Antennas Propag. 2012, 60, 21512156. Dickey, M. D.; Chiechi, R. C.; Larsen, R. J.; Weiss, E. A.; Weitz, D. A.; Whitesides, G. M. Eutectic Gallium-Indium (Egain): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature. Adv. Funct. Mater. 2008, 18, 10971104. Khan, M. R.; Trlica, C.; So, J.-H.; Valeri, M.; Dickey, M. D. Influence of Water on the Interfacial Behavior of Gallium Liquid Metal Alloys. ACS Appl. Mater. Interfaces 2014, 6, 22467-22473. Kim, 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 Hydrocloric Acid Vapor. ACS Appl. Mater. Interfaces 2013, 5, 179-185. Tang, S.-Y.; Sivan, V.; Petersen, P.; Zhang, W.; Morrison, P. D.; Kalantar-zadeh, K.; Mitchell, A.; Khoshmanesh, K. Liquid Metal Actuator for Inducing Chaotic Advection. Adv. Funct. Mater. 2014, 24, 5851-5858. Sivan, V.; Tang, S.-Y.; O'Mullane, A. P.; Petersen, P.; Kalantar-Zadeh, K.; Khoshmanesh, K.; Mitchell, A. Influence of Semiconducting Properties of Nanoparticle Coating on the Electrochemical Actuation of Liquid Metal Marble. Appl. Phys. Lett. 2014, 105, 121607:1-5. Ge, H.; Li, H.; Mei, S.; Liu, J. Low Melting Point Liquid Metal as a New Class of Phase Change Material: An Emerging Frontier in Energy Area. Renewable Sustainable Energy Rev. 2013, 21, 331-346. Li, H.; Yang, Y.; Liu, J. Printable Tiny Thermocouple by Liquid Metal Gallium and Its Matching Metal. Appl. Phys. Lett. 2012, 101, 073511:1-4.


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Table of Contents Graphic:


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149x107mm (300 x 300 DPI)


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Images of the gallium droplet being deformed by a 200 Hz AC waveform with DC offsets of (A) no voltage, (B) 0.5 V, (C) 1.1 V and (D) 1.5 V. (E) The deformation % and DC offset/frequency plot and (F) the AC current waveform when 200 Hz 0.95 V offset voltage is applied. 171x104mm (150 x 150 DPI)



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(A) Schematic showing the positioning of a laser source and CCD detector, (B) a close up intensity image of the reflected laser light 23 seconds after the voltage was applied showing a focal spot ~ 200 µm in diameter and (C) a sequence of images showing the reflection of the laser light from the deforming gallium taken from the CCD output measured at 1 frame per second with each image scale bar equal to 1 mm. 168x139mm (150 x 150 DPI)


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(A) Temperature vs time, showing when the voltage will be applied to the gallium in relation to the gallium freezing temperature, (B) reflectivity measurements from the rapidly solidified gallium droplet, (C) photo of a solidified gallium droplet using a slow temperature ramp rate (-5 °C.min-1) and (D) photo of a solidified gallium droplet using a fast temperature ramp rate (-11.8 °C.min-1). 83x173mm (300 x 300 DPI)



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(A) Surface profile of the solidified gallium when cooled rapidly with 200 Hz deformation voltage, (B) surface profile of the solidified gallium when cooled rapidly with 25 Hz deformation voltage and (C) 3D profile of the surface of slowly frozen gallium droplet with 25 Hz deformation voltage. 171x136mm (150 x 150 DPI)


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Controlled Electrochemical Deformation of Liquid-Phase Gallium

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