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Raymond Adam Bilodeau, Dmitry Y. Zemlyanov, and Rebecca K. Kramer*
More than six decades ago, liquid mercury was used to demonstrate the potential of room-temperature liquid metals assoft circuit elements.[1] This sparked an exploration into using
liquid metals to conduct both heat and electricity in systems
that are either physically compliant or reconfigurable. Generaluse of mercury is impeded by its toxicity, so in the past decadegallium-based room-temperature liquid metals have begun toreplace mercury for use in conductive circuitry applications.
The value of flexible and adaptable liquid metal conductors is
probably best exemplified in recent soft robotics design, development, and control applications.[2–5] In order to harness thefull potential of liquid metal circuitry, more investigation is necessary into the fundamental principles governing liquid metals’
behavior, as well as possible applications for these liquid metals.
Gallium-based alloys can be found in a liquid form at
both room and elevated temperatures. At high temperatures(>600 °C), researchers have controlled the motion of liquidgallium arsenide via chemical decomposition of the alloy, controlling the direction of motion via surface crystallinity of thesubstrate.[6–9] Other high-temperature gallium alloys have been
controlled and directed by surface roughness.[10] This previous
work demonstrates the feasibility of metallic droplet manipulation, but the high temperatures involved make it impractical forcommon, everyday use.
Gallium-indium-based room-temperature liquid metal alloys(usually eGaIn, a eutectic gallium-indium alloy, or galinstan,a gallium-indium-tin alloy) have become even more commonplace as researchers develop reconfigurable and soft electronic
devices. These gallium alloys have appeared in a number of soft
electronics[11] including self-healing electronics,[12] tunable electronics,[13] and stretchable sensors.[14] In these examples, theliquid metal is manipulated through encapsulation inside of aflexible polymer, enabling mechanical shape-giving and shapechanging of soft electronics.
There is also a growing interest in the ability to manipulate
the geometry and electric path of the liquid metal via nonmechanical techniques. By powder coating small droplets,
researchers created microscopic marbles,[15,16] which Tang et al.
moved by shining a light on the powder-coated surface.[17] Highfrequency magnetic fields can drive galinstan droplet motion,[18]R. A. Bilodeau, Prof. R. K. Kramer
Department of Mechanical Engineering
Purdue University
West Lafayette, IN 47907, USA
E-mail: rebeccakramer@purdue.edu
Dr. D. Y. Zemlyanov
Birck Nanotechnology Center
Purdue University
West Lafayette, IN 47907, USA

DOI: 10.1002/admi.201600913

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providing a second system for non-mechanical liquid metal
manipulation. Zhang et al. fed aluminum chips to a galinstan
droplet as fuel for self-propulsion,[19] and have since developed
non-contacting magnetic controls for the droplet.[20] Finally, alarge category of recent research has used electricity to create
relative motion and shape change in liquid metals,[21–23] enabling non-contacting pumps for microfluidic flow[24] andcooling,[25] self-destructive circuitry (path-destructive liquid
metal droplet motion),[26] and most recently as a source forrigid-body locomotion.[27]It is well known that gallium oxidizes rapidly in normal
atmospheric conditions, or in any environment with oxygen
levels above 1 ppm,[11,28,29] forming a gallium oxide that has
been studied in detail.[30–32] This oxide is solid at room temperature and forms a nanoscale stabilizing shell around
liquid gallium indium alloys, enabling the liquid metal to bepatterned onto, and subsequently adhere to, many different
material surfaces (via a plethora of techniques).[33] If the oxide
is removed, the liquid metal loses its adhesion to the substrate
and will reflow under the influence of gravity (or other forces),
enabling a fixed liquid metal pattern to change. In other words,
oxide removal enables real-time, non-mechanical manipulation of liquid metals. This has typically been done either innitrogen boxes (via passive oxide prevention) or in chemical
etchant baths (via active oxide removal).[18,19,21,24–26] Unfortunately, these methods require enclosed (air-tight or water-proof)
chambers that limit the practical applications for manipulationof liquid metal circuits on the fly, as the circuits must remain
inside the chamber.
The two main etchants used for aqueous oxide removal
are hydrochloric acid (HCl, a low-pH acid)[15,18,21,30,31,34] andsodium hydroxide (NaOH, a high-pH base).[19,21,22,24–26] Bothof these etchants mix readily with water and can be containedsafely at high concentrations. A non-aqueous alternative foroxide removal comes through exposing the liquid metals toconcentrated HCl vapor,[35–39] but practical applications of thisare generally limited to environments that can withstand thepresence of a highly corrosive gas.
In this work, we demonstrate control over the surface oxideof liquid metal droplets, which in turn controls the wetting andadhesion of those drops on a substrate, using purely environmental stimuli. We compare the use of highly acidic HCl andhighly alkaline NaOH (in solution) in removing the oxide torelease pinned droplets. We show that although both solvent
solutions result in high contact angles between the droplet
and the substrate, NaOH achieves this result at a rate that isorders of magnitude faster than HCl. We further explore theuse of neutral distilled water to regrow the oxide and manipulate the contact area of the liquid metal droplet on the surface,causing the droplet to readhere to the substrate. Finally, weapply this environmentally controlled depinning and repinning© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Liquid Metal Switches for Environmentally Responsive
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Table 1. Contact angles of galinstan droplets on various substrates,
immersed in either 1 m HCl or 1 m NaOH. ± value is one standard
deviation.
Etchant

Substrate

Contact angle [°]HCl (1 m)

Glass

164 ± 1.6

NaOH (1 m)

Silicon161 ± 1.3

PDMS

159 ± 4.6

Glass

165 ± 0.8

Silicon162 ± 0.5

PDMS

164 ± 2.1

technique, demonstrating its viability for creating both reconfigurable electronics and fully reversible liquid metal switches.
The methods presented herein enable the use of simpler
aqueous techniques for liquid metal manipulation, while subsequently permitting the final liquid metal morphology to be
removed from the aqueous medium.
Oxide Removal Rate Measured by Contact Angle: Since both
HCl and NaOH have been used to remove the gallium oxide
layer surrounding liquid metal droplets, we begin by comparing
the influence of the two chemical etchants. We measured thecontact angle of galinstan droplets on the surface of various
substrates while submerged in both HCl and NaOH solutionsat room temperature (see the Experimental Section for details).
Our results are presented in Table 1. All equilibrium contact
angles were roughly the same (≈160°) in 1 m HCL and NaOH,
which is lower than previously reported values.[30] For glass andsilicon substrates, data variance was low; but with a silicone
elastomer as the substrate (polydimethylsiloxane; PDMS), thedroplets had a noticeable variation in their final contact angles(with one droplet never getting higher than 153°). We suspectthat this is due to chemical interaction between the etchantand the PDMS, combined with batch variations inherent to
PDMS.[40] We also note that droplet buoyancy plays a role incontact angle when measured in aqueous solutions. Therefore,
the contact angles measured are higher than reported contact
angles of liquid metal droplets with the oxide removed via HClvapor treatment.[36]Comparing time stamps for each profile photograph, we
approximated the time-scale required to develop the maximum
equilibrium contact angle for each droplet. At the same concentrations (1 m), HCl took much longer than NaOH to develop the
maximum contact angle, even though the treatment method
was the same. The time difference was largest on a PDMS substrate (see Figure 1), likely due to the softer and more porous
nature of the substrate. When placed in an HCl bath on PDMS,
galinstan droplets consistently took several hours to develop
their maximum contact angle (like the droplet shown in HCl in
Figure 1). When placed in NaOH, the droplets would achieve amaximum contact angle in a very short amount of time (on theorder of minutes).
In the context of reconfigurable electronics, this has immediate ramifications on the configuration rate. The choice ofchemical etchant seems to have a large degree of influence on
the speed at which an adhered droplet can be released from asubstrate. Since oxide removal is the key to releasing a droplet,
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Figure 1. The evolution of the contact angle and droplet profile for agalinstan droplet placed on a flat PDMS in either a 1 m HCL or a 1 m
NaOH bath. It should be noted that the last frame of the NaOH shows
the droplet completely depinned (it has begun rolling).

we further investigated the speed at which the surface oxide isremoved by HCl and NaOH from a liquid metal droplet.
Oxide Removal Rate Measured Optically: To the authors’knowledge, there have been no studies on the chemical kinetics
of gallium oxide reacting with HCl and NaOH. To measure
the time it takes to remove the oxide by chemical etchants,
we developed a simple reflectance experiment, as shown in
Figure 2. We coated small, chemically inert platforms with
PDMS (for adhesion) and then, using a syringe, we placed verylarge galinstan droplets (diameter ≈ 8 mm) onto the PDMS.
The droplets sat for 24 h to encourage oxide adhesion. We thenremoved the liquid metal from inside of the droplet with asyringe, leaving the encapsulating oxide behind. As the surface
area of the droplet decreased from its original spherical area toan area approximately the size of the droplet’s circular profile,Figure 2. Experimental method for comparing galinstan oxide removal
rates by HCl and NaOH. a) Large droplets of galinstan are placed on a
PDMS substrate, and allowed to sit for 24 h. b) The liquid metal inside ofthe droplet is then removed, c) causing the solid oxide layer to develop avisible texture. d) When the textured oxide is immersed into a chemical
etchant bath, e,f) the exterior of the oxide begins to dissolve until theoriginal, reflective surface is restored. The time it takes to move from (d)to (e) depends on the chemical etchant used, and the insets for (c) and(f) show examples of textured droplets treated by either HCl or NaOH,
along with the time required to remove the visible oxide. g,h) We tested
the removal rate of the oxide at several concentrations, and present thedata in logarithmically scaled plots. The data gathered by Xu et al. using
rheometry (in HCl)[30] is presented alongside ours, as validation of our
method. Scale bar is 4 mm.

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our simple setup does, but we interpreted their data to develop
an oxide removal time versus concentration curve, and plotted
their data against ours. Via curve-fitting the b and m constantsof the power-law equation, we found that their data set andour HCl-treated data set had nearly identical slopes (ours withm = −1.417 compared to theirs of m = −1.501), with an offset on
the b value likely because their setup provided greater information about the complete oxide removal.
Combining this quantitative test with the qualitative work
shown in Figure 1, we can conclude that, at the same concentration, NaOH will release a droplet of pinned galinstan much
more rapidly than HCl will by dissolving away the gallium
oxide faster. It is not until the concentration of HCl is increased
to 2 m that the oxide is removed at a rate similar to very low
concentration NaOH (0.005 m).
Oxide Removal Rate Measured by Surface Tension: Surface
tension is a key part of both liquid–substrate adhesion (in thecontact angles developed by a liquid) and droplet mobility.
To compare galinstan droplets’ surface tension in either HClor NaOH baths, we performed simple goniometry measurements, and our results can be found in Table 2. We started bymeasuring the surface tension of galinstan droplets suspendedin oxygen-rich air. The surface tension we measured was
593 ± 46 mN m−1 (95% confidence), exhibiting a large variation,
but always greater than 500 mN m−1, as expected.[11]Unlike in our contact angle tests, we were not able to haveboth the HCl and the NaOH at 1 m concentrations. When we
extruded a galinstan droplet into a 1 m NaOH solution, thechemical reaction between the galinstan and the etchant was sovigorous that the droplet would disconnect from the extrusionneedle before we could begin measurements. Decreasing theconcentration to 0.1 m enabled rapid oxide removal and droplet
stabilization, while permitting measurements to be made. Thisrequired change in concentration for our goniometry tests provides a third demonstrable difference between HCl and NaOH
in chemically treating galinstan.
The final surface tension of galinstan in both the HCl
and NaOH baths was 470 ± 12 and 486 ± 6 mN m−1 (95%
confidence), respectively. Though the averages are slightly different, the confidence intervals overlap sufficiently to negate
any distinction between the two. We attribute the uncertainties
of the values in Table 2 to the highly spherical nature of thedroplets and therefore low bond numbers[41] (≈0.1). Althoughbond numbers <1 are desired when using drop shape analysis
to determine surface tension,[42] extremely low bond numbers (<0.1) cause errors in the fitting parameters when digitalsystems are in use.[43] Our droplets were close to this range,
causing a slightly larger variance in the measured values than
Table 2. Surface tension on galinstan droplets in various environments.
± value is 95% confidence on mean.
Surface tension[mN m−1]

Time to equilibriumAir

593 ± 46

85 minHCl (1 m)

470 ± 12

1.5 h

NaOH (0.1 m)

486 ± 60s

H2O

230–350

6h

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the solid oxide skin buckled and wrinkled, developing a dullgray hue. The buckled oxide scatters light instead of reflecting
it as a fresh oxide layer or the pure liquid metal does. Oncethe droplets are submerged in either NaOH or HCl baths, thebuckled oxide is chemically removed and a reflective surface isrestored to the droplet. Filming the process allowed us to record
the time it takes to change the droplets surface. This optical test
confirmed the difference between the speed at which HCl and
NaOH remove the gallium oxide layer as observed when studying contact angles.
We set up all of the droplets onto platforms simultaneously,
and randomly selected droplets for submersion in either a 1 m
HCl or 1 m NaOH solution. Each bath was prepared fresh andused only once, and the droplets were submerged with chemically inert tweezers to prevent any interaction of the tweezers
with the acid/base. Examples of the droplets are displayedin Figure 2, and a video of the tests is available in the Supporting Information. The droplets submerged in HCl took
between 9 and 13 s for their oxide layers to be removed (average
11.35 s ± 1.7 s standard deviation). The galinstan droplets
immersed in NaOH, however, had their oxide skins removed in
0.58 s (average) with a 0.045 s deviation—most of the deviationand error being caused by the frame rate of the camera used
to film the submersion instead of the actual chemical reaction.
This test provides further evidence for a comparative advantage of using NaOH over HCl in removing the gallium oxide
layer from gallium-based room-temperature liquid metals.
With the initial success of comparing the 1 m concentrations
using this technique, we then expanded our tests to include
various concentrations of HCl and NaOH. Since 1 m NaOH was
already very fast at processing, we proceeded to test 0.1, 0.01,
and 0.001 m. Since 1 m HCl was already acting slowly, we performed the test on a range of molarities between 2 m (to speed
the process up) and 0.1 m.
The final results of our optical tests are displayed in the loglog graphs of Figure 2. Each data point is an average and theerror bars represent a 95% confidence interval on the meantime. We fit a simple power-law curve to the data (y = bxm),
where the constant b represents the speed of oxide removal
at 1 m concentration and the constant m correlates the changeof the etchant concentration and the time required to remove
the oxide. The HCl fitted parameter m is −1.417, indicating alarge change in oxide removal time for small changes in concentration. For NaOH this parameter was found to be −0.268,
an order of magnitude smaller than the HCl, indicating minimal change in oxide removal times at even very low concentrations. Although the trend lines do not contact all of the confidence intervals, it is important to note that the plotted confidence intervals are representative of deviation in the measured
time only and do not take into account any uncertainty in themolarity of the chemical etchant baths.
To validate our procedure, we compared our data to the work
performed by Xu et al.[30] who used parallel-plate rheometry tocharacterize the influence of various concentrations of HCl on
gallium oxide. By actively removing the oxide with HCl during
the parallel-plate test, they were able to observe a change inthe mechanical properties of the oxide over time, until theyobserved complete mechanical breakdown, suggesting oxide
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we would have desired. Regardless, we conclude that the surface tension measured in both etchants is close enough to beequivalent.
Oxide Regrowth and Transformation Measured by Surface
Tension: Since a 1 m solution of either HCl or NaOH has asignificant water component, we wanted to determine the influence of clean water on the surface tension of liquid metals.
This would help us determine if the chemistry of water was
having a significant impact on the surface tension of the liquid
metal droplets, or if the dissolved acid (or base) was driving themeasured surface tension values. Following the same experimental procedure we used for HCl and NaOH, we measured
the surface tension of galinstan droplets in both distilled anddeionized water. In both types of water, the droplets registeredan initial surface tension profile at well over 500 mN m−1, but
over the period of hours it would drop between 40% and 60%,
sinking to a range anywhere from 350 to 230 mN m−1 (seeTable 2). This surface tension range is nearly half the value ofthe other mediums.
In goniometry, the time to achieve equilibrium surface tension depends primarily on the chemistry between the droplet
and the surrounding environment.[43] Droplets extruded into
1 m HCl took around 1.5 h to come to an equilibrium whereas
droplets extruded in the 0.1 m NaOH bath achieved an equilibrium surface tension nearly instantaneously. In water, however, the droplets often took over 6 h to come to an equilibrium
state. The extreme drop in surface tension over such a longtime can only be attributed to a slow chemical change of thesurface properties of liquid metal by the surrounding water.
This finding is supported by the work of Khan et al. who performed X-ray photoelectron spectroscopy (XPS) studies on theoxide layer and noted the generation of a gallium hydroxidealongside the normal gallium oxide when the droplet had been
exposed to water.[44] Using rheometry, they demonstrated thatthe hydroxide layer has a much lower modulus of elasticity thangallium oxide. This helps us to understand the results of ourgoniometry. When we initially extruded a droplet of liquid metal
into water, the liquid metal likely used the dissolved oxygen in
the water to create the characteristic thin gallium oxide layer.
Over time, however, the water reacted with the surface of thedroplet, changing the surface into the softer hydroxide. Since
the stiff, stabilizing oxide had been chemically changed toa weak hydroxide, the droplet distends under its own weight(see Figure S1, Supporting Information). This explains boththe long time required to develop the final equilibrium surface
tension and the large change in surface tension. The long time
was because of the slow chemical kinetics of the water reactingwith the gallium oxide (having to diffuse through the wholelayer of gallium oxide), and the large change in surface tension
is due to the newly formed, weaker, hydroxide exterior.
Previous studies into the effects of water on eGaIn–surface
interactions have mostly focused on the short-term effects ofa thin “slip-layer” of water.[23,44] Khan et al. observed that oxidized eGaIn could slide across a surface that it would normally stick to, when there is a layer of water present.[44] They
also showed that as the substrate dries, the oxide sticks to it
again. During our experiments, we observed similar slippingof liquid metal droplets, but only on short time scales. Given
longer time scales, we observed that a droplet of gallium-based
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liquid metal will adhere to a surface, regardless of the presenceof water.
Oxide Regrowth and Transformation Measured by XPS Analysis: We performed an XPS analysis to characterize the chemical effect of various surface treatments on galinstan. We tested
four different conditions: droplet submersion in HCl followed
by a quick rinse in water or a 24 h immersion in water, anddroplet submersion in NaOH followed by a quick rinse inwater or a 24 h immersion in water. Rinsed droplets weredipped in a clean bath for 2 s, and afterward were immediately
dried. Soaked droplets were immersed for 24 h to compare the
XPS results to the rinsed droplets and determine which surface properties were caused by the chemical etchant and which
results stem from the water baths. A list of all the spectra
analyzed for these results is contained in the Experimental
Section.
Figure 3 shows the XPS results of the four different conditions over the In 4d/Ga 3d/Sn 4d region. With the rinsed droplets, we anticipated observing an effect similar to that reported
by Kim et al.,[36] who demonstrated that eGaIn droplets treatedwith HCl vapor grow a gallium chloride and indium chloride
shell in place of the gallium oxide. However, we did not find any
trace of these chloride shells. Instead, it appeared that almost
all of the influences of either chemical etchant (HCl or NaOH)
were completely removed when the droplets were rinsed in
H2O, making the XPS results nearly identical. We suspectthat any alternative shells grown around the galinstan droplets by either the NaOH or the HCl were completely removed
during the rinsing process. Notably, the surface concentration

Figure 3. In 4d/Ga 3d/Sn 4d XPS spectra obtained from the samples thatwere treated with NaOH (top panel) and HCL (bottom panel). Spectrain each panel obtained following soaking for 24 h (top spectrum) and ashort rinse (bottom spectrum).

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Communicationof gallium was slightly enriched after the rinsing (85 at% vs
78 at% for ideal galinstan), and there was a significant amountof oxygen present in the spectrum (not shown in the figure).
Using the ratio between the metal and oxide components in
Ga 2p (not shown) and 3d, we calculated the thickness of the
regrown gallium oxide at about 3 nm. The details of such calculations can be found elsewhere.[45–48]
As seen in Figure 3, both rinsed samples showed the
Sn 4d (metal) peaks, but the soaking treatment resulted indisappearance of tin from the surface of the droplet: the Sn
4d (metal) peaks are barely visible. Indium concentration follows the same trend: the amount of indium visible from thesurface greatly decreased following soaking in H2O. Inversely,
there was an increase of covalently bonded gallium (shown asa shift of the Ga 2p and 3d peaks from the pure-metal energy
level), indicating more oxide had grown on the surface. Fromthese data, we estimated a new oxidized gallium layer thickness at 7 nm. This doubling of the exterior shells’ thickness
explains the disappearance of the tin and indium, as they wereno longer visible from the surface of the droplet. Though thechemical composition of gallium oxide is mainly Ga2O3,[11] thesoaked sample showed much higher amount of OH groups
as evidenced from the O 1s spectra (not shown), confirming achemical shift toward gallium hydroxide on the surface of thedroplet, as expected.[44,49,50]
From our XPS results, it is clear that gallium oxide thickness
correlates with water treatment: rinsing resulted in 40%–44%
of gallium on the surface in the form of oxide, whereas soaking
resulted in 90%–93%. Furthermore, these results show evidence that the droplet’s oxide layers, which had been chemically removed by the HCl or NaOH, were completely restoredby simply rinsing the droplets in water. This happened for boththe HCl and NaOH treated droplets, so we can easily conclude
that the original method of chemically treating the galinstan is
unimportant when oxide restoration is performed by rinsing
with water. Finally, there is a time element involved in exposing
liquid metals to water: further exposure to water (beyond
rinsing) results in the oxide layer growing in thickness whilechanging partially into a hydroxide layer.
Oxide Regrowth and Transformation Measured by Contact Area:
Since water removes the surface effects of HCl and NaOH,
we could use it as a method to reattach droplets that had been
removed via the chemical etchants. We tested this on a droplet
through a simple experiment: we soaked a galinstan droplet in
NaOH to remove its oxide, and then placed it onto a glass substrate immersed in distilled water. We tracked the profile of thedroplet over a 4 h period, and recorded the radius of the droplet’s contact area on the substrate. From this information, we
calculated the contact area as it changed with time and present
our findings in Figure 4. Although the droplet began to adhereon the order of minutes, the increasing contact area improved
the adhesion over the period of the test.
The contact angle did not change much during the test,
but the contact area of the droplet increased significantly withincreasing time submerged in H2O. As is visible in the inset
images in Figure 4, the original circular droplet shape collapsed
down into a flattened ellipsoid, maintaining a high contact
angle as the droplet spread outward and enabling a large
contact area on the substrate.

Figure 4. Changing contact area between a galinstan droplet and a glass
substrate in water. The droplet was initially removed from the substrateusing a 1 m NaOH solution. The NaOH was replaced with clean water,
and the droplet collapse was observed for 4 h. The inset shows the droplet’s initial and final states in the water.

As with the surface tension work reported previously, wehypothesize that the changing contact area (and adhesion to
the substrate) is due to the rapid triple effect during the initial submersion of the droplet. First, the water removed any
influence of the NaOH. Second, the dissolved oxygen grewa gallium oxide shell. Third, the water quickly changed
the outer surface of the shell to a hydroxide, leaving the restof the oxide untouched. While continuing to soak in the bath,the hydroxide slowly diffused through the whole thickness ofthe shell, weakening the droplet walls and causing the spherical droplet to collapse under its own weight. This process
resulted in a pulsing/shape changing effect on the droplet,as evidenced from Figure 4 between 50 and 150 min, and canbe seen in Video S2 (Supporting Information) between timestamps 0:50 and 1:00.
We note that after the water was removed, the droplet had
fully adhered to the substrate. We tested other techniques(details in the Supporting Information) to repin the droplet, butthey were not as successful as this method.
Liquid Metal Droplet Mobility: With droplet adhesion driven
by the gallium oxide layer on the liquid metal, we can free thedroplet by removing the oxide layer with either HCl or NaOH,
causing the droplet to roll freely on the substrate. Afterward, we
can repin the droplet to the substrate by rinsing it with water,
encouraging regrowth of either a gallium oxide or a galliumhydroxide layer on the surface of the droplet. These two chemical tools are all that is required to remove galinstan dropletsfrom a substrate and then fix them in a desired location, andwe now demonstrate the potential for this to be applied inmanipulating liquid-metal-based electronics.
Figure 5 shows our results being applied for environmentally stimulated liquid metal droplet mobility. We depinned adroplet from one location, enabling it to roll to a new location,and then pinned the drop back to the substrate. Figure 5a shows
the droplet of galinstan that has been adhered to a substrate in
an initial position. The droplet is then exposed to a 1 m NaOH
bath (Figure 5b), which causes the droplet to depin from thesubstrate and change location (Figure 5c) by gravitational forces.
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Figure 5. Controlling the location and fixity of a liquid metal droplet. a) The droplet is pinned to the substrate in air. b) Submerging the droplet in 1 m
NaOH causes the droplet to depin. c) The depinned droplet moves under gravitational force. d) The droplet is repinned to the substrate through an
H2O bath. e–h) Photographs of the active test, matching the diagrams in (a)–(d). Inset boxes are ≈3 mm × 3 mm.

able to re-attach to the substrate in the final position (Figure 5d).
The high contact angle on the droplet enables extremely rapid
droplet motion once the droplet has been depinned. Furthermore, this droplet mobility is controlled by the environment
alone, with gravity being the only source of force. We see future
opportunities to combine this process with environmentallycontrollable surface morphology, which would in turn enable
control over droplet locomotion direction, as well as applications
in connecting/disconnecting circuitry for reconfigurable logic.
Liquid Metal Switch: Using environmental stimuli, we also
demonstrate the ability to repeatedly disconnect and reconnect
a conductive pathway, as shown in Figure 6. We connected an
LED and a droplet of galinstan in series in a circuit (visible in
Figure 6e). The galinstan droplet was in a special environmental
chamber with two conductive tungsten leads emerging fromthe chamber walls for the droplet to rest on. We alternately
tested the resistance across the galinstan droplet and applied apower source to the whole circuit (Korad KA3005D DC power
supply, set to provide either a max of 3.3 V or 10 mA) to see if
the LED would illuminate. For each test, we cycled the fluid in
the environmental chamber, starting with air and then fillingthe chamber with a 1 m NaOH bath, continuing on to distilledwater, and then drying all the water out of the chamber with air
again. Figure 6a–d is a schematic representation of the cycle,
showing a profile view of the droplet and how it contacts theelectrodes based on the chemical bath in the chamber. A videoof the droplet profile during a cycle is available in the Supporting Information.
We were able to disconnect and reconnect the LED using the
NaOH bath to lift the droplet up, disconnecting the electrodes,

Figure 6. Opening and closing a circuit via chemical manipulation of a liquid metal droplet. a–d) Schematic representation of the side profile of thedroplet during one air-NaOH-water-air treatment cycle: a) initial state, b) submersion in 1 m NaOH disconnecting the circuit, c) replacing the NaOH
with water encourages the droplet to collapse, and d) the droplet is dried reconnecting the circuit. e) A photograph of the environmental chamber and
LED used for this test (for scale, the LED is 3 mm dia). h) A series of photographs of the LED with power supplied to the circuit during two consecutive cycles. i) Close-up views of the droplets in the chamber when power was supplied, along with f) the chamber environment and g) a measurementof the resistance across the chamber’s leads, during the two consecutive cycles. A video of the droplet profile during a third cycle is available in the
Supporting Information.

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Experimental Section
The three types of substrates primarily used were P-type silicon (Wafer
World), microscope slide glass (Home Science Tools), and Sylgard

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184 as PDMS (Dow Corning). These substrates were used as they are
representative of substrates typically utilized for liquid metal patterningsince galinstan readily adheres to them. Galinstan (68.5% Ga, 21.5%
In, 10% Sn by weight) was obtained from Sigma Aldrich, and used
as-received from the manufacturer. HCl (Fisher Chemical) and NaOH
(Macron Fine Chemicals) were obtained in either concentrated or dried
forms and mixed with water to get the desired concentrations.
Contact Angles: To measure contact angles, galinstan droplets
were set on a cleaned substrate for 3–5 min, covered. This setting
time allowed the droplets to adhere to the surface, so that the droplet
could be placed in the bath without dislodging it. The substrates anddroplets were then placed in a bath of either NaOH (1 m) or HCl (1 m).
Photographs of the droplet profile were taken regularly while the etchantslowly lifted the edges of the droplet and the contact angle maximized.
At least three different droplets of different sizes were profiled and theresults returned were an average of the different droplets. HCl and
NaOH were used in both 1 molar concentrations for the bath, and each
was tested on the following substrate types: glass, silicon, and PDMS.
To ensure that the PDMS substrate was flat, uncured PDMS was spincoated onto small glass slides (SCS G3-8 Spin Coater) at 2000 rpm for
2 min, before curing them at 60 °C for 2 h. Each substrate was used only
once for a single galinstan droplet, to prevent any acid/base etching of
the surface from effecting subsequent results. All tests were performedat room temperature (≈25 °C).
Goniometry: Pendent drop shapes were recorded and measured
in a goniometer (Ramé-Hart Instrument Co.) capable of performing
oscillating pendent drop measurements (though this feature was notenabled). Each test was performed in a similar manner, using a non-air
gap technique in which the water inside of the pipette tip directly contacts
the liquid metal. This removed any viscoelastic effects of a compressible
air pocket while extruding the liquid metal droplets. 5–8 mL droplets ofliquid metal were extruded directly into the medium being used for the
analysis (air, water, HCl, or NaOH) and briefly allowed to stabilize for up
to 30 s, to enable vibrations from the extrusion process to die off. At thispoint, the surface tension and volume of the droplet were recorded atintervals between 10 and 30 s (depending on the total length of the test)
to allow the droplet to achieve equilibrium surface tension. The surface
tension was then recorded as an average of the equilibrium tail of thedata. Initial droplet volumes varied, but had a negligible impact on therecorded results.
Droplets extruded in both air and 1 m HCl were allowed at least 2 h
to establish an equilibrium tail, although liquid metal droplets extruded
into air often did not require as long of a time as those in HCl. Droplets
extruded into 0.1 m NaOH were allowed to come to an equilibrium forat least 40 min, and upward of 2 h, although this length of time was notnecessary as the droplet had established its equilibrium surface tension
before the analysis even began. Liquid metal droplet extruded into water
were allowed at least 10 h to come to an equilibrium, though many wereable to establish an equilibrium before this time, the additional hours
enabled a proper determination of equilibrium status.
XPS and Sample Preparation: XPS measurements were performed
using a Kratos Axis Ultra DLD spectrometer with monochromatic Al
Kα radiation (hν = 1486.6 eV). The high-resolution Ga 2p3/2, O 1s, Sn
3d, In 3d, C 1s, Cl 2p, and Ga 3d/Sn 4d/In 4d spectra were collected
at constant pass energy (PE) with a PE of 20 eV. A built-in commercial
Kratos charge neutralizer was used to achieve better resolution. Binding
energy (BE) values refer to the Fermi edge and the energy scale wascalibrated using Au 4f7/2 at 84.0 eV and Cu 2p3/2 at 932.67 eV. The
photoemission peak positions were charge corrected to the adventitious
carbon signal of C 1s at 284.8 eV. Spectra were analyzed using the
CasaXPS software program, version 2.3.16 PR 1.6 (Casa Software Ltd.). A
Shirley background was subtracted from each region before curve fitting;metal components were fitted with asymmetric Gaussian–Lorentzianpeaks with tail dampening (CasaXPS Lineshapes ≈ LF(1,1.4,20,50))
and oxide components—with Gaussian–Lorentzian peaks (CasaXPS
Lineshapes ≈ SGL(10)). Since the Ga 3d/Sn 4d/In 4d region contains
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

wileyonlinelibrary.com(7 of 9) 1600913

Communicationand the water bath to collapse the droplet back down, reconnecting the electrodes. Figure 6f,g describes the current stageof the cycle and the resistance measured across the liquid metal
droplet, while Figure 6h,i is a series of simultaneous photographs of the LED and the galinstan droplet during each stageof the cycle. It is important to note that although the NaOH
bath disconnected the liquid metal droplet from the circuit, the
NaOH itself was able to conduct electricity via electrochemicalmeans. This is why although the resistance is very high withboth the NaOH and the water baths, the LED illuminates (partially) when power is applied during the NaOH portion of thecycle. Furthermore, the electrochemical interaction is whatcauses the gas bubbles visible around the galinstan droplet in
Figure 6i during this portion of the cycle (also visible in thevideo). Once we rinsed the NaOH away with distilled water, thewater no longer connected the electrodes and the LED ceased
to illuminate. We also note that, although not shown in the
figure, if we rinse out the NaOH from the chamber and thenrapidly dry the water from the chamber (with a small fan) the
droplet holds its new shape, keeping the circuit open. The circuit can then be closed at any time by soaking the droplet inwater.
Having successfully developed a system where we were able
to completely disconnect and reconnect the circuit via environmental stimulation, we cycled the circuit to ensure repeatability.
In Figure 6 we show two consecutive disconnect and reconnect
cycles, without changing the liquid metal droplet or any otherpart of the circuit. We cycled it a third time, while recording thevideo found in the Supporting Information. In the video, we
demonstrate the mobility of the chamber after the droplet had
dried, proving that liquid metals can be manipulated, reconfigured, and then pinned in place so that circuits built from theseliquid metals can be removed from environmental control
chambers.
In this paper, we have quantified the influence of acidic(HCl), alkaline (NaOH), and neutral (water) aqueous environments on liquid metal droplets prone to surface oxidation.
Our results demonstrate the use of environmental stimuli tocontrol the removal, formation, and transformation of surface
oxide on liquid metal droplets, thereby controlling the adhesionof 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
differ by orders of magnitude at similar concentrations, with
NaOH emerging as the more aggressive etchant. Furthermore,
we have shown that rinsing the liquid metal droplet in water
reverses the effects of HCl and NaOH, allowing formationof a surface oxide at short timescales and surface hydroxide
at long timescales, which differ in mechanical properties but
both encourage liquid metal adhesion to the substrate. Finally,
we have employed our findings to demonstrate liquid metal
droplet mobility on-demand and environmentally responsive
liquid metal switches.

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Communicationwww.advmatinterfaces.de
Sn 4d (4d5/2 and 4d3/2), Ga 3d (3d5/2 and 3d3/2), and In 4d (4d5/2
and 4d3/2) electron levels were subject to spacing constraints of 1.10,
0.45, and 0.85 eV, respectively. The intensity ratio of the spin–orbit
coupling doublets for the d levels (d5/2 and d3/2) was fixed to be 3:2.
The atomic concentrations of the chemical elements on the near-surface
region were estimated after the subtraction of a Shirley-type background,taking into account the corresponding Scofield atomic sensitivity factors
and inelastic mean free path (IMFP) of photoelectrons using standard
procedures in the CasaXPS software.
The following protocol was used to prepare samples for the XPS
measurements. (a) Glass substrates were exposed to oxygen plasma to
encourage the galinstan droplets to adhere to the surface. (b) Dropletsof galinstan were immediately placed on the substrates, and wereallowed to sit for 2 d to maximize adhesion between the droplet and thesubstrate (so the droplets do not move during treatment and testing).
(c) Samples were then briefly rinsed in a chemical etchant bath of NaOH
(1 m) or HCl (1 m) to alter the droplet's surface chemistry for 20 and 45 s,
respectively. This is longer than is needed to create a visible change ofthe droplets’ surface chemistry, but the extra time was to ensure that alloxide had been removed. (d) The two samples that were rinsed in water
were quickly dried first by wicking away the majority of the remaining
water (via absorption), with the remaining water forcibly evaporated
by passing dry nitrogen across the substrate and droplet. As-preparedsamples were loaded to the XPS spectrometer through a load-lock.

Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
The authors would like to thank Prof. Kendra Erk (Materials Science
Engineering, Purdue University) for her assistance with the surface
tension goniometry. This project was funded by a NASA Early Career
Faculty award (Grant No. NNX14AO52G). Any opinion, findings, andconclusions or recommendations expressed in this material are thoseof the authors and do not necessarily reflect the views of the National
Aeronautics and Space Administration.
Received: September 22, 2016
Revised: October 26, 2016
Published online: January 5, 2017

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