Communication 1600913 and ena- and Both [24] Unfortu- Finally, a Finally, [21–23] If the oxide (1 of 9) [20] [33] [19,21,22,24–26] [15,18,21,30,31,34] [18,19,21,24–26] www.advmatinterfaces.de and have since developed and have since developed [19] but practical applications of this - tem room at solid is oxide This forming a gallium oxide that has and most recently as a source for and most recently [35–39] [26] [30–32] [27] wileyonlinelibrary.com [11,28,29] self-destructive circuitry (path-destructive liquid self-destructive [25] In this work, we demonstrate control over the surface oxide The two main etchants used for aqueous oxide removal The two main etchants used for aqueous It is well known that gallium oxidizes rapidly in normal It is well known that gallium oxidizes are generally limited to environments that can withstand the presence of a highly corrosive gas. of liquid metal droplets, which in turn controls the wetting and adhesion of those drops on a substrate, using purely environ- use of highly acidic HCl and compare the mental stimuli. We highly alkaline NaOH (in solution) in removing the oxide to that although both solvent show release pinned droplets. We solutions result in high contact angles between the droplet and the substrate, NaOH achieves this result at a rate that is further explore the orders of magnitude faster than HCl. We use of neutral distilled water to regrow the oxide and manipu- late the contact area of the liquid metal droplet on the surface, we Finally, substrate. the to readhere to droplet the causing apply this environmentally controlled depinning and repinning perature and forms a nanoscale stabilizing shell around perature and forms a nanoscale stabilizing liquid metal to be liquid gallium indium alloys, enabling the to, many different patterned onto, and subsequently adhere material surfaces (via a plethora of techniques). non-contacting magnetic controls for the droplet. non-contacting magnetic cooling, providing a second system for non-mechanical liquid metal system for non-mechanical liquid metal providing a second galinstan to a chips fed aluminum et al. Zhang manipulation. droplet as fuel for self-propulsion, nately, these methods require enclosed (air-tight or water-proof) or water-proof) these methods require enclosed (air-tight nately, for manipulation chambers that limit the practical applications as the circuits must remain of liquid metal circuits on the fly, inside the chamber. acid) are hydrochloric acid (HCl, a low-pH atmospheric conditions, or in any environment with oxygen atmospheric conditions, or in any environment levels above 1 ppm, of these etchants mix readily with water and can be contained safely at high concentrations. A non-aqueous alternative for oxide removal comes through exposing the liquid metals to concentrated HCl vapor, metal droplet motion), is removed, the liquid metal loses its adhesion to the substrate is removed, the liquid metal loses its adhesion gravity (or other forces), and will reflow under the influence of In other words, enabling a fixed liquid metal pattern to change. manipula- oxide removal enables real-time, non-mechanical in either done been typically has This metals. liquid of tion or in chemical nitrogen boxes (via passive oxide prevention) etchant baths (via active oxide removal). bling non-contacting pumps for microfluidic flow microfluidic for pumps non-contacting bling rigid-body locomotion. large category of recent research has used electricity to create to electricity used has research recent of category large shape change in liquid metals, relative motion and been studied in detail. in studied been sodium hydroxide (NaOH, a high-pH base).

[18] High- [17] tunable elec- This previous [12] [10] which Tang et al. et Tang which [15,16] In these examples, the In order to harness the 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2017 WILEY-VCH Verlag [14] © [2–5] This sparked an exploration into using This sparked an exploration [1] 4, 1600913 including self-healing electronics, including self-healing Other high-temperature gallium alloys have been Other high-temperature gallium alloys have [11] and stretchable sensors. [6–9]

[13] C), researchers have controlled the motion of liquid °C), researchers have controlled the motion

Gallium-indium-based room-temperature liquid metal alloys room-temperature Gallium-indium-based Gallium-based alloys can be found in a liquid form at alloys can be Gallium-based There is also a growing interest in the ability to manipulate www.advancedsciencenews.com Adv. Mater. Interfaces 2017, Mater. Adv. DOI: 10.1002/admi.201600913 R. A. Bilodeau, Prof. R. K. Kramer Department of Mechanical Engineering Purdue University Lafayette, IN 47907, USA West E-mail: [email protected] Zemlyanov D. Y. Dr. Birck Nanotechnology Center Purdue University Lafayette, IN 47907, USA West work demonstrates the feasibility of metallic droplet manipula- work demonstrates the feasibility of metallic make it impractical for tion, but the high temperatures involved common, everyday use. or galinstan, alloy, a eutectic gallium-indium (usually eGaIn, even more common- a gallium-indium-tin alloy) have become electronic and soft place as researchers develop reconfigurable in a number of soft devices. These gallium alloys have appeared electronics tronics, liquid metals to conduct both heat and electricity in systems both heat and electricity in systems liquid metals to conduct General compliant or reconfigurable. that are either physically decade past the in so toxicity, its by impeded is mercury of use to liquid metals have begun gallium-based room-temperature use in conductive circuitry applications. replace mercury for is and adaptable liquid metal conductors The value of flexible - robotics design, devel probably best exemplified in recent soft opment, and control applications. full potential of liquid metal circuitry, more investigation is nec- full potential of liquid metal circuitry, liquid metals’ essary into the fundamental principles governing as well as possible applications for these liquid metals. behavior, high temperatures At both room and elevated temperatures. (>600 con- of the alloy, gallium arsenide via chemical decomposition the of crystallinity surface via motion of direction the trolling substrate. moved by shining a light on the powder-coated surface. moved by shining a light on the powder-coated - ago, liquid mercury was used to dem More than six decades as of room-temperature liquid metals onstrate the potential circuit elements. soft Electronics K. Kramer* and Rebecca Zemlyanov, Bilodeau, Dmitry Y. Raymond Adam Liquid Metal Switches for Environmentally Responsive Responsive Environmentally for Metal Switches Liquid controlled and directed by surface roughness. liquid metal is manipulated through encapsulation inside of a of inside encapsulation through manipulated is metal liquid and shape- shape-giving mechanical enabling polymer, flexible electronics. changing of soft the geometry and electric path of the liquid metal via non- droplets, small coating powder By techniques. mechanical marbles, microscopic created researchers frequency magnetic fields can drive galinstan droplet motion, Communication 1600913 deviation. PDMS. and thePDMS,combinedwithbatchvariationsinherentto that thisisduetochemicalinteractionbetweentheetchant (with onedropletnevergettinghigherthan153°).We suspect droplets had a noticeable variation in their final contact angles elastomer as the substrate (polydimethylsiloxane; PDMS), the silicon substrates, data variancewaslow; but with asilicone vapor treatment. angles of liquid metal droplets with the oxide removed via HCl the contactanglesmeasuredarehigherthanreported contact anglewhenmeasuredinaqueoussolutions.Therefore, which islowerthanpreviouslyreportedvalues. angles wereroughlythesame(≈160°)in1 Our resultsarepresentedinTable at room temperature (see the Experimental Section for details). substrates whilesubmergedinbothHClandNaOHsolutions contact angleofgalinstandropletsonthesurfacevarious the influenceoftwochemicaletchants. We measuredthe layer surroundingliquid metal droplets,we begin bycomparing HCl andNaOHhavebeenusedtoremovethegalliumoxide removed fromtheaqueousmedium. sequently permittingthefinalliquidmetalmorphologytobe aqueous techniquesforliquidmetalmanipulation,whilesub- The methodspresentedhereinenabletheuseofsimpler figurable electronicsandfullyreversibleliquidmetalswitches. technique, demonstratingitsviabilityforcreatingbothrecon- HCl (1 Etchant 1 Table immersed ineither1 trations (1 equilibrium contact angle for each droplet. At the same concen - approximated thetime-scalerequiredtodevelopmaximum substrate. Since oxideremovalisthekeytoreleasingadroplet, the speedatwhichanadhered dropletcanbereleasedfroma chemical etchantseemstohave alargedegreeofinfluenceon diate ramificationson theconfiguration rate.The choice of order ofminutes). maximum contact angle inaveryshortamount of time(onthe Figure 1). When placed in NaOH, the droplets would achieve a their maximumcontactangle(likethedropletshowninHCl in galinstan dropletsconsistentlytookseveralhourstodevelop nature ofthesubstrate.WhenplacedinanHClbathonPDMS, strate (seeFigure was thesame.Thetimedifference was largest onaPDMS sub- maximum contactangle,eventhoughthetreatmentmethod NaOH (1 www.advmatinterfaces.de Oxide RemovalRateMeasured byContactAngle:Since both Comparing timestampsforeachprofilephotograph,we In thecontextofreconfigurable electronics,thishasimme- m (2 of9) ) . [40] m Contact anglesofgalinstandropletsonvarioussubstrates, ) We alsonotethatdropletbuoyancyplaysarolein m ), HCltookmuchlongerthanNaOHtodevelopthe [36] 1), likelyduetothesofter andmoreporous m HClor1 wileyonlinelibrary.com Substrate Silicon Silicon PDMS PDMS Glass Glass m NaOH.±valueisonestandard 1. Allequilibriumcontact m HCLandNaOH, [30] For glassand Contact angle[°] © 2017WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim 164 ±2.1 162 ±0.5 165 ±0.8 164 ±1.6 159 ±4.6 161 ±1.3 an areaapproximatelythesizeofdroplet’s circularprofile, area ofthedropletdecreasedfromitsoriginalsphericalto syringe, leavingtheencapsulatingoxidebehind.Assurface removed theliquidmetalfrominsideofdropletwitha The dropletssatfor24htoencourageoxideadhesion.We then large galinstandroplets(diameter≈8mm)ontothePDMS. adhesion) and then, using a syringe, we placed very PDMS (for Figure we developedasimplereflectanceexperiment,asshownin the timeittakestoremoveoxidebychemicaletchants, of gallium oxide reacting with HCl and NaOH. To measure knowledge, therehavebeennostudiesonthechemicalkinetics removed byHClandNaOHfromaliquidmetaldroplet. we furtherinvestigatedthespeedatwhichsurfaceoxideis method. Scalebaris4mm. galinstan dropletplacedonaflatPDMSineither1 Figure 1. rheometry (inHCl) data inlogarithmicallyscaledplots. ThedatagatheredbyXuetal.using the removal rate oftheoxide at severalconcentrations, and present the along with the time required to remove the visible oxide. g,h) We tested showexamplesoftextured droplets treated byeither HCl or NaOH, (f) to (e) depends on the chemical etchant used, and the insets for (c) and original, reflectivesurfaceisrestored.Thetimeittakestomovefrom (d) theexteriorofoxidebeginstodissolveuntil the etchant bath,e,f) visible texture.d)Whenthetexturedoxideisimmersedintoachemical the dropletisthenremoved,c)causingsolidoxidelayertodevelop a PDMS substrate,andallowedtositfor24h.b)Theliquidmetalinside of rates byHClandNaOH.a)Largedropletsofgalinstanareplacedon a Figure 2. the dropletcompletelydepinned(ithasbegunrolling). NaOH bath.Itshouldbenotedthatthelastframeofshows Oxide RemovalRateMeasured Optically:To theauthors’ 2. We coated small, chemically inert platforms with Experimental methodforcomparinggalinstanoxideremoval The evolutionofthecontactangleanddropletprofilefora [30] ispresentedalongsideours,asvalidation ofour Adv. Mater. Interfaces www.advancedsciencenews.com m HCLora1 2017, 4, 1600913 m

Communication

- 1600913 (95% −1 0 s 6 h 1.5 h 85 min (3 of 9) [11] Time to equilibrium . We started by 2. We (≈0.1). Although [41] www.advmatinterfaces.de NaOH solution, the m 1.501), with an offset on −1.501), with an offset concentrations. When we , as expected. = m ] −1 extremely low bond num- −1 [42] ). 486 ± 6 230–350 470 ± 12 593 ± 46 m [mN m Surface tension Our droplets were close to this range, enabled rapid oxide removal and droplet enabled rapid oxide removal and droplet wileyonlinelibrary.com m [43] (95% confidence), exhibiting a large variation, (95% confidence), exhibiting a large variation, −1 ) m Surface tension on galinstan droplets in various environments. ) that the oxide is removed at a rate similar to very low that the oxide is removed . m m 1.417 compared to theirs of m −1.417 compared to theirs ± 46 mN m value likely because their setup provided greater informa- b value likely because : Surface : Surface Tension by Surface Measured Oxide Removal Rate Unlike in our contact angle tests, we were not able to have to able not were we tests, angle contact our in Unlike The final surface tension of galinstan in both the HCl The final surface tension of galinstan Combining this quantitative test with the qualitative work test Combining this quantitative = O 2 tension is a key part of both liquid–substrate adhesion (in the of both liquid–substrate adhesion (in the tension is a key part mobility. droplet and liquid) a by developed angles contact surface tension in either HCl galinstan droplets’ compare To measure- goniometry simple performed baths, we NaOH or Table ments, and our results can be found in suspended droplets measuring the surface tension of galinstan The surface tension we measured was in oxygen-rich air. 593 extruded a galinstan droplet into a 1 bers (<0.1) cause errors in the fitting parameters when digital systems are in use. bond numbers <1 are desired when using drop shape analysis to determine surface tension, causing a slightly larger variance in the measured values than ferent, the confidence intervals overlap sufficiently to negate to negate sufficiently ferent, the confidence intervals overlap attribute the uncertainties any distinction between the two. We of the 2 to the highly spherical nature of the values in Table droplets and therefore low bond numbers confidence), respectively. Though the averages are slightly dif confidence), respectively. both the HCl and the NaOH at 1 chemical reaction between the galinstan and the etchant was so chemical reaction between the galinstan and from the extrusion vigorous that the droplet would disconnect Decreasing the needle before we could begin measurements. concentration to 0.1 concentration NaOH (0.005 concentration NaOH stabilization, while permitting measurements to be made. This made. be to measurements permitting while stabilization, goniometry tests pro- required change in concentration for our between HCl and NaOH vides a third demonstrable difference in chemically treating galinstan. ± 6 mN m and NaOH baths was 470 ± 12 and 486 but always greater than 500 mN m our simple setup does, but we interpreted their data to develop interpreted their data setup does, but we our simple and plotted concentration curve, time versus an oxide removal b and m constants curve-fitting the against ours. Via their data and set data their that found we equation, power-law the of (ours with identical slopes data set had nearly our HCl-treated m NaOH (0.1 the oxide removal. tion about the complete that, at the same concen- 1, we can conclude shown in Figure a droplet of pinned galinstan much tration, NaOH will release will by dissolving away the gallium more rapidly than HCl until the concentration of HCl is increased It is not oxide faster. to 2 Environment Table 2 ± value is 95% confidence on mean. Air HCl (1 H

- ), m m bx = (to speed m NaOH was m concentrations m 1.417, indicating a indicating −1.417, is m 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2017 WILEY-VCH Verlag © who used parallel-plate rheometry to . m HCl was already acting slowly, we per slowly, acting HCl was already [30] m 4, 1600913 . Since 1 . Since NaOH solution. Each bath was prepared fresh and fresh prepared was bath Each solution. NaOH m m concentration and the constant m correlates the change m To validate our procedure, we compared our data to the work validate our procedure, we compared our data to To The final results of our optical tests are displayed in the log- The final results of our optical tests are This test provides further evidence for a comparative advan- This test provides further evidence for a We set up all of the droplets onto platforms simultaneously, all of the droplets onto platforms simultaneously, set up We www.advancedsciencenews.com Adv. Mater. Interfaces 2017, Mater. Adv. large change in oxide removal time for small changes in con- NaOH this parameter was found to be −0.268, centration. For an order of magnitude smaller than the HCl, indicating min- imal change in oxide removal times at even very low concentra- all of the confi- the trend lines do not contact tions. Although dence intervals, it is important to note that the plotted confi- dence intervals are representative of deviation in the measured the in uncertainty any account into take not do and only time molarity of the chemical etchant baths. performed by Xu et al. log graphs of Figure 2. Each data point is an average and the log graphs of Figure interval on the mean error bars represent a 95% confidence (y data the to curve power-law simple a fit We time. of the etchant concentration and the time required to remove parameter fitted HCl The oxide. the formed the test on a range of molarities between 2 formed the test on a range of molarities the process up) and 0.1 represents the speed of oxide removal where the constant b represents the speed of oxide removal at 1 HCl or 1 or HCl - the droplets were submerged with chemi used only once, and to prevent any interaction of the tweezers cally inert tweezers Examples of the droplets are displayed with the acid/base. - is available in the Sup 2, and a video of the tests in Figure took HCl in submerged droplets The Information. porting be removed (average between 9 and 13 s for their oxide layers to droplets 11.35 s ± 1.7 s standard deviation). The galinstan had their oxide skins removed in immersed in NaOH, however, of the deviation 0.58 s (average) with a 0.045 s deviation—most of the camera used and error being caused by the frame rate chemical reaction. to film the submersion instead of the actual the gallium oxide tage of using NaOH over HCl in removing liquid metals. layer from gallium-based room-temperature the initial success of comparing the 1 With and randomly selected droplets for submersion in either a 1 a either in submersion for droplets selected randomly and the solid oxide skin buckled and wrinkled, developing a dull wrinkled, developing skin buckled and the solid oxide of reflecting light instead The buckled oxide scatters gray hue. does. Once pure liquid metal oxide layer or the it as a fresh baths, the either NaOH or HCl are submerged in the droplets surface is and a reflective is chemically removed buckled oxide allowed us to record the process Filming restored to the droplet. the droplets surface. This optical test the time it takes to change at which HCl and between the speed confirmed the difference - oxide layer as observed when stud NaOH remove the gallium ying contact angles. already very fast at processing, we proceeded to test 0.1, 0.01, already very fast at processing, we proceeded and 0.001 characterize the influence of various concentrations of HCl on HCl of concentrations various of influence the characterize actively removing the oxide with HCl during gallium oxide. By the parallel-plate test, they were able to observe a change in the mechanical properties of the oxide over time, until they observed complete mechanical breakdown, suggesting oxide removal. Their results do not have the timescale resolution as using this technique, we then expanded our tests to include to tests our expanded then we technique, this using 1 Since various concentrations of HCl and NaOH. Communication 1600913 sinking to a range anywhere from 350 to 230 mN m over theperiodofhoursitwoulddropbetween40%and60%, an initialsurfacetensionprofileatwellover500mNm deionized water. Inbothtypesofwater, thedropletsregistered the surfacetensionofgalinstandropletsinbothdistilledand mental procedureweusedforHClandNaOH,measured measured surface tension values. Following the same experi- metal droplets, or if the dissolved acid (or base) was driving the having asignificantimpactonthesurfacetensionofliquid This wouldhelpusdetermineifthechemistryofwaterwas ence ofcleanwateronthesurfacetensionliquidmetals. significant watercomponent,wewantedtodeterminetheinflu- droplets extrudedinthe0.1 longer timescales,weobserved thatadropletofgallium-based of liquidmetaldroplets,butonly onshorttimescales.Given again. Duringourexperiments, weobservedsimilarslipping also showedthatasthesubstrate dries,theoxidestickstoit and thesurroundingenvironment. sion depends primarily on the chemistry between the droplet the othermediums. Table 2). This surface tension range is nearly half the value of mally stickto, when thereisalayerofwaterpresent. dized eGaIn couldslideacrossasurfacethatitwouldnor Tension: Since a1 equivalent. face tensionmeasuredinbothetchantsiscloseenoughtobe we wouldhavedesired.Regardless,concludethatthesur exposed towater. alongside thenormalgalliumoxidewhendroplethadbeen oxide layer and noted the generation of a gallium hydroxide formed X-ray photoelectronspectroscopy(XPS)studiesonthe This findingissupportedbytheworkofKhanetal.whoper surface properties of liquid metal by the surrounding water. time canonlybeattributedtoaslowchemicalchangeofthe state. Theextremedropinsurfacetensionoversuchalong ever, the droplets often took over 6 h to come to an equilibrium librium surface tension nearly instantaneously. In water, how- 1 a thin“slip-layer” ofwater. interactions havemostlyfocusedontheshort-termeffects of is duetothenewlyformed,weaker, hydroxideexterior. layer ofgalliumoxide),andthelargechangeinsurfacetension with thegalliumoxide(havingtodiffuse throughthewhole was because of the slow chemical kinetics of the water reacting tension and the large change in surface tension. The long time the longtimerequiredtodevelopfinalequilibriumsurface (see Figure S1, Supporting Information). This explains both a weakhydroxide,thedropletdistendsunderitsownweight the stiff, stabilizingoxidehadbeenchemicallychangedto droplet, changingthesurfaceintosofter hydroxide.Since Over time,however, thewaterreactedwithsurfaceof the watertocreatecharacteristicthingalliumoxidelayer. into water, theliquidmetallikelyuseddissolvedoxygenin goniometry. Whenweinitiallyextrudedadropletofliquidmetal gallium oxide.Thishelpsustounderstandtheresultsofour the hydroxidelayerhasamuchlowermodulusofelasticitythan www.advmatinterfaces.de m In goniometry, thetimetoachieveequilibriumsurfaceten- Oxide RegrowthandTransformation Measured bySurface Previous studiesintotheeffects ofwateroneGaIn–surface HCltookaround1.5htocomeanequilibriumwhereas (4 of9) [44] m Usingrheometry, theydemonstratedthat solutionofeitherHClorNaOHhasa wileyonlinelibrary.com [23,44] m NaOHbathachievedanequi- Khanetal.observedthatoxi- [43] Dropletsextrudedinto © 2017WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim [44] −1 −1 They (see , but - - - by Kimetal., lets, we anticipated observing an effect similar to that reported tions overtheIn4d/Ga 3d/Sn4dregion.With therinseddrop- Section. analyzed fortheseresultsiscontainedintheExperimental results stemfromthewaterbaths.Alistofallspectra face propertieswerecausedbythechemicaletchantandwhich XPS results to the rinsed droplets and determine which sur dried. Soakeddropletswereimmersedfor24htocomparethe dipped inacleanbathfor2s,andafterward wereimmediately water ora24himmersioninwater. Rinseddropletswere droplet submersioninNaOHfollowedbyaquickrinse by aquickrinseinwateror24himmersionwater, and four different conditions:dropletsubmersioninHClfollowed ical effect ofvarioussurfacetreatmentsongalinstan.We tested ysis: We performedanXPSanalysistocharacterizethechem- of water. liquid metalwilladheretoasurface,regardlessofthepresence during therinsingprocess.Notably, thesurfaceconcentration lets byeithertheNaOHorHClwerecompletelyremoved that anyalternativeshellsgrownaroundthegalinstandrop- H were completelyremovedwhenthedropletsrinsedin all oftheinfluenceseitherchemicaletchant(HClorNaOH) trace ofthesechlorideshells.Instead,itappearedthatalmost shell inplaceofthegalliumoxide.However, wedidnotfindany with HClvaporgrowagallium chloride andindium short rinse(bottomspectrum). in eachpanelobtainedfollowingsoaking for24h(topspectrum)anda were treated with NaOH (top panel) and HCL (bottom panel). Spectra Figure 3. 2 Figure Oxide RegrowthandTransformation Measured byXPSAnal- O, makingtheXPSresultsnearlyidentical.We suspect In 4d/Ga3d/Sn4dXPSspectraobtained fromthesamplesthat 3 shows the XPSresultsoffourdifferent condi- [36] whodemonstratedthateGaIn dropletstreated Adv. Mater. Interfaces www.advancedsciencenews.com 2017, 4, 1600913 - Communication - 1600913 NaOH m (5 of 9) www.advmatinterfaces.de wileyonlinelibrary.com shows our results being applied for environmen- NaOH solution. The NaOH was replaced with clean water, was replaced with clean water, NaOH solution. The NaOH 5 m Changing contact area between a galinstan droplet and a glass Changing contact area between We note that after the water was removed, the droplet had note that after We droplet adhesion driven Droplet Mobility: With Liquid Metal Figure As with the surface tension work reported previously, we we with the surface tension work reported previously, As Figure 4. removed from the substrate The droplet was initially substrate in water. using a 1 and the droplet collapse was observed for 4 h. The inset shows the drop- and the droplet collapse was observed for 4 h. let’s initial and final states in the water. ical droplet to collapse under its own weight. This process weight. This process ical droplet to collapse under its own on the droplet, effect resulted in a pulsing/shape changing 4 between 50 and 150 min, and can as evidenced from Figure between times- Information) S2 (Supporting be seen in Video tamps 0:50 and 1:00. tested other techniques fully adhered to the substrate. We to repin the droplet, but Information) (details in the Supporting they were not as successful as this method. by the gallium oxide layer on the liquid metal, we can free the NaOH, or HCl either with layer oxide the removing by droplet we Afterward, substrate. the on freely roll to droplet the causing by rinsing it with water, can repin the droplet to the substrate encouraging regrowth of either a gallium oxide or a gallium chem- two These droplet. the of surface the on layer hydroxide ical tools are all that is required to remove galinstan droplets from a substrate and then fix them in a desired location, and we now demonstrate the potential for this to be applied in manipulating liquid-metal-based electronics. adepinned We mobility. droplet metal liquid stimulated tally droplet from one location, enabling it to roll to a new location, shows5a Figure the substrate. to back drop the pinned then and the droplet of galinstan that has been adhered to a substrate in an initial position. The droplet is then exposed to a 1 the 5b), which causes the droplet to depin from bath (Figure forces. 5c) by gravitational substrate and change location (Figure the droplet is by replacing the NaOH bath with water, Finally, hypothesize that the changing contact area (and adhesion to (and adhesion to hypothesize that the changing contact area - during the ini effect the substrate) is due to the rapid triple any removed water the First, droplet. the tial submersion of NaOH. Second, the dissolved oxygen grew the influence of quickly changed a gallium oxide shell. Third, the water leaving the rest the outer surface of the shell to a hydroxide, to soak in the bath, of the oxide untouched. While continuing through the whole thickness of the hydroxide slowly diffused causing the spher the shell, weakening the droplet walls and the [11] , 3 O 2 O. Inversely, O. Inversely, 2 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2017 WILEY-VCH Verlag © O. As is visible in the inset the in visible is As O. 2 [45–48] 4. Although the droplet began to adhere 4, 1600913 [44,49,50] Oxide Regrowth and Transformation Measured by Contact Area: Measured Oxide Regrowth and Transformation From our XPS results, it is clear that gallium oxide thickness From The contact angle did not change much during the test, As seen in Figure 3, both rinsed samples showed the 3, both rinsed As seen in Figure www.advancedsciencenews.com Adv. Mater. Interfaces 2017, Mater. Adv. there was an increase of covalently bonded gallium (shown as of covalently bonded gallium (shown there was an increase from the pure-metal energy 2p and 3d peaks of the Ga a shift From surface. the on grown had oxide more indicating level), thick- layer gallium oxidized new a estimated we data, these thickness shells’ ness at 7 nm. This doubling of the exterior indium, as they were explains the disappearance of the tin and droplet. Though the no longer visible from the surface of the mainly Ga chemical composition of gallium oxide is correlates with water treatment: rinsing resulted in 40%–44% correlates with water treatment: rinsing whereas soaking of gallium on the surface in the form of oxide, these results show evi- resulted in 90%–93%. Furthermore, oxide layers, which had been chemi- dence that the droplet’s completely restored cally removed by the HCl or NaOH, were This happened for both by simply rinsing the droplets in water. can easily conclude the HCl and NaOH treated droplets, so we the galinstan is that the original method of chemically treating performed by rinsing unimportant when oxide restoration is there is a time element involved in exposing Finally, with water. further exposure to water (beyond liquid metals to water: in thickness while rinsing) results in the oxide layer growing changing partially into a hydroxide layer. of HCl and NaOH, water removes the surface effects Since we could use it as a method to reattach droplets that had been tested this on a droplet removed via the chemical etchants. We through a simple experiment: we soaked a galinstan droplet in NaOH to remove its oxide, and then placed it onto a glass sub- tracked the profile of the We strate immersed in distilled water. of the drop- droplet over a 4 h period, and recorded the radius this information, we contact area on the substrate. From let’s calculated the contact area as it changed with time and present our findings in Figure on the order of minutes, the increasing contact area improved the adhesion over the period of the test. but the contact area of the droplet increased significantly with H in submerged time increasing Sn 4d (metal) peaks, but the soaking treatment resulted in but the soaking treatment resulted Sn 4d (metal) peaks, from the surface of the droplet: the Sn disappearance of tin fol- concentration Indium visible. barely are peaks (metal) 4d from the visible of indium amount trend: the the same lows following soaking in H surface greatly decreased of gallium was slightly enriched after the rinsing (85 at% vs the rinsing after was slightly enriched of gallium amount there was a significant ideal galinstan), and 78 at% for the figure). (not shown in present in the spectrum of oxygen in and oxide components ratio between the metal Using the the thickness of the 3d, we calculated 2p (not shown) and Ga - calcu such of details The nm. 3 about at oxide gallium regrown elsewhere. lations can be found soaked sample showed much higher amount of OH groups soaked sample showed much higher amount shown), confirming a as evidenced from the O 1s spectra (not toward gallium hydroxide on the surface of the chemical shift droplet, as expected. images in Figure 4, the original circular droplet shape collapsed 4, the original circular droplet shape collapsed images in Figure down into a flattened ellipsoid, maintaining a high contact angle as the droplet spread outward and enabling a large contact area on the substrate. Communication 1600913 droplet during one air-NaOH-water-air treatment cycle: a) initial state, b) submersion in 1 Figure 6. chamber withtwoconductivetungstenleadsemergingfrom Figure 6e).Thegalinstandropletwasinaspecial environmental LED andadropletofgalinstaninseriescircuit(visible a conductive pathway, as shown in demonstrate theabilitytorepeatedlydisconnectandreconnect in connecting/disconnectingcircuitryforreconfigurablelogic. control overdropletlocomotiondirection,aswellapplications controllable surfacemorphology, whichwouldinturnenable opportunities tocombinethisprocesswithenvironmentally alone, withgravitybeingtheonlysourceofforce.We seefuture more, thisdropletmobilityiscontrolledbytheenvironment droplet motiononcethehasbeendepinned.Further The highcontactangleonthedropletenablesextremelyrapid able to re-attach the substrate in the final position (Figure 5d). H NaOH causesthedroplettodepin.c)Thedepinnedmovesundergravitationalforce.d)isrepinnedsubstratethroughan Figure 5. Supporting Information. of theresistanceacrosschamber’s leads,duringthetwoconsecutivecycles.Avideoofdropletprofile duringathirdcycleisavailableinthe the chamber environment and g) a measurement tive cycles. i) Close-up views of the droplets in the chamber when power was supplied, along with f) LED usedforthistest(forscale,the LEDis3mmdia).h)Aseriesofphotographsthewithpower suppliedtothecircuitduringtwoconsecu- with waterencouragesthedropletto collapse,andd)thedropletisdriedreconnectingcircuit.e)Aphotograph oftheenvironmentalchamberand www.advmatinterfaces.de 2 O bath.e–h)Photographsoftheactivetest,matchingdiagramsin(a)–(d).Insetboxesare≈3mm×mm. Liquid Metal Switch:Usingenvironmentalstimuli,wealso (6 of9) Controlling thelocationandfixityofaliquidmetaldroplet.a)Thedropletispinnedtosubstrateinair. b)Submergingthedropletin1 Opening and closing a circuit via chemical manipulation of a liquid metal droplet. a–d) Schematic representation of the side profile of the wileyonlinelibrary.com Figure 6. We connected an © 2017WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim - the chamberwitha1 the environmental chamber, starting with air and then filling the LEDwouldilluminate.For eachtest,wecycledthefluidin supply, settoprovideeitheramaxof3.3Vor10mA)seeif power sourcetothewholecircuit(KoradKA3005DDC tested theresistanceacrossgalinstandropletandapplieda the chamberwallsfordroplettoreston.We alternately NaOH bathtolift thedropletup,disconnectingelectrodes, porting Information. of thedropletprofileduringacycleisavailablein Sup- electrodes basedonthechemicalbathinchamber. Avideo showing aprofileviewofthedropletandhowitcontacts again. Figure 6a–disaschematicrepresentation of thecycle, water, andthendryingallthewateroutofchamberwithair We wereabletodisconnectandreconnecttheLEDusing m NaOH disconnecting the circuit, c) replacing the NaOH m NaOHbath,continuingontodistilled Adv. Mater. Interfaces www.advancedsciencenews.com 2017, 4, 1600913 m

Communication ). m 1600913 (7 of 9) ) or HCl (1 m ≈ LF(1,1.4,20,50)) www.advmatinterfaces.de HCl were allowed at least 2 h m °C). wileyonlinelibrary.com NaOH were allowed to come to an equilibrium for NaOH were allowed to come to an equilibrium m = 1486.6 eV). The high-resolution Ga 2p3/2, O 1s, Sn

ν ≈ SGL(10)). Since the Ga 3d/Sn 4d/In 4d region contains : To measure contact angles, galinstan droplets measure contact angles, galinstan Angles: To Contact : Pendent drop shapes were recorded and measured Goniometry: Pendent drop shapes were recorded and Droplets extruded in both air and 1 XPS and Sample Preparation: XPS measurements were performed radiation (h α to establish an equilibrium tail, although liquid metal droplets extruded metal droplets tail, although liquid an equilibrium to establish Droplets HCl. in those as time a of long as require not did often air into extruded into 0.1 184 as PDMS (Dow Corning). These substrates were used as they are substrates were used These (Dow Corning). 184 as PDMS patterning metal liquid for utilized typically substrates of representative Ga, 21.5% them. Galinstan (68.5% readily adheres to since galinstan used and Aldrich, Sigma from obtained was weight) by Sn 10% In, and NaOH HCl (Fisher Chemical) from the manufacturer. as-received dried or concentrated either in obtained were Chemicals) Fine (Macron to get the desired concentrations. forms and mixed with water substrate for 3–5 min, covered. This setting were set on a cleaned to adhere to the surface, so that the droplet time allowed the droplets and The substrates it. dislodging bath without in the placed be could in a bath of either NaOH (1 droplets were then placed Photographs of the droplet profile were taken regularly while the etchant profile were taken regularly while the etchant Photographs of the droplet of the droplet and the contact angle maximized. slowly lifted the edges the profiled and were different sizes of different droplets three At least an average of the different droplets. HCl and results returned were 1 molar concentrations for the bath, and each NaOH were used in both substrate types: glass, silicon, and PDMS. was tested on the following flat, uncured PDMS was spin- ensure that the PDMS substrate was To at 2000 rpm for Coater) coated onto small glass slides (SCS G3-8 Spin 2 min, before curing them at 60 °C for 2 h. Each substrate was used only any acid/base etching of once for a single galinstan droplet, to prevent All tests were performed the surface from effecting subsequent results. at room temperature (≈25 capable of performing in a goniometer (Ramé-Hart Instrument Co.) this feature was not oscillating pendent drop measurements (though using a non-air manner, enabled). Each test was performed in a similar contacts directly tip pipette of the inside water the which in technique gap effects of a compressible the liquid metal. This removed any viscoelastic of droplets mL 5–8 droplets. metal liquid the extruding while pocket air being used for the liquid metal were extruded directly into the medium HCl, or NaOH) and briefly allowed to stabilize for up water, analysis (air, process to die off. At this to 30 s, to enable vibrations from the extrusion droplet were recorded at point, the surface tension and volume of the total length of the test) intervals between 10 and 30 s (depending on the surface The tension. surface equilibrium to achieve the droplet allow to equilibrium tail of the tension was then recorded as an average of the negligible impact on the data. Initial droplet volumes varied, but had a recorded results. at least 40 min, and upward of 2 h, although this length of time was not at least 40 min, and upward of 2 h, although this surface tension necessary as the droplet had established its equilibrium water into extruded droplet metal Liquid began. even analysis the before were allowed at least 10 h to come to an equilibrium, though many were able to establish an equilibrium before this time, the additional hours enabled a proper determination of equilibrium status. using a Kratos Axis Ultra DLD spectrometer with monochromatic Al K 3d, In 3d, C 1s, Cl 2p, and Ga 3d/Sn 4d/In 4d spectra were collected A built-in commercial at constant pass energy (PE) with a PE of 20 eV. Kratos charge neutralizer was used to achieve better resolution. Binding edge and the energy scale was energy (BE) values refer to the Fermi The calibrated using Au 4f7/2 at 84.0 eV and Cu 2p3/2 at 932.67 eV. photoemission peak positions were charge corrected to the adventitious Spectra were analyzed using the carbon signal of C 1s at 284.8 eV. Ltd.). A Software version 2.3.16 PR 1.6 (Casa software program, CasaXPS Shirley background was subtracted from each region before curve fitting; metal components were fitted with asymmetric Gaussian–Lorentzian Lineshapes (CasaXPS dampening peaks with tail Gaussian–Lorentzian peaks (CasaXPS and oxide components—with Lineshapes the contribution of all three metal of interest, we focused qualitative/ quantitative analysis on this region. Spin–orbit coupling doublets of the - - 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2017 WILEY-VCH Verlag © 4, 1600913 Having successfully developed a system where we were able successfully developed Having we have quantified the influence of acidic In this paper, www.advancedsciencenews.com Adv. Mater. Interfaces 2017, Mater. Adv. Experimental Section The three types of substrates primarily used were P-type silicon (Wafer and Sylgard microscope slide glass (Home Science Tools), World), and the water bath to collapse the droplet back down, recon- the droplet back down, bath to collapse and the water current stage 6f,g describes the electrodes. Figure necting the liquid metal the measured across resistance and the the cycle of photo- is a series of simultaneous 6h,i Figure droplet, while each stage droplet during the LED and the galinstan graphs of to note that although the NaOH of the cycle. It is important droplet from the circuit, the metal liquid bath disconnected the electrochemical via electricity to conduct was able NaOH itself although the resistance is very high with means. This is why the water baths, the LED illuminates (par both the NaOH and tially) when power is applied during the NaOH portion of the applied during the NaOH portion of the tially) when power is is what the electrochemical interaction cycle. Furthermore, in visible around the galinstan droplet causes the gas bubbles cycle (also visible in the 6i during this portion of the Figure the the NaOH away with distilled water, video). Once we rinsed the electrodes and the LED ceased water no longer connected in the also note that, although not shown to illuminate. We the chamber and then figure, if we rinse out the NaOH from (with a small fan) the rapidly dry the water from the chamber circuit open. The cir droplet holds its new shape, keeping the in droplet the soaking by time any at closed be then can cuit water. circuit via environ- to completely disconnect and reconnect the to ensure repeatability. mental stimulation, we cycled the circuit reconnect and disconnect consecutive two show we 6 Figure In any other the liquid metal droplet or cycles, without changing cycled it a third time, while recording the part of the circuit. We In the video, we Information. video found in the Supporting the droplet had after mobility of the chamber demonstrate the reconfig- dried, proving that liquid metals can be manipulated, built from these ured, and then pinned in place so that circuits control liquid metals can be removed from environmental chambers. aqueous environ- (HCl), alkaline (NaOH), and neutral (water) to surface oxidation. ments on liquid metal droplets prone stimuli to Our results demonstrate the use of environmental of surface control the removal, formation, and transformation the adhesion oxide on liquid metal droplets, thereby controlling of the droplets on various surfaces. While previous studies have shown liquid metal oxide removal in HCl and NaOH baths, our results indicate the timescales for these removals at similar concentrations, with by orders of magnitude differ NaOH emerging as the more aggressive etchant. Furthermore, we have shown that rinsing the liquid metal droplet in water of HCl and NaOH, allowing formation reverses the effects of a surface oxide at short timescales and surface hydroxide in mechanical properties but at long timescales, which differ Finally, to the substrate. metal adhesion encourage liquid both metal liquid demonstrate to findings our employed have we droplet mobility on-demand and environmentally responsive liquid metal switches. Communication 1600913 samples wereloadedtotheXPSspectrometerthroughaload-lock. by passingdrynitrogenacrossthesubstrateanddroplet.As-prepared water (viaabsorption),withtheremainingforciblyevaporated were quicklydriedfirstbywickingawaythemajorityofremaining oxide hadbeenremoved.(d)Thetwosamplesthatwererinsedinwater the droplets’ surface chemistry, but the extratimewastoensurethat all respectively. Thisislongerthanneededtocreateavisiblechangeof Aeronautics andSpaceAdministration. of theauthorsanddonotnecessarilyreflectviewsNational conclusions or recommendations expressed in this material are those Faculty award(GrantNo.NNX14AO52G).Anyopinion,findings,and tension goniometry. Thisproject was fundedbyaNASAEarlyCareer Engineering, Purdue University) for her assistance with the surface The authorswouldliketothankProf.KendraErk(MaterialsScience Acknowledgements from theauthor. Supporting InformationisavailablefromtheWileyOnlineLibraryor Supporting Information (1 (c) SampleswerethenbrieflyrinsedinachemicaletchantbathofNaOH substrate (sothedropletsdonotmoveduringtreatmentandtesting). allowed tositfor2dmaximizeadhesionbetweenthedropletand of galinstanwereimmediatelyplacedonthesubstrates,and encourage the galinstan droplets to adhere to the surface. (b) Droplets measurements. (a)Glasssubstrateswereexposedtooxygenplasma procedures intheCasaXPS software. and inelasticmean free path(IMFP) of photoelectrons using standard taking intoaccountthecorrespondingScofieldatomicsensitivityfactors region were estimated after thesubtraction of a Shirley-typebackground, The atomicconcentrationsofthechemicalelementsonnear-surface coupling doubletsforthedlevels(d5/2andd3/2)wasfixedtobe3:2. 0.45, and0.85eV, respectively. Theintensityratioofthespin–orbit and 4d3/2)electronlevelsweresubjecttospacingconstraintsof1.10, Sn 4d(4d5/2and4d3/2),Ga3d(3d5/23d3/2),In [10] www.advmatinterfaces.de [9] [8] [7] [6] [5] [4] [3] [2] [1] m The followingprotocolwasusedtopreparesamplesfortheXPS ) orHCl(1 E. Lundgren, A.Mikkelsen,NanoLett. E. 2013, S. Kanjanachuchai, C.Euaruksakul, ACS Appl.Mater. Interfaces PLoS ONE J. Wu, Z.M. Wang, A.Z. Li,M.Benamara,S.G.J.Salamo, Res. Dev. W. X. Tang, C.X. Zheng,Z.Y. Zhou, D.E. Jesson, J.Tersoff,IBM J. J. Tersoff, D.E. Jesson,W. X. 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