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Dec 21, 2018 - Electric Actuation of Liquid Metal Droplets in Acidified Aqueous. Electrolyte. Stephan Handschuh-Wang, Yuzhen Chen, Lifei Zhu, Tianshen...
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Cite This: Langmuir 2019, 35, 372−381

Electric Actuation of Liquid Metal Droplets in Acidified Aqueous Electrolyte Stephan Handschuh-Wang, Yuzhen Chen, Lifei Zhu, Tiansheng Gan, and Xuechang Zhou* College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, P. R. China

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S Supporting Information *

ABSTRACT: The electric actuation of room-temperature liquid metals, such as Galinstan (gallium−indium−tin), has largely been conducted in alkaline electrolyte. Addition of surface-active anions and a proper acidic pH are expected to influence the interfacial tension of the liquid metal due to a high surface charge density. Hence, it should be possible to actuate liquid metals in such acidic environments. To ascertain this, at first, the dependence of the interfacial tension of Galinstan in NaOH, acidified KI, and acidified NaCl electrolyte on the concentration of the surface-active anions OH−, I−, and Cl−, respectively, were studied. Subsequently, a systematic study of the actuation of Galinstan in acidified KI electrolyte was executed and compared to actuation in alkaline medium. In the presence of HCl and acidified NaCl electrolyte, the interfacial tension of Galinstan is only marginally altered, while acidified KI solution reduced the interfacial tension of Galinstan significantly from 470.8 ± 1.4 (no KI) to 370.6 ± 4.1 mN/m (5 M KI) due to the high surface charge density of the electric double layer. Therefore, in acidified electrolyte in the presence of surface-active anions, the electrically actuated motion of LM can be realized. In particular, the actuation of Galinstan achieves a higher average and maximum speed at lower applied voltage and power consumption for acidified KI electrolyte. The formation of high surface charge density in acidified environments signifies a paradigm shift and opens up new possibilities to tune interfacial tension and controlled LM droplet motion of room-temperature liquid metals.

1. INTRODUCTION Room-temperature liquid metals (LM), metals and alloys that are liquid near room temperature, have recently attracted renascent attention of the scientific community due to their extraordinary properties,1,2 such as low toxicity3 compared to mercury, electric and thermal conductivity, and intrinsic flowability. Therefore, liquid metals are believed to be ideal candidates for flexible electrics and thermal conductors.4−9 Furthermore, liquid metal (LM) offers great opportunities as chemical reaction environments,10−13 as catalysts,14−16 in biomedical applications,17−19 as energy-storage devices,20−22 as sensors,23−26 and in microfluidics.27−29 Application of gallium-based liquid metals is challenging due to the high surface/interfacial tension (IFT)30 and oxide formation, which leads to high yield stress and adhesion overcoming the cohesion of the LM with an oxide skin.31 Although this oxide skin can be favorable for some application,9 often a precise control of surface chemistry and surface/interfacial tension is of paramount importance for realizing applications, especially in research areas related to filling and printing of filigree features and generation of nano-/microdroplet dispersions.27,32−37 Notably, Khan et al. showed that the surface tension of LM can be controlled by application of an electric potential from 509 mN/m to near zero,38 yet unfavorable surface oxidation was present at potential difference ≥−1.3 V © 2018 American Chemical Society

between working electrode and reference electrode. In contrast to an electric approach, the interfacial tension (IFT) of Galinstan varied with chemical environment due to the formation of an electric double layer (EDL).30,39 The integrated form of the Lippmann equation describes the decline of interfacial tension from the IFT (γ0) at point of zero charge (PZC) (φ0) to IFT (γφ) at known potential (φ) by modeling the metal immersed in ionic solution as a parallelplate capacitor.40,41 Experimental and theoretical values of the IFT versus dφ show mirror symmetry with the PZC as vertex, while the parabolic curve of IFT versus φ is approximately shaped like an inverted U. γφ = γ0 −

c (φ − φ0)2 2

(1)

where c denotes EDL capacitance per unit area. Movement and shape transformation can be induced by generation of external forces.42−45 These external forces include electric field, magnetic field, and ultrasound.46,47 For example, due to exertion of an electric field, the EDL charge density is redistributed, yielding a variation of the interfacial Received: October 9, 2018 Revised: December 7, 2018 Published: December 21, 2018 372

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Figure 1. (a) Course of the IFT of Galinstan/aqueous NaOH solution over time at varying NaOH concentrations. Measurements marked with a “*” were measured after removal of air (oxygen) from the solution. (b) Dependence of the IFT on the concentration of NaOH in the aqueous solution. (c) Course of IFT of Galinstan over time in a 0.005 M NaOH solution. 1st, 2nd, and 3rd denote, respectively, the first, second, and third droplets generated in the same solution (cuvette 1 × 1 cm2, 2 mL solution). (d) Galinstan droplet shape in 0.00001 M NaOH, 0.001 M NaOH (oxidation), and 5 M NaOH solution after 50 s. Measurement marked with a “*” was measured after removal of air (oxygen) from the solution.

For this purpose, at first, the variation of interfacial tension with increasing concentration of NaOH, HCl, KI, and NaCl is evaluated and compared to the dependence of the interfacial tension of mercury in aqueous electrolyte. Acidified electrolyte, which exhibits a strong impact on the IFT (KI solution), was employed for the actuation of LMD. Here, especially the critical voltage and the dependence of the velocity on the applied voltage are scrutinized and discussed. The actuation in the acidified medium has the advantages that lower voltages are needed for actuation, and the LMD movement is faster while lower concentration of acid is required compared to base. In contrast to actuation in alkaline conditions, in acidic conditions also, other ions can be employed to form high surface charge density sufficient for LM actuation, such as bromide.

tension across a liquid metal droplet (LMD), which can be utilized for their transport.28 In this regard, Tang et al. showed that the velocity of LM actuation in NaOH solution is dependent on the applied voltage and can be altered by addition of nanoparticles to the surface.40 Wang et al. discovered that the velocity of actuation and the critical voltage depend on the size of the LMD and the concentration of NaOH in the electrolyte.48 Yang et al. explored the application of an AC voltage for generation of oscillating LMDs, which was adopted for liquid metal mixer and pump.49 Similarly, Wissman et al. and Yang et al. showed that electrically actuated LM in NaOH solution can be utilized for electric and thermal switches, respectively.50,51 Very recently, pulsating of liquid gallium has been demonstrated in NaOH and HCl solution at applied DC voltage, which originated from a self-regulating cycle of oxidation and oxide removal, affording IFT changes.52 Recent research concerned with electric actuation of LM and LMD, however, suggests that electric actuation is only possible in alkaline solution due to the formation of gallate (Ga[OH]4−) at the interface of the LM and in solution,40,49 which is crucial for the formation of an EDL. Although low surface charge density of the electric double layer in an acidic environment was suggested by Daeneke el al., which was coupled to Ga+3 localized at the LM interface,1 it has to be mentioned that Ga3+ is only generated at pH values near or lower than 1.53 The goal of the present experimental study is to demonstrate that LM actuation is possible in acidified solutions due to addition of surface-active ions, which are responsible for the generation of high surface charge density of the electric double layer, hereby confirming the suggestion made by Daeneke et al.

2. EXPERIMENTAL SECTION 2.1. Materials. Galinstan (gallium (∼68.5%)−indium (∼21.5%)− tin (∼10%), 6.440 kg/L) was purchased from Wochang (China). The dilution series of hydrochloric acid was prepared out of a purchased stock solution (36.46%, Huachangda, China), while the sodium hydroxide dilution series was prepared out of NaOH pellets (purity 96%, Macklin, China). KI (≥99%, Aladdin) and NaCl (99.8%, Macklin) were used to increase the ionic strength and change the charge density of the electric double layer. Deionized (DI) water (18 MΩ·cm) was used unless stated otherwise. A standard dumbbellshaped poly(tetrafluoroethylene) (PTFE) mold (length of channel 3 cm, width of channel 4 mm and height of channel 4 mm), which typically is used for the preparation of samples for tensile test, was utilized for the actuation experiments. 2.2. Measurement and Analysis of Interfacial Tension. The interfacial tension measurements were conducted with a computer373

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Figure 2. (a) Dependence of the IFT of Galinstan on concentration of HCl. (b) Dependence of the IFT of Galinstan in KI solutions with different composition and varying KI concentration. Four different KI solutions were prepared, namely, alkalized KI, oxygen-free KI, acidified KI (prepared ca. 3 h before use), and acidified KI (prepared directly before use) solution. The red line depicts a logarithmic fitting, while the dashed red line depicts the tentative experimental IFT behavior at high concentrations (>1 M KI). (c1−3) Schematic illustration of a Galinstan droplet in oxygenfree water, 0.1 M HCl, and 0.1 M NaOH, respectively. feature article by Berry et al.54 For the fitting process, the differences in density of the liquids were considered. In Table S1 in the Supporting Information, a list of the employed liquid densities is given. For measurements with low acid or base concentration, ≤0.1 M for HCl and ≤0.005 M for NaOH, oxygen was removed from the continuous phase by the freeze−thaw−pump method (see below). 2.3. Oxygen Removal by Freeze−Thaw−Pump Method. The solvent was poured into a thoroughly cleaned two-neck round-bottom flask and frozen in liquid nitrogen. After the liquid was fully frozen, vacuum was applied with an oil pump for 10 min. Subsequently, the stopcock was closed and the flask removed from the liquid nitrogen, which allowed the liquid inside to thaw slowly. During thawing, gas bubbles developed. After the thawing process was complete, residual gas cavities at the glass/air interface were removed by carefully shaking. Then, the round-bottom flask was again immersed into the liquid nitrogen for another freezing/vacuum/thawing cycle. To obtain oxygen-free solvent, at least three, typically four freezing/vacuum/ thawing cycles were performed. Finally, an inert gas (N2) was added to the solvent and withdrawal of solvent was done while a countercurrent flow of N2 was applied. 2.4. Actuation of Liquid Metal Droplets. The actuation of liquid metal droplets (3 ± 0.05 mm diameter) was conducted in a commercially available PTFE mold. At both ends of the mold (channel), copper contacts were placed and connected with a Keithley 2400 Multimeter as power source. Into the channel, the electric conductive aqueous solvent was poured (ca. 5 mL). Subsequently, the liquid metal droplet was added into the channel. The liquid metal droplet was actuated by applying a voltage of 1−21 V, while the movement was captured with a high-speed camera at a resolution of

based contact angle microscope (SDC-200, Sindin, China). The measurements of IFT were conducted in an air-conditioned room (temperature between 22 and 24 °C) in a quartz cuvette (path length, 1 cm; Jiangsu Jingbo, China). The cuvette was filled with 2 mL of the continuous phase (solvent), and then the needle (inner diameter, 1.35 mm; outer diameter, 1.95 mm, fluorocarbon) was immersed into the solvent (approx. 2 mm). In contrast to our previous measurements of IFT,30 the first droplet was always dismissed as this shortened the time for achieving equilibrium (see Figure 1a and compare with time scales of 20−200 s in the above-mentioned article). A second droplet with a volume of around 22 μL, roughly corresponding to a Worthington number of 0.4 (a measure of droplet size and accuracy, which is in contrast to the Bond number independent of the needle diameter, a Worthington number of 0.4 affords a high enough accuracy for IFT measurements; see also Supporting Information and ref 54), was generated and the measurement started. The measurements were conducted for 200−400 s, and images of the liquid metal droplet in contact with the continuous phase were acquired with 0.33−1 Hz (typically 400 s, 0.5 Hz), depending on the time needed for achieving equilibrium. Prior to analysis of the pictures, the contrast and brightness of the images were improved where appropriate with the software ImageBatch (version 5.7.1, freeware). This improved the edge detection of the surface (interfacial) tension analysis software, yet the value of the IFT was merely affected by this (typical change of IFT due to this procedure was lower than 1 mN/m). The images of the liquid metal droplet shape were analyzed by the software OpenDrop v1.1 by fitting the Young−Laplace equation to the experimental images in a batch process. For further information about the fitting process and the OpenDrop software, refer to the intriguing 374

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Langmuir 768 × 770 (Phantom Miro Lab 110) and an acquisition rate of 200 Hz. Special care was taken to avoid contact of the liquid metal droplet with the copper contacts. After each measurement, the liquid metal droplet was repositioned to the starting position, where it could be reused.

swift gas formation (H2) at the interface, as gas formation impedes the shape analysis. The lowest concentration, which inhibits oxide formation, was determined to be 0.1 M HCl (461.2 ± 2.1 mN/m). Overall, the IFT between Galinstan and oxygen-free HCl solution is higher at around 471 mN/m compared to the electrolyte at the same HCl concentration (oxygen present). The IFT is between 10−4 and 0.1 M HCl (oxygen absent) virtually constant, and is diminished slowly to 467.4 ± 1.7 mN/m (1 M HCl), as shown in Figure 2a. The minute dwindle of IFT in HCl solution can be explained by the low surface activity of the chloride anion. As a result of this low variance in IFT, solely HCl exhibits low surface charge density of the electric double layer and electrowetting approaches yield in low changes in IFT between the two LMD sides, leading to low driving forces and the fact that electric LM actuation is nearly impossible under these conditions. The reactivity of gallium-based liquid metals toward acids, bases, and in water is important for understanding the surface termination of the liquid metal in these environments. In alkaline environment, the well-known gallate (Ga(OH)4−) is generated at the surface, which is dissolvable in water. In contrast, at strong acidic conditions (near and below pH 1), Ga3+ is generated at the liquid−metal interface,53 while gas formation (H2) can be observed. The formed Ga3+ is soluble in water, for example, as GaCl3, leading to slow dissolution of liquid metal at strong acidic pH. If the liquid metal is exposed to water near neutral pH, the properties of the surface depend on the presence of oxygen in the solvent. If oxygen is available in the solvent, a thin and passivating oxide skin will be built up. This oxide skin is detrimental to IFT measurements (see Supporting Information) and influences mechanical properties and the adsorption of ions. In oxygen-free aqueous solution near neutral pH, oxo-hydroxide/hydroxides species exist,56,57 such as gallium hydroxides Ga(OH)xy, with x = 1−3 and y = 3 − x, and GaO(OH), Ga(OH)3 and GaO(OH) passivating the liquid metal. 3.2. Utilizing the Electric Double Layer in Acidified Aqueous Media. Inspired by work of Grahame,55 we investigated the influence of KI on the IFT of Galinstan. Potassium iodide showed strong surface activity on mercury and was able to diminish the IFT between mercury and KI electrolyte substantially. Yet, the high reactivity1 of Galinstan (toward oxygen and water) complicates a measurement of the IFT between Galinstan and solely KI. To measure the IFT change without surface oxides, at first, alkalized (0.1 M NaOH) KI electrolyte was employed. The measured dependence of IFT with concentration of KI, however, does not show the typical course of IFT (compare Figures 1b and 2a) affected by changing surface charge density of the EDL. Similarly, the course of IFT of Galinstan in oxygen-free KI solution does not follow the typical curve. In both cases, the buildup of high surface charge density of the EDL is impeded by either gallate or GaO(OH),30 probably due to electrostatic repulsion (see also Figure 2c). To generate a surface to which iodide ions can adsorb and generate high surface charge density of the EDL, acidified (0.1 M HCl) KI solutions were utilized, as 0.1 M HCl removes the oxide layer and reliably protects the liquid metal from reoxidation. This concentration of acid was already successfully employed by Frumkin et al. in their work concerning electrocapillary phenomena on gallium.58 KI was able to adsorb to this interface with a high surface charge density, which is evidenced by the steep decline in IFT with increasing concentration from 469.7 ± 0.8 (no KI) to 403.0 ±

3. RESULTS AND DISCUSSION 3.1. Effect of the Electric Double Layer on Interfacial Tension of Galinstan. Typically, electrocapillary curves (interfacial tension vs applied potential) are employed to ascertain the influence of ions on the interfacial tension and point of zero charge. However, with the measurement of interfacial tension vs ion concentration, we can easily see whether ions show no obvious surface activity or strong surface activity, which is needed for electric liquid metal actuation. Therefore, we determined the effect of ion concentration in aqueous solution on the interfacial tension of LMD. The effect of NaOH concentration on IFT of Galinstan in NaOH electrolyte is shown in Figure 1a,b. After 20 s, equilibrium was reached, which is much faster than our previous measurement.30 This short equilibrium time is related to the pristine interface of the LMD, as it was generated directly in the liquid and the previous (contaminated interface) LMD was discarded. With increasing concentration of NaOH, the IFT of Galinstan dwindled from around 491.8 ± 5.5 mN/ m (10−5 M) to 370.3 ± 1.2 mN/m (10 M). This decline in IFT is not linear, rather it shows a downward curvature with increasing concentration in a semilog plot, as shown in Figure 1b. In principle, the IFT for a specific concentration of NaOH can be calculated by the fit shown in Figure S4. The behavior observed is consistent with previously published IFT values on the electric double layer (EDL) of mercury.55 As the IFT still decreased with increasing NaOH concentration for concentrations larger than 0.3 M, the assumption of Wang et al., who predicted a saturated EDL charge density at concentrations higher than 0.3 M NaOH,48 was disproven. To visualize the effect of oxygen in the NaOH solution, measurements of the IFT in oxygen-free 0.001, 0.01, and 0.1 M NaOH were made. For high concentrations of NaOH, the difference between both IFT values is marginal (≤5 mN/m), while the difference increases with decreasing concentration up to ≥10 mN/m at 0.001 M, which is due to slower oxide removal at the Galinstan interface. The lowest concentration for a reliable and stable IFT measurement was determined to be 0.005 M NaOH (oxygen present), as shown in Figure 1c, which exhibited a stable IFT for ca. 5000 s. A second and third droplet generated in the same solution exhibited a shorter equilibrium time and higher IFT, both related to the lower amount of NaOH left in the solution. Interface oxidation for 0.001 M NaOH solution (oxygen present) is visible in the droplet shape (Figures 1d and S3), as the fitting yielded significant error and deviation and the fact that the equilibrium IFT was only stable for a very short time (see Figure S3a). In contrast, for the measurements of the oxygen-free aqueous electrolyte at low concentrations, no oxidation can be observed (compare Figure 1a,d). In contrast to the results obtained for NaOH, the IFT of Galinstan in HCl is virtually constant between 0.1 and 1 M solution, as the IFT dwindles only by 1 mN/m. By increasing the concentration to values higher than 1 M, the IFT was reduced significantly to 458.2 ± 0.6, 455.8 ± 9.3, and 442.0 ± 9.9 mN/m in 2, 5, and 10 M HCl solution, respectively. The accuracy of values at 5 and 10 M HCl, however, is low due to 375

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Figure 3. Scheme of Galinstan actuation (a) in alkaline solution and (b) in acidified KI solution as electrolyte. Sequential snapshots of Galinstan movement in (c) 1 M NaOH at 6 V and (d) acidified 0.5 M KI at 3 V.

2.3 mN/m (1 M KI). In this concentration range, the change in IFT(Δγ) due to the presence of KI at specific concentration (C) in an acidified solution can be easily calculated by the empirical eq 2, which may as well be used as a means to determine the concentration of iodide ions in an acidified aqueous solution. Δγ = 103.7 mN/m·ln(C + 1)

NaOH, and HCl on the interface and its implication for the generation of an EDL. 3.3. Galinstan Actuation in Acidified Aqueous Media. Figure 3 shows a schematic illustration of Galinstan actuation in alkaline and acidic solutions with KI as the electrolyte. In alkaline solution, a gallate layer is built up on the LM droplet/ NaOH solution interface.48 By application of an electric field, the negative charges of the gallate [Ga(OH)4]− are displaced toward the hemisphere facing the anode (+). Concomitantly, the positively charged ions (Na+) are drawn toward the hemisphere of the LM with highly negative charge density (hemisphere facing the anode). Due to the higher charge density, the IFT at the anode facing hemisphere is smaller (effect of EDL, see eq 1) than that of the cathode facing one. The driving force for the actuation in NaOH toward the anode as the change in IFT between the LMD hemispheres facing anode and cathode generates a pressure difference (Δp), which can be expressed by eq 3.

(2)

For concentrations higher than 1 M, this equation is not valid, as the values of IFT at higher concentration deviates from the fit. For example, 5 M KI solution yields a measured IFT of 370 ± 4.2 mN/m, while the calculated one is 285 mN/m. This may be related to a saturation effect. Due to the formation of a high surface charge density, acidified KI solutions are promising candidates for electric actuation of LMDs. In contrast, NaCl ions do not show a strong effect on the IFT, the IFT declined only by 10 mN/m from 0.01 M to 5 m NaCl (see Figure S5), which can be related to the minute surface activity of chloride ions (compare to small change of IFT of Galinstan in HCl solutions, Figure 2a). The scheme in Figure 2c summarizes the effect of water,

1 zy 1 zyz ji 1 ji 1 Δp = γA jjj + zzz − γCjjj + z jr z j r2 { r2 * zz{ k1 k r1 *

376

(3)

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Figure 4. Time−displacement curves of Galinstan in (a) 1 M NaOH, (b) acidified 1 M KI, and (c) acidified 0.5 M KI. (d) Comparison of the maximum velocity of the LM droplet in the PTFE channel with different aqueous electrolytes.

f1 = 6πrηυ0

where r1 and r2 denote the principal radii of curvature of the LM droplet, while γA and γC denote the IFT of the LM at the LMD hemisphere facing the anode and cathode, respectively. The * in eq 3 reflects the difference in principle radii of curvature on the two droplet hemispheres, as was demonstrated in contact angle measurements during chemical actuation by Zavabeti et al.39 In Figure 3c, such actuation of a Galinstan droplet by an applied voltage of 6 V in 1 M NaOH is shown. In contrast to the movement from the cathode to the anode in NaOH, Galinstan moves in acidified KI solution toward the cathode (−), as shown in Figure 3b. By applying an electric field to the LMD in acidified KI solution, the positive charges1 at the LMD interface are more abundant at the hemisphere facing the cathode (−), while the hemisphere facing the anode (+) has little positive charges. This influences the charge density of the anions near the LMD interface, yielding IFT and pressure difference (see eq 3). Therefore, the mechanism for LMD actuation, albeit reversed, remains the asymmetric distribution of charge density generated by the electric field. Sequential snapshots of the LMD actuation in acidified 0.5 M KI solution are shown in Figure 3d, depicting this movement from the anode toward the cathode. It has to be noted that against the driving force of liquid metal actuation, retardation forces, such as viscous friction between the droplet and its surrounding electrolyte, friction on the substrate, and chemical reactions at the LM interface (i.e., hydrogen or I2 generation), are acting.48 The viscous force on a spherical particle is dependent on the radius of the particle (r), viscosity of the surrounding medium (η), and the velocity of the particle through the medium (υ0), as shown in eq 4.

(4)

The friction force between the droplet and the substrate is given by the product of density difference between the LMD and the electrolyte (ρdiff), the volume of the droplet (υ), gravitational constant (g), and the friction coefficient between LMD and substrate (β), and is depicted in the following equation. f2 = ρdiff υgβ

(5)

The acceleration of an LMD (a) can then be expressed by the difference between the driving force (F) induced by applied voltage (Velectrode) and the sum of the retardation forces.48 a= =

F − (f1 + f2 ) ij 9ηυ ρ gβ yz Velectrode − jjj 2 0 + diff zzz j ρ z{ − 2πr ) L k 2r ρ

m 9q0Acurrent r 2 ρ(3Acurrent

2

(6)

In this equation, ρ is the density of the LM, m depicts the mass of the LMD, q0 is the initial charge on the EDL, and Acurrent is the cross-sectional area of the current path, while L is its total length. According to eq 6, the acceleration is dependent on the square of the radius of the LMD, leading to higher critical voltage, the voltage at which the LMD starts moving, necessary.48 Therefore, the size of the LM droplet was maintained for all measurements at 3 ± 0.05 mm (diameter). The acceleration and thus the droplet velocity are increased by higher Velectrode, yet faster droplet motion increases viscous 377

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Langmuir Table 1. Comparison of Recent Gallium-Based Room-Temperature Liquid Metal Actuation Literature actuation principle hydrostatic pressure or electric field electric field electric field (0−15 V) electric field different chemical environment electric field electric actuation electric field

electrolyte(s)

most auspicious results

1 M NaOH

LM actuation for thermal conductor51

0−5 wt % NaOH (0−1.32 M NaOH) 0.5 M NaOH 0−0.8 M NaOH 0.3−3 M NaOH + 0.3−3 M HCl

LM actuation for electric switch, controlled coalescence, and separation of LMDs50 LMD movement speed dependent on applied voltage, critical voltage>2 V, LM marble actuation40 LMD movement speed and critical voltage dependent on diameter of LMD4 exploitation of IFT gradient due to the formation of EDL (NaOH) and actuation of LMDs39

0−0.6 M NaOH 1 M NaOH, HCl acidified 0.5−1 M KI and other aqueous electrolytes

LMD oscillation due to AC electric field49 voltage-stimulated heartbeat effect52 LMD movement under acidic aqueous conditions, faster and lower critical voltage compared to NaOH actuation (this work)

electrochemical reaction (H2 generation) may occur, yielding at voltages equal to or higher than 15 V to a standstill. A similar course of maximum velocity versus voltage was obtained for both acidified KI solutions. However, the peak of the maximum velocity is shifted to lower voltages. Especially, for the lower concentration of 0.5 M KI, the highest maximum speed is already obtained at 4 V (7.1 ± 0.4 cm/s) instead of 6 and 9 V for 1 M KI and 1 M NaOH, respectively. Therefore, it appears that the separation of the charge density on the Galinstan interface is easier for lower KI concentration, which in turn leads to a bigger Δγ. The highest maximum velocity of 8.0 ± 0.4 cm/s of LMDs was obtained in acidified 1 M KI at 6 V. At voltages higher than 15 V, no LMD movement was observed, which may be related to generation of iodine or other surface reactions. This generation was already observed at voltages higher than 6 V at the droplet hemisphere facing the cathode. As iodine is generated, the charge density at this LMD hemisphere (facing cathode) is decreased due to the removal of I− ions and therefore the Δγ will be smaller. The most favorable condition for actuation of LMDs remains acidified 0.5 M KI at 4 V. As we have shown, the liquid metal motion is dependent on several parameters, such as ion concentration, voltage, and diameter of the droplet. The velocity of droplet motion is linearly dependent on the applied voltage up to a voltage of around 6 V. At higher voltages, detrimental effects, such as surface reactions (I2, H2) and oxide formation, may occur.48 Similar to the dependence on voltage, the droplet motion appears to be linearly dependent on initial surface charge density (q) at low concentrations.48 However, this dependency does not hold for high ion concentrations, as higher external forces are needed to separate the charges at the surface. The diameter of the LMDs influenced the actuation velocity strongly. Although we were able to actuate big droplets (≥3 mm) with low voltages (1.3 V), their velocity was comparably low. With decreasing diameter down to 1.2 mm, the droplet speed increased up to 21 mm/s (see Figure S7). The motion of big droplets is dominated by high friction (heavy) and viscous drag (see also eq 6), slowing them substantially. A KI solution acidified with 0.01 M HCl could not be actuated with a reasonable voltage, which we attribute to either gallium oxide (oxygen present) or GaO(OH) (oxygen-free) layer built up at these low concentrations. This might be the reason for the difference in interfacial tension of HCl and NaOH solutions at infinite dilution, which were determined to be 470.6 ± 1.8 and 491.8 ± 5.5 mN/m, respectively (see also Figures 1b and 2a). Higher concentrations of acid (i.e., 1 M HCl), on the other hand, are detrimental to actuation due to

forces, which is detrimental to droplet acceleration by the electric field. Therefore, the maximum velocity for the droplet motion can be determined. The critical voltage determined for actuation of LMD in 1 M NaOH solution was 3 V, while the critical voltage in acidified 1 M KI was 2 V. As shown earlier, the addition of 0.1 M HCl has only a minute effect on the change in IFT of Galinstan. Therefore, the effect of HCl is nearly exclusively limited to removal of the oxide layer and prevention of oxidation of the Galinstan interface. In this regard, it is worth noting that electrical actuation of liquid metal droplets (3 mm diameter) in (solely) 0.1 and 1 M HCl was not possible. Interestingly, by decreasing the concentration of KI to 0.5 M, the critical voltage was reduced to 1.3 V (see Figure 4d). At this concentration, we measured the critical voltage dependent on the diameter of the liquid metal droplet. For large droplet size (≥3 mm), the critical voltage is virtually constant at 1.3 V, while reduction in droplet size below this value necessitates greater voltages to actuate a droplet, i.e., 6 V for 1.2 mm diameter. Thus, it is more difficult to induce a sufficient surface tension difference in smaller liquid metal droplets.48 Further decline in KI concentration to 0.1 M KI lead to an unfavorable increase in the critical voltage to 9 V. The lowest critical voltage was obtained for acidified 0.5 M KI (Δγd ≈ 44 mN/m), which exhibited a lower IFT change (Δγd) due to EDL formation than liquid metal droplets actuated in higher concentrations of electrolyte (i.e., 1 M NaOH (Δγd ≈ 78 mN/m) and acidified 1 M KI (Δγd ≈ 69 mN/m)). This leads to the conclusion that crucial for the LMD actuation is not Δγd, but the electrically induced interfacial tension difference (Δγ) between both hemispheres of the LM droplet (see also eq 3). Thus, the needed force to displace the charges at the interface of the LMD needs to be high to ascertain a strong enough driving force to overcome the retardation forces. In this context, it appears to require less force to generate a big enough Δγ in acidified 0.5 M KI than in 1 M NaOH and acidified 1M KI. Figure 4a−c shows time−displacement curves of Galinstan in 1 M NaOH, acidified 1 M KI, and 0.5 M KI. Similarly, Figure 4d depicts the maximum velocity obtained for an LMD dependent on the applied voltage. For 1 M NaOH, the maximum velocity of the LMD rose from 2 cm/s at 3 V to around 6.5 cm/s at 9 V. Further incline in voltage yielded a decline in velocity (12 V, 6 cm/s), while no droplet movement was observed for voltages equal to or higher than 15 V. This absolute maximum can be related to the fact that the driving force Δγ, the difference in IFT between anode and cathode facing hemisphere of the droplet, increases with increasing voltage (see eq 6), but at voltages equal to or higher than 9 V, 378

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comparison, NaCl showed only a very weak effect on IFT and thus a low affinity to form an EDL. In contrast to previous articles reporting on LMD actuation in alkaline media,28,40,49−51 LMD actuation was feasible in acidic solution due to achieving high surface charge density of the EDL in acidified KI. Due to the higher mobility of its charge density compared to the one in alkaline environment, the actuation in KI solution was found to be superior to the actuation in NaOH. Lower voltages could be utilized for droplet actuation (4 V compared to 9 V in NaOH) and droplet movement was faster. It was found that the iodide anion can be substituted by other surface-active anions like bromide and still afford LMD actuation. The formation of high surface charge density in acidified environments opens up new possibilities to tune the interfacial properties of liquid metals for applications related to mixing, i.e., microfluidics and flexible conductors, and denotes a paradigm shift in controlled LM actuation, which might find application in novel pump and switch applications.

the corrosive nature (gas formation and dissolution of the liquid metal). The results shown here can be compared to previously obtained results for gallium-based liquid metal actuation, which are related to the asymmetric change in IFT over the LMD, as listed in Table 1. Electric actuation of LMs was typically done in ≈0.5 M NaOH solution. Compared to the here obtained results, actuation in NaOH needs higher voltages (3 V compared to 1.3 V) and LMD actuation is slower, when considering the same LMD diameter.4,40 The applications, such as thermal switch, electric switch, and oscillating droplet, are strongly dependent on the charge density of the EDL and were only shown in NaOH.49−51 HCl has been, however, utilized in three applications of LM action. First, it was utilized for actuation of LMD due to ionical imbalance.39 Yet, this approach utilizes the IFT difference between the LMD hemispheres located in NaOH solution (high charge density of the EDL) and HCl solution (low charge density of the EDL). Although this approach is intriguing, controlled actuation is by this means very challenging. Second, HCl could be utilized to achieve a voltage-driven heartbeat effect.52 In this approach, the LMD was actuated by contacting the electrode, resulting in swift oxidation, and fast oxide removal in NaOH or HCl. Although with HCl LMD actuation was possible, it was much slower due to the slower oxide removal by HCl30 while the mechanism for actuation is not based on charge density separation due to application of an electric field. Similarly, the motion and transformation reported by Liang et al. is induced by direct contact with an electrode and not an electric field.59 In contrast to these methods, in this article, actuation of LMDs in acidic aqueous electrolyte by applying an electric field was shown for the first time, which was more sensitive to applied voltage and yielded higher velocities compared to actuation in NaOH solutions. Due to the fact that a high surface charge density was formed for acidified KI solution, this solution can substitute alkaline solutions where necessary. Furthermore, we were able to achieve LMD motion even as the iodide anion was substituted by bromide anions. As a proof of principle, the motion of an LMD (diameter around 3 mm) in acidified potassium bromide is shown in Supporting Information video 5. The velocity of LMD motion is lower, and the critical voltage is 9 V (1 M KBr in 0.1 M HCl) higher compared to the motion in acidified KI solution, which can be related to the lower surface activity of bromide anion compared to the iodide anion, yet it shows the diversity of this approach.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b03384. A discussion of the interfacial measurement parameters and fitting procedure and also interfacial tension of Galinstan in acidified NaCl solution (PDF)

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LMD actuation in acidified 1 M KI solution, 3 V; LMD actuation in acidified 0.5 M KI solution, 3 V; LMD actuation in acidified 0.5 M KI solution, 1.5 V; LMD actuation in 1 M NaOH solution, 3 V; LMD actuation in acidified 1 M KBr solution, 9 V; all videos are slow motions, where 1 s is equal to 0.15 s real time (Videos 1−5) (AVI) (AVI) (AVI) (AVI) (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuechang Zhou: 0000-0002-1558-9423 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (21674064) and the Shenzhen Science and Technology Foundation (KQJSCX20170727100240033) for financial support of this work. S. Handschuh-Wang acknowledges China Postdoctoral Science Foundation Grant (2018M633144) for financial support. S. Handschuh-Wang thanks Dr. Tanju Yildirim for fruitful discussion(s).

4. SUMMARY AND CONCLUSIONS On the basis of previous studies regarding the interfacial tension of Galinstan,30 we demonstrated that IFT of Galinstan can be varied due to the formation of an EDL, which was applied to actuation of liquid metal droplets by an electric field. NaOH was found to afford a high surface charge density, while HCl showed only a minute change in IFT due to a weak tendency to form an EDL. Furthermore, it became evident that NaCl and KI only formed an EDL if no oxide, gallate, or Ga(O)OH layer was present. This was achieved by acidifying the aqueous solution with HCl with at least 0.1 M. However, we anticipate that other acids, such as HI and HBr, can also be utilized to remove and protect from surface oxides while reducing the need for anion addition. KI exhibited a strong effect on the IFT of Galinstan, which is related to a high surface activity and high surface charge density in the EDL. In

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