From Nanoscale Wetting Mechanisms to Contact Printing of Gallium

May 21, 2014 - dominated by the intimate contact between new oxide and substrate. We also show that by ..... across a silicon wafer to adhere for imag...
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Different Shades of Oxide: From Nanoscale Wetting Mechanisms to Contact Printing of Gallium-Based Liquid Metals Kyle Doudrick,†,‡ Shanliangzi Liu,† Eva M. Mutunga,§ Kate L. Klein,§ Viraj Damle,† Kripa K. Varanasi,∥ and Konrad Rykaczewski*,† †

School for Engineering of Transport, Matter and Energy, Arizona State University, Tempe, Arizona 85287, United States Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States § University of the District of Columbia, Washington, D.C. 20008, United States ∥ Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡

S Supporting Information *

ABSTRACT: Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because, despite having surface tension 10 times higher than water, they strongly adhere to a majority of substrates. This unusually high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In this work, we demonstrate that, dependent on dynamics of formation and resulting morphology of the liquid metal−substrate interface, GaInSn adhesion can occur in two modes. The first mode occurs when the oxide shell is not ruptured as it makes contact with the substrate. Because of the nanoscale topology of the oxide surface, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate’s surface energy or texture. In the second mode, the formation of the GaInSn−substrate interface involves rupturing of the original oxide skin and formation of a composite interface that includes contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide. We demonstrate that in this latter mode GaInSn adhesion is dominated by the intimate contact between new oxide and substrate. We also show that by varying the pinned contact line length using varied degrees of surface texturing, the adhesion of GaInSn in this mode can be either decreased or increased. Lastly, we demonstrate how these two adhesion modes limit microcontact printing of GaInSn patterns but can be exploited to repeatedly print individual sub-200 nm liquid metal drops.

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and contacts,16,17 droplet-based microswitches,18 microsyringes for cells,19 radio-frequency switches,20 magneto-hydrodynamic pumps,21 stretchable antennas,22 resonators,23 and tunablefrequency selective surfaces.24 Because of this wide application space, several new routes of manufacturing gallium-based liquid metal devices have been proposed. These methods are intended to replace the current syringe injection-molding fabrication approach that is labor intensive, nonscalable, and limits achievable device geometries.25,26 The recent alternatives include fabrication of liquid metal structures using vacuum-induced patterning,4,8 contact printing,25,27,28 roller-ball pen,29,30 direct writing,31 masked deposition,32 microfluidic flow focusing,33 co-electrospinning,34 freeze-casting,35 airbrushing,36 and 3D printing.37 However, fabrication of well-defined geometrical features (i.e.,

INTRODUCTION The development of stretchable conductors is of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices.1−4 The use of roomtemperature liquid metals is particularly attractive because, in contrast to semiconductor-based stretchable electronics,2 circuits comprising of these materials are intrinsically soft and remain functional even when stretched to several times their initial length.5 One of the earliest examples of liquid-phase electronics is the Whitney strain gauge,6 which measures strain of a mercury-filled rubber tube by measuring variation in electric resistance of the metal due to change in its geometry. Recently, mercury has been replaced with safer roomtemperature liquid gallium alloys such as gallium−indium (GaIn) or gallium−indium−tin (GaInSn also known as Galinstan).5 Besides polymer encapsulated strain sensors,7−12 gallium-based liquid metals have also been proposed for a number of other applications including microelectronic heat sinks,13,14 thermal interface materials,15 electrical interconnects © 2014 American Chemical Society

Received: March 31, 2014 Revised: May 20, 2014 Published: May 21, 2014 6867

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Figure 1. Optical images of advancing and receding contact angles of GaInSn on (a, b) glass, (c) tungsten, (d) PTFE, and (e) PDMS surfaces measured using volume (VV) and substrate height variation (HV) methods in air at about 98 kPa. The scale bar corresponds to 230 μm.

not randomly distributed nanofibers34) with sizes significantly below 10 μm has proven to be difficult. The limited spatial resolution of most of these novel microfabrication techniques is attributed to complex rheology and wetting of the gallium-based liquid metals7,38−41 caused by a thin oxide shell that rapidly forms when these liquids are exposed to an oxygen concentration above 1 ppm (or in other reports, a dose of 1.8 × 10−4 Torr·s).42−45 Although in high vacuum the surface composition of GaIn and GaInSn is dominated by indium,7,44 the shell of both alloys primarily consists of Ga2O3 because indium is slow to oxidize and thermodynamically Ga2O3 is preferential over In2O3.7,38,44 Also, despite a thickness of only 0.5−2.5 nm,7,42−45 this thin film is robust enough to mechanically stabilize large deformations,37 which leads to macroscopic viscoelastic behavior of the liquid metals in the presence of air (critical surface yield stress of about 0.5 N/m).7,45 The oxide shell has also been reported to strongly adhere to almost any surface,45,46 making manipulation and transfer of the liquid metals challenging. Several routes of mitigating the high adhesion of the oxide including embedding of metal droplets in hydrophobic nanoparticles (i.e., liquid marbles),47 use of textured metal-phobic surfaces,46 hydrochloric acid liquid or vapor treatment,7,38 and acid-impregnated surfaces48,49 have been proposed. However, thus far, none of these methods have aided to increase GaIn and GaInSn patterning spatial resolution. As in the case of microscale droplets of molten solder,50−53 a better understanding of the phenomena underlying the liquid metal adhesion and wetting could lead to development of improved fabrication methods with higher spatial resolution. To this end, we have performed a multiscale study aiming at understanding the fundamental mechanisms governing wetting of gallium-based liquid metals. In particular, we used macroscale dynamic contact angle measurements coupled with in situ nano- to microscale experimentation within a focused ion beam-scanning electron microscope (FIB-SEM)54 to relate

macroscopic drop adhesion to morphology and formation dynamics of the liquid metal−surface interface. We establish the contribution of the bare liquid metal adhesion, the oxide layer growth, and morphology as well as the substrate’s surface energy, mechanical properties, and texture to the wetting process. Lastly, we relate how dynamics of GaInSn wetting of a master stamp currently limits patterning with microcontact printing, and demonstrate a new dipping approach that enables printing of individual liquid metal nanodrops.

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RESULTS AND DISCUSSION Solid−liquid adhesion can be quantified through contact angle hysteresis (CAH), which is the difference between the contact angles that a drop makes while advancing on (θA) and receding (θR) from a surface.55 To study the impact of the oxide shell on GaInSn adhesion, we measured the liquid metal’s θA and θR on variety of surfaces using two methods. In the first approach, the position of the surface was fixed while the volume of the drop forming at tip of a syringe was varied. Specifically, the volume was increased as drop made contact with and expanded over the surface and subsequently decreased until the drop retracted from the surface (see Figure 1a). This approach, which we refer to as the volume variation method, is commonly used to measure the CAH of liquids,55 including gallium alloys.46 Increasing the GaInSn volume creates new surface area and leads to exposure of bare liquid metal followed by initiation of new oxide growth within microfractures of the old oxide shell. Thus, the CAH measured using the volume variation method is a measure of the adhesion of the composite interface that includes oxide of various ages and potentially bare liquid metal. To probe the adhesive properties of GaInSn without rupturing of the oxide skin, we used an alternative approach, which we refer to as the height variation method. In this second approach, to avoid breaking the oxide shell, the sample surface was gently brought in contact with and retracted from a liquid 6868

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metal drop having a fixed volume. Thus, the CAH measured using this approach quantifies the adhesion between the sample and surface of the unbroken oxide (i.e., exposure of bare liquid metal followed by oxide regrowth is minimized; see Supporting Information for further discussion of oxide shell morphology during this measurement). To ensure that the second approach was applicable to measuring of dynamic contact angles, we measured and obtain a close match for the CAH of water on a flat hydrophobic surface using both approaches (see Supporting Information for further details). Contrasting of images shown in Figure 1a,b reveals that dramatically different receding contact angles of GaInSn are measured on a glass substrate using the two methods. While the θR measured using the height variation approach is about 125° (CAH of 8°), the θR measured by reducing droplet volume is about 46° (CAH of 85°). A nearly identical trend is observed for tungsten foil (Figure 1c) and polytetrafluoroethylene surfaces (PTFE; Figure 1d). Only the receding contact angles of GaInSn measured on polydimethylsiloxane (PDMS) are in close agreement with about 78.6° (CAH 49.0°) and 73.4° (CAH 67.7°) measured using the volume and height variation methods, respectively (all measured contact angles are listed in Table S1 of the Supporting Information). Thus, with exception of the highly viscoelastic PDMS, the interfacial adhesion between the old, unruptured oxide and the studied flat surfaces observed using the sample height variation method is low. In this case the surface energy of the substrate, γ, does not play a significant role (γPTFE = 18 mJ/m2, γPDMS = 19.8 mJ/m2, γSiO2 = 593 mJ/m2, γW(110) = 3320 mJ/m2).56 In contrast, exposure of bare GaInSn followed by oxidation resulting from old oxide shell rupturing during the volume addition method led to very high adhesion to all examined surfaces. Based on the CAH experiments, our hypothesis is that the liquid metal adhesion to flat surfaces is dominated by morphology and dynamics of formation of the oxide−surface interface and as a result can occur in two modes. The schematic in Figure 2a illustrates the first mode that occurs during the height variation experiments and leads to formation of a solid− solid rather than solid−liquid interface. According to the Johnson, Kendall, and Roberts theory,57 the adhesion force (i.e., pull-off force), Fad, between a flat substrate of material 1 and a sphere of material 2 with radius R within a medium 3 is equal to58 3 Fad = − πRW132 2

Figure 2. (a) Schematic illustration of the rough oxide−surface interface responsible for low adhesion measured using the height variation method and SEM images showing (b) microscale and (c) nanoscale roughness of the old, air-formed, top surface of an oxidized GaInSn drop formed at tip of a syringe (the drop was lightly sheared across a silicon wafer to adhere for imaging). (d) Schematic illustration of the smooth oxide−surface interface formation around perimeter of drops formed using the volume addition method (a composite interfacial area consisting of fractured pieces of old oxide, new oxide, and bare GaInSn is likely present underneath the drop).

An understanding of the mechanisms causing the high CAH in the second mode measured using the volume variation approach is more difficult because of the composite nature of the liquid metal−surface interfacial area and its perimeter (contact line). As the GaInSn expands over the substrate, multiple microfractures within the old oxide shell continually expose bare GaInSn. This process leads to formation of a composite interfacial area between the drop and the substrate that likely includes pieces of the old oxide, new oxide, and bare liquid metal. The latter two components could be in intimate contact with the substrate because of the positive pressure driving the drop expansion. The presence of microscale pieces of unruptured old oxide on the base area and also around the contact line could cause locally minimal adhesion to the substrate. The schematic in Figure 2d shows that, in the areas with exposed liquid metal, these microscale dynamics lead to formation of a smooth, conformable, liquid-like joint between the substrate surface and the fresh oxide around the perimeter of the drops. The surface roughness of the oxide might increase with age due strain from handling, but not at the already formed 1−2 nm thick oxide− substrate junction. Since the contact line of the GaInSn drops comprises of this intimate interface, it can have a significant contribution to the observed large CAH. To determine the contribution of the bare liquid metal and oxide with varied roughness to the adhesion mechanisms in mode 2, we use in situ nano- to microscale FIB-SEM experimentation. In particular, we use this approach to observe GaInSn behavior during oxide shell removal as well as inherent solid−solid adhesive properties of old and freshly regrown oxide.

(1)

where W132 is the work of adhesion between material 1 and 2 in the presence of medium 3. If one of the two involved solids has a surface roughness with a root-mean-square value, εrms, the adhesion force is reduced by58 Fad(ε) = Fad(0)e−εrms / ε0

(2)

where Fad(0) is the adhesion force on a flat surface and ε0 is a constant. Because of the exponential nature of eq 2, even roughness on the order of a few nanometers can significantly lower the adhesion force.58 The SEM images in Figure 2b,c show that the old oxide surface has both microscale and nanoscale topological features that likely form due to straininduced during handling. Thus, the low CAH measured for mode 1 using the height variation method is due to the old oxide roughness (eq 2), not inherently low work of adhesion between gallium oxide and the studied solid surfaces (eq 1). 6869

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Figure 3. FIB secondary ion images of GaInSn behavior under continuous Ga+ and He+ ion irradiation: (a) schematic illustration of directional nature of ion beam irradiation, (b) oxide-skin removal and liquid metal dewetting from silicon wafer and (c) subsequent liquid metal wetting of silicon wafer caused by Ga+ irradiation and implantation in the substrate, (d) oxide-skin removal, liquid metal dewetting, and sputtering on silicon wafer due to He+ irradiation; effects of Ga+ irradiation on GaInSn on (e) tungsten foil, (f) PTFE, and (g) PDMS. The vacuum level in the Ga+ and He+ FIBs is 0.13−1.3 mPa and 0.013 mPa, respectively.

GaInSn Wetting Behavior during Oxide Removal. To decouple the adhesion of the oxide shell and wetting of bare liquid metal, we removed the oxide using two approaches: FIB abrasion and HCl vapor treatment. Application of FIB has the advantage of simultaneous high precision milling and imaging of the material’s response. Because of the small interaction volume of impinging ions, secondary electrons created during the ion−sample collision can provide significantly better resolution and contrast between surface features than SEM.59 Previously, only a wide-beam argon and helium ion irradiation have been used to quantify near surface composition of GaIn,7,38,44 but not morphological response of the material. We used two commercially available FIB instruments: one with a gallium (Ga+) and one with a helium (He+) column. We note that the Ga+ FIB is a dual column FIB-SEM equipped with a mechanical nanoprobe and energy-dispersive X-ray spectrosco-

py (EDS). The schematic in Figure 3a illustrates that ion beam irradiation is directional and thus only affects the top and not the bottom of the drop (i.e., the liquid metal acts as a protective mask). The sequence of images in Figure 3b shows the response of a GaInSn island (formed by manually spreading GaInSn over a silicon wafer) to Ga+ beam scanning. As the oxide skin begins to fracture under ion beam milling, the high surface-tension liquid metal proceeds to rapidly dewet the silicon surface. Within 15 s of irradiation at 0.92 nA and 30 keV (equivalent flux of 0.08−0.32 A/m2), the initial island with contact angle lower than about 15° transforms into a lightly pinned drop with a contact angle of about 80° and base diameter of 40 μm (see also Movie 1 in Supporting Information). We also observed film-to-drop dewetting of GaInSn due to Ga+ irradiation on tungsten and PTFE (Figure 3e,f), demonstrating low adhesion of the bare GaInSn to these 6870

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surfaces. In contrast, images in Figure 3g show that while ion irradiation of GaInSn on PDMS did lead to oxide removal, it was not followed by dewetting of the liquid metal. This behavior is due to either high adhesion of PDMS to residual pieces of the old oxide on the elastomer−drop interface or to formation of oxide at the PDMS interface due to high oxygen permeability through the polymer.60 A striking switch in wetting behavior of the GaInSn on silicon is observed after the ion irradiation is continued, resulting in a high total ion exposure dose. The sequence of images in Figure 3c shows that increasing the ion flux (i.e., increased magnification) on a dewetted GaInSn drop causes it to completely wet the exposed area within 1 s. This is shown by the flat liquid metal square in the low-magnification image (also see Movie 1). Furthermore, GaInSn also flows from “reservoir” drops to completely fill neighboring areas that are exposed to high Ga+ doses (see Movie 1 and Figure S1). We observed that prolonged Ga+ irradiation led to superwetting by GaInSn of silicon and tungsten as well, but we could not confirm the superwetting behavior on glass, PTFE, and PDMS due to excessive charging and/or rapid milling of these surfaces. The mechanism responsible for the high Ga+ exposuretriggered complete wetting of silicon by GaInSn can be inferred by comparing the liquid metal dynamics under Ga+ and He+ irradiation. Abrasion by both of the ion beams causes removal of the oxide skin and liquid metal dewetting from silicon. However, GaInSn drops under He+ ion irradiation are slowly sputtered away, instead of spreading on the substrate (Figure 3d and Figure S2). Thus, the likely mechanism for superwetting by GaInSn under Ga+ irradiation is gallium implantation in the silicon substrate, with a threshold exposure of about 30 ions/ nm2 (flux of 20 A/m2 for 0.2 s). Since Ga+ FIB can selectively implant ions with about a 5 nm spatial resolution, inducing preferential superwetting of GaInSn can be used to fabricate nanoscale liquid metal patterns. We successfully demonstrated this nanofabrication technique; however, we do not expand on it because this approach is nonscalable, and the liquid metal only fills pre-exposed patterns during Ga+ irradiation (see Figure S1 and further discussion in Supporting Information). To complement the ion abrasion experiments, we also removed the oxide shell using HCl vapor.38,40 Images in Figure 4 show that this treatment causes dramatic changes to the GaInSn morphology. In particular, the sporadic oxide-stabilized thin films dewet the surface and form nearly spherical “balls”. The dewetting process was expected, as spectroscopic analysis has previously shown that HCl vapor treatment results in replacement of the outer oxide layer with GaCl3 and some InCl3.38 However, the topography of GaInSn surface after the HCl vapor treatment has not been previously investigated. Our results show that the thin oxide skin is replaced with an approximately 1 μm thick shell with multiple fracture lines. EDS revealed that this shell consists primarily of gallium and chlorine with small amounts of oxygen. The exposed liquid-like material in-between the cracked shell consists mostly of gallium with a trace amount of surface oxygen (see Supporting Information for further discussion of the shell composition and mechanical properties). Thus, as previously reported,38 oxide shell does not regrow after the HCl vapor treatment. However, the HCl treatment introduces too many morphological and chemical alterations to the liquid metal to provide insight into wetting of untreated GaInSn. Adhesive Properties of Newly Formed and Aged Gallium Oxide. Dewetting of the GaInSn from silicon,

Figure 4. SEM images of GaInSn on silicon wafer after HCl vapor treatment and disruption of the chloride shell.

tungsten, and PTFE during Ga+ irradiation implies that wetting and adhesion of the liquid metal on these solids are dominated by the oxide−substrate interactions. The vacuum level of the Ga+ FIB-SEM and He+ FIB is at 0.13−1.3 mPa (10−6−10−5 Torr) and about 0.013 mPa (10−7 Torr), respectively, which provides sufficient residual oxygen to regrow the oxide skin (both microscopes were pumped down from air environment, corresponding to about 20% oxygen content by volume).42,43 To observe the oxide shell regrowth dynamics, we cycled the ion irradiation on and off (or by selectively increasing and decreasing the magnification) while focused on a large GaInSn film surface. The image in Figure 5 shows that this cyclic ion exposure procedure leads to formation of multiple fractures in the shell surface. The various observed shades correlate to the age of the oxide, with the darkest being freshly exposed GaInSn and the lightest being the original oxide (see also Movie 2). We

Figure 5. FIB secondary electron image of multiple thicknesses of regrowing oxide achieved by cycling Ga+ irradiation on and off at vacuum level of 0.13−1.3 mPa (different shades correspond to different ages, and with that thicknesses, of the oxide). 6871

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Figure 6. Nanoprobing of GaInSn oxide in vacuum level of 0.13−1.3 mPa: “poking” of original oxide skin with (a) Si microparticle attached to the nanoprobe, (b) unmodified tungsten nanoprobe, (c) PTFE-coated nanoprobe, (d) PDMS-coated nanoprobe, and (e) observation of residual interface contact line formed through dipping, waiting for 160 s, and retracting a tungsten nanoprobe from GaInSn whose original oxide skin removed via ion irradiation.

also observed the same oxide regrowth process while cycling He+ irradiation (see Figure S2), which shows that Ga+ irradiation did not introduce significant artifacts. However, the oxide regrowth under the lower pressure in He+ FIB was significantly slower (hundreds of seconds) than in the Ga+ FIBSEM (less than 10 s). The relatively slow shell regrowth dynamics provide sufficiently enough time to probe adhesive properties of fresh and old oxide but not to quantify elemental evolution of the surface using in situ EDS (insufficient time to acquire a set of spectra). To qualitatively observe the adhesive properties of the oxide, we imaged the response of the shell to gentle “poking” with the tungsten nanoprobe with and without modification with silica, PTFE, and PDMS. The x−y−z movement of the nanoprober can be controlled with a high precision down to a rate of 200 nm/s. To avoid ion abrasion artifacts, the experiments were imaged using the electron beam. To capture the dynamics of the process, images were saved with 1 Hz frequency. The sequence of images in Figure 6a show a nanoprobe with a silicon microparticle (attached to the probe using ion beam induced deposition of platinum “glue”) gently approaching, contacting, and retracting from the “old” oxide shell formed in air. Approaching the surface slowly, via steps of 200 nm enabled us to perform this experiment without piercing of the shell. While deflection of the shell under probing was observed, no sticking or other signs of adhesion of the metal oxide to the silicon microparticle (Figure 6a), bare tungsten (Figure 6b), or PTFE covered (Figure 6c) nanoprobes were observed. In turn, retraction of the nanoprobe with PDMS coating leads to formation of about a 500 nm wide neck between the probe tip and the oxide surface (Figure 6d). The nanoneck formation corroborates the results of the CAH experiments (i.e., Figure 1e) and confirms an inherently high adhesion between the aged oxide shell and PDMS. The high adhesion of PDMS is likely

due to its viscoelastic nature, which enables formation of intimate contact with even nanotextured surfaces.58 We also developed a procedure to use the nanoprobe to qualitatively observe the adhesive strength of freshly formed oxide−tungsten interface. In particular, the SEM images in Figure 6e show that we first swiftly dipped the tungsten nanoprobe into a freshly FIB-milled region of the GaInSn surface (i.e., oxide skin removed). Subsequently, we allowed the oxide to regrow for over 2 min and then slowly retracted the nanoprobe. We selected the uncoated tungsten probe for this experiment because its surface is relatively smooth. Furthermore, the ion irradiation experiments demonstrated that bare GaInSn readily dewets tungsten. The strongly concave liquid metal meniscus observed during dipping of the probe (i.e., Figure 6e at 24 s) further corroborates the GaInSn-phobic nature of tungsten. However, after 2 min of oxide regrowth, the liquid metal surface strongly adheres to the nanoprobe. The strong bond between the new oxide and the tungsten surface is illustrated by the large deformations of the oxide surface around the probe during its slow retraction (200 nm/s). Continuing the nanoprobe withdrawal led to abrupt retraction of the stretched oxide surface (i.e., Figure 6e, at 165 s). The close-up image of the nanoprobe surface shows residue of the oxide− tungsten contact line, implying that the fracture occurred within the oxide surface not at the interface. Thus, the strength of the oxide−tungsten interface formed during oxide regrowth is higher than the oxide surface yield strength of 0.5 N/m.7 GaInSn Wetting on Textured Surfaces. Adhesion of GaInSn to textured substrates is highly dependent on the dynamics of oxide−substrate interface formation. If during this process the oxide skin is not fractured or the microfractures heal prior to making contact with substrate, adhesion will be governed through solid−solid contact mechanics. Therefore, addition of surface roughness will decrease the adhesion force according to eq 2. Indeed, a significant reduction in CAH of 6872

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the liquid−substrate contact line around the perimeter of the drop base.55 With addition of surface texture, the adhesion force of the interface will be altered in proportion to the change in the perimeter contact line length. Paxson and Varanasi61 showed that for a drop sitting on a surface with a single tier ordered texture (such as shown in Figure 7d), the ratio of the lengths of the textured surface and projected (i.e., flat surface) contact lines, referred to as the pinned ratio ϕ, is equal to

GaInSn on viscoelastic PDMS due to surface texturing of the polymer has been previously reported.46 However, fracturing of the oxide shell during spreading over a textured substrate, with air trapped in-between geometrical features, should lead to formation of an oxide film underneath the drop (i.e., Figure 7a).

ϕ=

NpP l

=

P τ

(3)

where l is the length of the project contact line and Np is the number of peripheral texture features, while P and τ are the perimeter and the pitch of individual features. Thus, a ϕ < 1 and a ϕ > 1 correspond to decreased and increased adhesion, respectively, as compared to a flat surface. The same analysis applies to the case of the GaInSn spread over a textured surface, but with the contact line representing the oxide shell−solid interface, not the liquid−solid interface. If the force applied to detach the drop is higher than the critical surface yield force, equal to the product of l and critical surface strength σs, surface fracture rather than interface failure will occur. This leads to formation of extra surface area or residual material on the surface. To demonstrate the effect of the pinning fraction on GaInSn adhesion, we measured contact angle hysteresis of surfaces with ϕ < 1 and ϕ > 1 using the volume variation method. In particular, SEM images Figure 7d,e show a square silicon micropillar surface (square width of 10 μm and pitch of 15 μm) with ϕ = 2.7 and silicon nanograss with ϕ = 0.18.61 Both of these surfaces were modified with low-energy octadecyltrichlorosilane (OTS) coating to render them superhydrophobic. The GaInSn adheres strongly to the micropillar surface with CAH of 80.0° (θA = 150.4 ± 3.8° and θR = 72.3 ± 11.2°). In contrast, the CAH of GaInSn on the nanograss surface was only 1.3° (θA = 150.4 ± 3.8° and θR = 151.7 ± 2.0°), which is the lowest value for all studied surfaces. Thus, surface texture can be designed to either increase or decrease adhesive properties of GaInSn (see Movie 3 and Figure S6 for more on GaInSn dewetting from the nanograss and micropillar surfaces, respectively; see Table S1 for contact angle values for all surfaces). Since for both flat and textured surfaces the interfacial base area and the contact line is a composite that includes fractured pieces of the old oxide that locally have minimal substrate adhesion (with exception of PDMS), eq 3 should also include a factor accounting for reduction of the pinned contact line length due to its presence. Implications of Wetting Mechanics to Nano- to Microscale Contact Printing of GaInSn. Soft lithography is a simple and inexpensive method of transferring nanoscale and microscale patterns from a “stamp” to a desired substrate.62,63 This attractive fabrication approach is currently used for applications ranging from patterning of self-assembled monolayers to site-selective transfer of biological materials.25 However, extension of this technique to patterning of galliumbased liquid metals has only worked for serial transfer of individual microscale droplets with smallest feature diameter and pitch of 340 and 202 μm, respectively,25 and has failed for full pattern transfers (i.e., parallel fabrication). Here we discuss the impact of wetting and adhesion modes of GaInSn for parallel feature transfer and ultimate spatial resolution of an individual feature transfer.

Figure 7. (a) Schematic and (b) FIB secondary electron images of GaInSn dewetting during Ga+-induced fracturing of top oxide shell on OTS-coated nanograss surface at vacuum level of 0.13−1.3 mPa, (c) corresponding SEM images showing the residual oxide film attached to the nanograss; advancing and receding contact angles of GaInSn on OTS-coated (d) micropillar and (e) nanograss superhydrophobic surfaces using the volume variation method in air at 98 kPa.

The presence of this film is demonstrated during Ga+ irradiation of GaInSn spread over a superhydrophobic nanograss surface (Figure 7b,c). Ion-induced fracturing of the top oxide shell leads to the pulling together of the GaInSn liquid. However, after retraction of the liquid metal a residual thin oxide film remains on the textured surface. SEM images in Figure 7c show that this thin film outlines the preirradiation perimeter of the drop and is in contact only with top of the topological features. Thus, the high adhesion of this oxide film comes from the oxide−solid contact line formed around perimeter of each nanopillar under the liquid metal layer (see schematic in Figure 7a). Dependent on its geometry, surface texture can either increase or decrease the liquid metal drop adhesion. In general, dewetting of Newtonian liquids is dominated by dynamics of 6873

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Figure 8. (a) Schematic of and (b) optical images of PDMS microgroove stamp contact printing of GaInSn in air at 98 kPa; the force applied to the liquid metal reservoir while pressing it against the PDMS stamp was varied so that the liquid metal (b) made contact only with microgroove tops, (c) penetrated partially, and (d) penetrated microgrooves fully; (e) optical images of the stamp surface showing nonuniform transfer of the liquid metal after full penetration of the grooves.

The first step of contact printing is transferring of GaInSn from a reservoir to tops of PDMS stamp features. The schematic in Figure 8a shows the possible complications introduced to this process by the two GaInSn wetting modes. Images in Figure 8b show that gently approaching a pool of GaInSn to a PDMS stamp with microridges did not fracture the oxide skin. This enabled easy retraction of the stamp, but it did not result in liquid metal transfer. Thus, we confirm that despite intrinsically high adhesion to PDMS, oxide adhesion to the elastomer can be dramatically reduced due to surface texture (following solid−solid adhesion mechanics specified by eq 2).46,48 In the current version of the contact printing process, only the force used to push the stamp can be increased to try to induce better transfer of the liquid metal. However, images in Figure 8c show that the PDMS grooves resisted flooding by the liquid metal with a moderate force increase. Compared to Figure 8b, more pronounced pinning of the GaInSn to the PMDS ridges occurred as the stamp was retracted. However, only minor, isolated transfer of the liquid metal was observed. In turn, increasing the applied force beyond this point caused flooding of the texture and strong, but spatially nonuniform adhesion (Figure 8d). This resulted in a significant but nonuniform transfer of GaInSn to the stamp (Figure 8e). The strong adhesion can be due to a combination of increasing old oxide−PDMS surface contact, formation of new oxide− PDMS interface, and bare GaInSn−PDMS contact (which likely leads to formation of new oxide because of high oxygen permeability of PDMS). Thus, transfer of liquid metal from the reservoir to the PDMS stamp is challenging because a light pressing force only establishes solid−solid contact with reduced adhesion (i.e., mode 1), but if the force is increased sufficiently to break the oxide, the PDMS grooves are flooded causing a

dramatic increase in the GaInSn adhesion (i.e., mode 2) and too much material is transferred. Lastly, we explore how wetting and adhesion of GaInSn impact maximal spatial resolution of an individual feature transfer. This can be achieved by three routes: (1) direct adhesive solid−solid contact, (2) fracturing followed by formation of new interface, and (3) scooping the old oxide. To transfer liquid metal through solely probe−oxide contact, the adhesion force must be greater than Pσs (P is the perimeter of the feature) so that the oxide skin yields. However, images in Figure 6d show that we did not observe oxide-fracturing during withdrawal of with the PDMS nanoprobe, implying that this approach will not work for a nanoscale feature transfer. The second approach, in which the nanoprobe pierces the oxide− shell and new oxide−probe interface is formed, does lead to fracturing of the oxide skin during pull-off (i.e., Figure 6e). However, in this case the detachment of the oxide from the nanoprobe is abrupt and repeatable material transfer using this approach proved to be difficult. Instead, we found that more GaInSn can be transferred through a scooping approach that involves piercing of the shell followed by lateral movement and retraction of the nanoprobe (Figure 9a). Using this approach, we were able to use liquid metal adhered to a PDMS-modified nanoprobe to imprint sub-200 nm diameter drops onto a silicon wafer (Figure 9b,c).

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CONCLUSIONS Because of the rapid oxidation of Ga-based liquid metals, an oxide shell forms over the liquid and dominates the drop− substrate interactions. Using nano- to macroscale experimentation, we demonstrated that, depending on the formation process and resulting morphology of the liquid metal−substrate interface, GaInSn adhesion can occur in two modes. First, if the 6874

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liquid metal drops due to presence of air in between the surface texture during the interface formation. However, drop depinning is only affected by the contact line around the peripheral features. Thus, as in the case of water,61 the liquid metal drop adhesion to textured surfaces depends on the pinning ratio of the contact line. We also demonstrated how the described adhesion modes impact contact printing of GaInSn patterns. We showed that transfer of liquid metal from a reservoir to the PDMS stamp is challenging because application of a light pressing force only establishes solid−solid contact with reduced adhesion (i.e., mode 1), but if the force is increased sufficiently to fracture the oxide skin, the PDMS grooves are flooded, causing dramatic increase in the GaInSn adhesion (i.e., mode 2) and too much material is transferred. However, we also showed that by “scooping up” GaInSn, we can repeatedly print individual sub200 nm liquid metal drops. Thus, nanocontact printing of GaInSn is feasible, but in order to scale up this nanofabrication technique, routes of mitigating nonuniformities in the GaInSn transfer to multiple features need to be identified. The new insight into the fundamental mechanisms governing GaInSn wetting and adhesion developed here may also help to improve the spatial resolution of other liquid metal fabrication techniques.

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Figure 9. SEM images of nanoscale contact printing using a PDMSmodified nanoprobe at vacuum level of 0.13−1.3 mPa: (a) GaInSn transfer through “scooping” the oxide and nanodot imprinting on silicon and (b, c) examples of fabricated sub-200 nm GaInSn dots.

METHODS

Materials. The GaInSn (68.5% Ga, 21.5% In, 10% Sn) was purchased from Rotometals. To chemically remove the oxide skin, concentrated hydrochloric acid (HCl) (37%, Sigma-Aldrich 258148) was used. Formation of concentrated HCl vapors were accelerated by adding 100 μL of HCl to a 50 mL beaker and heating it to 50 °C. GaInSn spread onto a silicon wafer was exposed to the vapors by holding the sample above the beaker for 5 s. The tungsten nanoprobes were used as purchased (Ted Pella, Omniprobe AutoProbe 460-105). Four substrates were chosen to examine wetting properties of GaInSn including precleaned glass (Thermo Scientific, 2950-001), PDMS (Dow Corning, Sylguard 182, silicone elastomer kit), tungsten foil (Sigma-Aldrich 267538 99.9%), and 4,5-difluoro-2,2-bis(trifluorormethyl)-1,3-dioxole, and PTFE polymer: tetrafluoroethylene (DBD) (DuPont, AF1600, 601S2100-6) and fluorinated solvent (3M, FC3283). Substrate and Sample Preparation. Glass and tungsten substrates were clean by rinsing with acetone and water. The PDMS substrate was prepared by mixing the elastomer base and curing agents in a 10:1 ratio. The mixture was spread onto a glass slide and then cured at 100 °C for 20 min. Polytetrafluoroethylene (PTFE) was prepared using the procedure described by Park et al.64 Briefly, the PTFE precursor (i.e., DBD) was diluted with a fluorinated solvent to a concentration of 6 wt %, spin-coated onto a glass slide, and then annealed at 120 °C for 1 h. The PDMS microridge patterns were formed from acrylonitrile−butadiene−styrene (ABS) masters with fixed line density of 2.5 and 10 mm−1. The ABS masters were 3D printed using Makerbot Replicator 2X. The nanoprobes were coated with PDMS through dip coating in the mixture described above. The PTFE coating of the nanoprobes was achieved using plasma-enhanced chemical vapor deposition (Blue Lantern plasma reactor and etcher, Integrated Surface Technologies, Inc.). The coating was achieved by filling the chamber with Water-Shield fluorocarbon precursor gas to a pressure of about 300 mTorr, a plasma power of 75 W, and a deposition time of about 15 min. GaInSn was manually spread over the flat samples using either a microsyringe or PTFE tweezers. See Supporting Information for atomic force microscopy characterization of surface roughness of the utilized “flat” surfaces. The fabrication and OTS coating procedures for the silicon micropillars and nanograss are described elsewhere.65 SEM-FIB Imaging and Manipulation Procedure. A majority of the in situ experiments were conducted using Nova 200 FIB-SEM from

oxide shell is not ruptured as it makes contact with the substrate, only a solid−solid contact is established. The natural nanoscale and microscale topology of the oxide surface results in minimal adhesion between the liquid metal and most solids, regardless of substrate’s surface energy or texture. Only viscoelastic and air permeable PDMS, which can readily fill the space in between the nano/microscale topological features of the oxide, had high adhesion to the unruptured oxide. However, the high adhesion of PDMS can be reduced through texturing of its surface. In the second adhesion mode, the formation of the GaInSn−substrate interface involves rupturing of the original oxide skin. Thus, a composite interface including contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide is formed. Liquid metal dewetting upon ion irradiation demonstrated that, with exception of PDMS, GaInSn adhesion in this mode is dominated by the intimate contact between newly formed oxide and the surface. We also showed that oxide regrowth is oxygen concentration dependent and occurs on a time scale ranging from seconds to hundreds of seconds at pressures of 0.13−1.3 mPa and 0.013 mPa, respectively. Quantifying the oxide regrowth time scale in higher pressures (e.g., lab environment) is challenging. However, a qualitative match in adhesion trends observed between the macroscale experiments performed at 98 kPa and nano- to microscale experiments performed in vacuum implies that, regardless of the pressure, the exposed bare liquid metal forms locally intimate interface with the substrate prior to full regrowth of the oxide shell. Thus, the high adhesion of GaInSn in this mode stems from the smoothness of the new oxide− substrate contact. We demonstrated that by varying the pinned contact line length using varied degrees of surface texturing, the adhesion of GaInSn can be either decreased or increased. On textured surfaces a new oxide film is formed underneath the 6875

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FEI with Ga+ column and field-emission electron gun. This instrument is equipped with an Omniprobe nanomanipulator and EDS detector. The SEM imaging was performed at 5 kV and 0.4−1.6 nA with dwell time between 0.3 and 3 μs. To remove the oxide skin, FIB at 30 kV and 0.92−3 nA with a dwell time of 0.3 μs and raster size of 512 × 471 or 1024 × 943 pixels was used. The corresponding images were automatically saved with a frequency of 0.2−1 Hz. For the nanoprobe “poking” and imprinting experiments, the samples were mounted at an angled of about 60° with respect to the SEM column. He+ Ion Microscope Milling and Imaging Procedure. The helium ion exposure experiments were performed using a Zeiss Orion helium ion microscope under the following conditions: 30 keV beam energy, 2.7 pA beam current, and 7 mm working distance. A 1 μm2 area containing a GaInSn nanodroplet was exposed to the beam scanning 256 by 256 pixels with a 1 μs dwell time per pixel and a total dose of 3 × 1018 ions/cm2. Images were acquired periodically throughout the exposure with a 1024 × 1024 pixel density and a 30 μs dwell time. The oxide skin regrowth experiments were carried out on a larger droplet surface where a 1 μm2 area was exposed to 6 × 1017 ions/cm2 dose, which resulted in the oxide skin removal. Subsequently, the beam was blanked for 17 min at a chamber pressure of 1.4 × 10−7 Torr to allow the oxide skin to regrow. Following the regrowth period, a 0.25 μm2 subarea was re-exposed and imaged to reveal the regrown oxide shell. Dynamic Contact Angle Measurements. The dynamic contact angles were measured using sample height and drop volume variation methods. In order to obtain a high magnification with a large depth of field and good lighting, the dynamic contact angle measurements were done using a custom 3D printed horizontal apparatus coupled with a high-magnification optical microscope (Zeiss Zoom Microscope Axial V16, 1.5× lens). To avoid effects of gravity, the GaInSn drop size was kept significantly below the capillary length (3.7 mm). The setup consisted of a 10 μL syringe (Hamilton 7635-01, 22 gauge blunt 9.52 mm needle), a 3D printed syringe holder with guides for the syringe plunger, a 90° aluminum substrate holder, and a micrometer X−Y−Z motion control stage (see schematic in Figure S5). Prior to contact angle measurements, the syringe was first cleaned with isopropanol and acetone and then loaded with GaInSn. The contact angles were measured using the angle tool plugin in ImageJ software. The reported values are averages and standard deviations of 6−10 measurements performed images from different stages of either the advancing or receding drop motion. Additional control experiments were carried out to ensure that major rupturing of the oxide shell did not occur during the height variation measurement; see discussion in Supporting Information and Figures S7−S9. Microcontact Printing Procedure. To image the imprinting process, both the PDMS microgroove stamp and uniformly spread GaInSn layer (“pool”) were mounted on glass slides in a vertical position. The two surfaces were brought in contact and pressed against each other using the 3D printed apparatus described in the previous section. The process was imaged using a Zeiss optical microscope.

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MIT. The authors also acknowledge the use of facilities in the John M. Cowley Center for High Resolution Electron Microscopy at ASU. E.M. and K.K. thank A. E. Vladar for access to the helium ion microscope at the National Institute of Standards and Technology in Gaithersburg, MD.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S10 and Movies 1−3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.R. acknowledges startup funding from Ira A. Fulton Schools of Engineering at ASU and insightful conversations with Prof. Marcus Herrmann from ASU and Dr. Sushant Anand from 6876

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