Roughness-Induced Chemical Heterogeneity Leads to Large

Article pubs.acs.org/JPCC

Roughness-Induced Chemical Heterogeneity Leads to Large Hydrophobicity in Wetting-Translucent Nanostructures Joseph Eugene Andrews, Yanbin Wang, Shayandev Sinha, Peter W. Chung, and Siddhartha Das* Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: Hydrophobic graphene marries the usefulness of graphene with the benefits of a hydrophobic surface leading to applications in oil−water separation, wettability switching, electromagnetic radiation shielding, fabrication of smart actuators and wearable electronics, tissue engineering, and many more. In current practices, such hydrophobicity in graphene or graphene-based materials or GBMs (e.g., graphene oxide) has been introduced by either chemical treatment or by enforcing graphene or GBM into a physical structure that triggers a hierarchical roughness enforcing the water drop into a Cassie−Baxter (CB) state. Here, through molecular dynamics (MD) simulations, we describe a new route to graphene-based hydrophobicity. We demonstrate that the wetting translucency property of graphene ensures that the surface-periodic features such as nanopillars can trigger a roughness-induced chemical heterogeneity. Therefore, an uneven coating of graphene on a hydrophilic surface (e.g., gold) will lead to a chemical heterogeneity across this graphene coating. This heterogeneity, in turn, enforces a wettability jump that ultimately results in contact line pinning and triggering of large hydrophobicity. Influence of this hydrophobicity-triggering mechanism can be so significant that one may witness a highly hydrophobic, fully wetted Wenzel state that is equally as hydrophobic as the corresponding CB state.

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INTRODUCTION Over the past decade, graphene has emerged as a most remarkable 2-D material that exhibits properties such as exceptionally large elastic modulus, high thermal and electrical conductivities, large thermal/chemical stabilities, and so on.1−3 Hydrophobic surfaces fabricated with graphene or graphenederived materials (such as graphene oxide) marry the benefits of these properties unique to graphene to the advantages intrinsic to superhydrophobic surfaces. Such surfaces have been used in applications such as oil−water separation,4,5 heavy-ion removal for water treatment,4 wettability switching,6,7 development of advanced fabrics,8 tissue engineering,9 electromagnetic shielding,10−12 fabrication of specialized optical and optoelectronic coating,13−16 transparent electrodes,17 smart actuators,14 and so on. The first approach of fabricating graphene-based hydrophobic and superhydrophobic surfaces consists of chemically modifying graphene or graphene oxide (e.g., by surface functionalization,13,17−21 light-induced chemical reaction,22,23 doping,24 etc.). The second approach involves triggering a physical alteration of graphene or graphene oxide (e.g., causing a crumpling of graphene nanosheets14 or preparing a grapheneoxide-aerogel by freeze-drying25), enforcing the water drop into hierarchical-roughness-induced superhydrophobic Cassie− Baxter (CB) configuration. Also, there have been extensive Molecular Dynamics (MD) simulation studies that have demonstrated how graphene nanopillars may lead to hydro© 2017 American Chemical Society

phobic states due to triggering of either Wenzel or CB configurations,26−29 the Wenzel state,30 for liquid drops on rough surfaces refers to the case when the drops are in a fully wetted condition (i.e., in full contact with the grooves), while in the CB state,31 the liquid drops are merely in contact with the tips of the roughness features trapping air pockets between the grooves (please see Figure 1e,f). MD simulations have established how water attains hydrophobic CB and Wenzel states on pillars made of graphene layers on a bulk graphite surface on account of increasing the number of water molecules.28 Such a phenomenon is equivalent to the development of advancing contact angle and occurs with the contact line remaining pinned on the nanopillars. If the number of water molecules is increased further, depinning occurs and the water drop spreads, thereby lowering the contact angle. Such a behavior is purely a hysteretic action of the water nanodrop on nanopillars. Other similar studies have reported transition and coexistence between the CB and Wenzel states on graphene nanopillars.27,29 For reasons related to difficulty in materials integration, bulk graphite substrates may be impractical to use or simply not available. In such situations, it would be highly desirable to enable these mechanisms using Received: March 8, 2017 Revised: April 12, 2017 Published: April 21, 2017 10010

DOI: 10.1021/acs.jpcc.7b02222 J. Phys. Chem. C 2017, 121, 10010−10017

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The Journal of Physical Chemistry C

Figure 1. (a−c) Demonstration of wetting translucency effect of graphene. Single layer of graphene on bare gold produces a contact angle (75°), see (b), that is the intermediate of that on bare gold (29°), see (a), and unsupported graphene monolayer (90°).34 However, on bilayer graphene supported on gold, the contact angle is similar to that on unsupported graphene monolayer and is barely affected by the underlying solid (goldsupported graphene monolayer), see (c). These figures are reproduced with permission from ref 34. Copyright 2016 PCCP Owner Societies. (d) Roughness-induced chemical heterogeneity and hence wettability gradient on graphene-nanostructure-coated solid, which in turns leads to contactline-pinning-driven hydrophobicity. Schematic of drops in Wenzel, see (e), and CB, see (f), states on nanopillars of nonwetting-translucent solids.

hydrophilic substrate materials. The wetting translucency of graphene may afford this capability. The wetting translucency of graphene signifies that the wetting characteristic of a particular graphene layer depends on the wettability (with respect to the original wettability of this graphene layer) of the underlying substrate.32−36 This is explained by the Molecular Dynamics (MD) simulation results of our previous paper34 and also shown in Figure 1a−c. For example, for a monolayer of graphene supported on bare gold, the water drop contact angle is around 75°, which is between the water drop contact angles on unsupported graphene (∼90°)34 and bare gold (∼29°).34 Of course, this water-drop contact angle on bare gold is possibly due to the adsorption of some contaminants, given that one would typically expect a zero water-drop contact angle on an absolutely pure metal surface.37 The fact that we witness a water-drop contact angle on gold-supported graphene monolayer that is between the contact angle values on unsupported graphene and bare gold stems from the large van der Waals (vdW) attraction between gold and water (quantified by a large gold-water Hamaker constant of interaction determined to be in the range of 25 × 10−20−40 × 10−20 J),38−40 which ensures that the relatively weak vdW interaction between graphene and water (quantified by a relatively small graphite-water Hamaker constant of interaction of 9 × 10−20 J)33 cannot solely decide the water drop contact angle despite the gold being “one graphene layer” away from the water drop. On the contrary, for a water drop on bilayer graphene the contact angle is very similar to that of unsupported monolayer graphene; this stems from the fact that the contact angle for the supporting solid below (for this case, the supporting solid is the gold-supported graphene monolayer) is 75°, which is not too much deviated from 90°. The present paper uses the wetting translucency of graphene to demonstrate how controlled screening using graphene nanostructures on a graphene monolayer sitting on a hydrophilic surface can lead to chemical heterogeneities that eventually triggers large hydrophobic behaviors.

Here we employ Molecular Dynamics (MD) simulations to study the wetting of gold surfaces covered with graphene nanostructures (see Figure 1d). We show that the wetting translucency property of graphene together with the surface coverage by partial sheets of additional layers of graphene (roughness) induces a chemical heterogeneity that permits pinning of the edges of water droplets leading to extraordinary contact angles. The idea is illustrated in the left figure in Figure 1d. Consider graphene roughness (or graphene nanopillar) equivalent to one graphene layer on a gold-supported graphene monolayer. Therefore, at the location of the roughness there are two layers of graphene on gold, whereas at the location where there is no roughness, there is just one layer of graphene on gold. Wetting translucency effect would imply, therefore, that the water contact angle at location of the graphene nanopillar (or roughness) is ∼90°, whereas the water contact angle ∼75° at the location where there is no roughness (see the above paragraph). Thus, most remarkably, coating a substrate with a chemically homogeneous structure (i.e., the wetting translucent material graphene) and then roughening it using nanopillars introduces a chemical heterogeneity that uniquely enables large hydrophobic behavior. We are not aware of any previous example where roughness introduces such a chemical heterogeneity on a rough substrate of a single material, and we ascribe such a phenomenon to the wetting translucency property of graphene. Such chemical heterogeneity, of course, is equivalent to the presence of a wettability gradient. Finally, we establish that this wettability gradient ensures the attainment of hydrophobic fully wetted states due to the occurrence of highly stable contact line pinning, this is the central finding of the present paper. Such chemical heterogeneity-mediated contact line pinning has been previously reported for chemically heterogeneous surfaces with periodic patches of two distinct materials;41 however, this present case is possibly the first example where a single material triggers chemical heterogeneity across it (and hence contact line pinning and hydrophobicity) due to the interplay of 10011


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Figure 2. Water-drop contact angles on nanopillar graphene layers (with n denoting the number of layers) supported on gold-supported graphene monolayer for n = 1−3. The results are shown for those number of water molecules (N) that ensure the attainment of advancing contact angles for each of the three systems (i.e., N = 5500, 4500, and 4500 for n = 1, 2, and 3, respectively; see ref 28 for the definition of an advancing contact angle of an MD-simulated nanodrop on a pillared surface).

contact angles. Following ref 28, we define the receding and the advancing contact angles, corresponding to the wetting of a given number of pillars, as the angle made by the drop containing N = Nmin and N = Nmax number of water molecules, respectively. Nmin and Nmax are defined as follows: for N < Nmin, the water nanodrop at equilibrium fails to wet the chosen number of pillars, while for N > Nmax, the water nanodrop at equilibrium wets more number of pillars than the chosen number. For the present study, this chosen number of pillars are 4, 3, and 3 for n = 1, 2, and 3 (please see the captions of Figures 3−5).

roughness and wetting translucency. In this context, it is useful to distinguish our findings from the classical situation of roughness/nanostructure-induced generation of hydrophobicity in standard nonwetting-translucent solids. For such cases, the drop may attain Wenzel or CB states (see Figure 1e,f) enforcing large hydrophobic contact angles. However, the generation of these corresponding hydrophobic states is solely due to the roughness effect with no role of any chemical heterogeneity. On the contrary, our study establishes how the layered nature of wetting-translucent-solid like graphene enforces roughness/nanostructure induced chemical heterogeneity on even hydrophilic substrates, enforcing large hydrophobic angles on such substrates.

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MOLECULAR DYNAMICS SIMULATIONS The molecular dynamics simulations presented in this work largely follow from our previous study of graphene wetting dynamics.34 Here, we provide a summary of the main simulation details and describe any differences from earlier simulations. All simulations are carried out in the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)42 software package. The Open Visualization Tool (OVITO)43 is used to generate the simulation renders, and the postprocessing is performed in MATLAB. As detailed in our previous study,34 we employ a quasi-2D simulation geometry to eliminate the curvature-induced line tension at the three-phase contact line. This effect arises as a consequence of the excess free energy per unit length associated with the formation of a contact line where three phases meet.44 For 3D, spherical-cap drops of nanometer scale, the line tension is responsible for a deviation of the observed contact angle (θ) from the contact angle predicted by Young’s equation (θ∞), as given by44 τ cos(θ) = cos(θ∞) − γR

Figure 3. Snapshots of MD simulations showing the dynamics of wetting of water nanodrop on monolayer graphene nanopillar on goldsupported graphene monolayer. In the inset of the snapshot for t = 4 ps, we show the different forces experienced by the contact line, while in the insets of the snapshot for t = 83 ps (representing a pinned contact line), we show only the forces that cause the pinning. In this figure corresponding to the case of the pinned contact line (t = 83 ps), the yellow arrow denotes the pinning force, the blue arrow denotes the force from direct contact with graphene nanopillar, and the green arrow denotes the force from indirect substrate contact (i.e., gap between pillars). In terms of the magnitude of the force that they represent, blue arrow > green arrow, ensuring a net positive value of the yellow arrow leading to a pinning behavior. Simulations are carried out for 5500 water molecules, which ensure attainment of an advancing contact angle (we consider wetting of four adjacent pillars).

where τ is the line tension, γ is the liquid−vapor surface tension, and R is the radius of the contact line. By implementing the quasi-2D, cylindrical drop configuration, the contact line becomes straight, thereby eliminating the system-size-dependence of the contact angle.33,44 Water is modeled by the extended simple point charge (SPC/E) model.45 In this model, water molecules interact with each other via a 12−6 Lennard-Jones (LJ) potential as well as Columbic electrostatic interactions. The LJ site is localized on the oxygen atom (with LJ parameters εOO = 0.650 kJ/mol, σOO = 0.3166 nm), while both the oxygen atom and the hydrogen atoms carry charges (qO = −0.8476e and qH = 0.4238e). For the present study, for each geometry we carry out the simulations by varying the number of water molecules in order to ensure that our simulations yield both the advancing and the receding

Water−graphene interactions are modeled via a C−O LJ potential. The LJ parameters come from the model of Werder et al.46 and are εCO = 0.392 kJ/mol and σCO = 3.19 Å. The hydrophilic substrate is taken as gold where the Au(111) surface is aligned with the graphene lattice, which is stretched by ∼1.5%. Gold−water interactions are modeled via the Au−O LJ potential of Merabia et al.,47 which has parameters εAu−O = 2.469 kJ/mol and σAu−O = 3.6 Å. All LJ interactions have a cutoff radius of 10 Å. We refer the reader to our previous study for a more detailed description of these models.34 Substrate atoms are held fixed at their lattice positions throughout the simulations. This simplification is employed in similar MD studies of drop wetting phenomena33,46 and has been shown to 10012


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Figure 4. Snapshots of MD simulations showing the dynamics of wetting of water nanodrop on bilayer graphene nanopillar on gold-supported graphene monolayer. We provide insets to illustrate the forces that cause pinning. Same color convention for the arrows, as in Figure 3, is used. Simulations are carried out for 4500 water molecules, which ensure attainment of an advancing contact angle (we consider wetting of three adjacent pillars).

formed, equilibrated drop. In the data collection phase, this equilibrated drop is brought to within ∼3 Å of the substrate. The system is then evolved forward in time again for 500 or 1000 ps, allowing the drop to spread on the surface. Choices of the dimensions of the nanopillars, size of the time step, and the total simulation time have been motivated by previous similar studies.26−28 To calculate equilibrium contact angles, atomic position coordinate data is imported into MATLAB. The data is then sorted into 2 Å by 2 Å square bins in the x−z plane. This allows markers to be placed along the edge of the drop, identifying the drop profile. A circle is then fit to these markers using a leastsquares regression. The line tangent to the circle where it intersects with the horizontal upper surface of the substrate is then calculated analytically, and the contact angle is easily determined from this tangent line. This procedure is used to calculate all equilibrium contact angles reported in this work.

Figure 5. Snapshots of MD simulations showing the dynamics of wetting of water nanodrop on trilayer graphene nanopillar on goldsupported graphene monolayer. We provide insets to illustrate the forces that cause pinning. Same color convention for the arrows, as in Figure 3, is used. Simulations are carried out for 4500 water molecules, which ensure attainment of an advancing contact angle (we consider wetting of three adjacent pillars).

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RESULTS AND DISCUSSION MD-Simulated Equilibrium Drop Profiles: RoughnessInduced Chemical Heterogeneity Leading to Hydrophobicity. In Figure 2, we show the MD-simulated equilibrium drop profiles for 2-D drops on graphene nanopillars (number of layer of nanopillars are identified as n) on gold-supported graphene monolayer. These equilibrium angles are advancing equilibrium contact angles. We also obtain the equilibrium receding contact angles for the same geometries. We have followed the approach of ref 28. in order to obtain the advancing and receding contact angles for the present case of nanodrop dynamics on a nanopillared substrate. The difference between the advancing and the receding values is the contact angle hysteresis summarized in the Table 1 (more details on how to obtain the advancing and the receding contact angles for different n are provided in the Molecular Dynamics Simulations).

substantially reduce computational expense without significantly affecting observed contact angles. Nanopillars are formed by first creating bulk graphene having the desired number of layers and then removing C atoms from specified locations, leaving the nanopillars behind. Both the width and spacing between the nanopillars is 13 Å. This value is chosen as it is greater than the molecular size of water but smaller than the water drop size (initial drop radius is ∼40 Å). Additionally, this value ensures that all nanopillar edges have the “zig-zag” configuration, which is known to be one of the stable edge configurations of a graphene sheet.48 Simulations are carried out in two phases−drop equilibration and data collection. Both phases are performed in the NVT ensemble with a temperature control via the Nosé−Hoover thermostat49 and a time step of 1 fs. For the drop equilibration phase, water molecules are first initialized in an ordered, boxlike configuration far from the substrate. The system temperature is then increased from 1 to 300 K in 50 K increments. Intermediate temperatures are held for 50 ps intervals and the final 300 K is held for 200 ps. This procedure produces a well-

Table 1. Contact Angle Hysteresis for Different n

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n

receding angle (°)

advancing angle (°)

hysteresis (°)

1 2 3

120 121 128

130 138 138

10 17 10


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simultaneously at t = 83 ps on the nanopillar. In the inset of Figure 3 showing the snapshot corresponding to t = 83 ps, we show the direction of the net pinning force (as well as the attractive forces that cause this pinning force), while the net force on the contact line is always zero. Of course, at the nanopillar edge (i.e., where pinning occurs), the contact line is under the attractive force from both the nanopillar (shown in blue) and from the gold-supported monolayer graphene substrate (shown in green). The attractive force from the nanopillar is more here since the underlying graphene substrate is at a distance of “one graphene layer” from the contact line, while there is a much less difference in the hydrophilicity between the gold-supported mono and bilayer graphene. This justifies why the pinning force (shown in yellow) is opposite to the spreading directions on both the contact lines, eventually ensuring the pinning and stopping of the contact line. We carry out simulations for much longer time (t ∼ 1000 ps) and witness no depinning post t = 83 ps. The capillary and the viscous time

The details of the simulations as well as the choices of the simulation geometry and parameters are also provided in the Molecular Dynamics Simulations. The equilibrium contact angles are denoted below each figure. We clearly witness the attainment of large hydrophobic contact angles. In fact, for n = 1 and 2, such large contact angles are witnessed even for the fully wetted Wenzel states, while the CB state is attained for n = 3. Most intriguingly, we witness that the contact angle in the CB state is very similar to that in the Wenzel state (for n = 2). The pinning mechanism (discussed in detail below) introduced by the roughness-induced-chemical heterogeneities, caused by the wetting translucency effect of graphene, leads to this large hydrophobic behavior. Attainment of such large hydrophobic contact angles in graphene nanostructure covered gold surfaces is the central result of this study. Drop Dynamics on Monolayer Graphene Nanopillar on Gold-Supported Monolayer Graphene: Hydrophobic Wenzel State. In Figure 3, we show the dynamics of a water nanodrop on monolayer graphene nanopillar on goldsupported, graphene monolayer (also see Supporting Information, Video S1). We consider those many water molecules (N = 5500) that ensure the attainment of advancing contact angle (see ref 28 for the definition). Snapshot at t = 4 ps shows the onset of the spreading process. Studying the forces on the contact line during this spreading process is central to pinpointing how pinning sets in and stalls the contact line. Of course, force can be exerted on a mass and not on a line. Therefore, by “force on the contact line”, one implies the force on a mass of liquid enclosed by the contact line,50 as shown in the inset of the snapshot for t = 4 ps. This mass element in the vicinity of the three phase contact line will be subjected to four different forces, as shown in the inset. The forces FLV and FSL are the forces at the liquid−vapor and solid−liquid interfaces (and not on the three phase contact line). As explained by Marchand et al.,50 these two forces are primarily associated with the anisotropy associated with the introduction of the second phase (solid phase or vapor phase) in the bulk liquid. For a drop on a homogeneous solid, the net horizontal attractive force on the contact line from the solid is zero. Therefore, it is the difference between FLV and FSL that causes the spreading. Nature of the attractive force from the solid changes the moment there is a chemical or structural heterogeneity. Chemical heterogeneity exists in the present case due to the fact that the drop demonstrates different wettability between gold-supported graphene monolayer (at the location where there is no nanopillar or no roughness) and gold-supported graphene bilayer (at the location where there is a nanopillar and hence roughness). Structural heterogeneity may give rise to diffusion barriers, such as the Ehrlich-Schwoebel barrier,51−53 which can contribute to the forces experienced by molecules that are pinned at entrant corners and terraces. In the present study, the contact line is always under unequal attractive forces (hence, the unequal horizontal component of these attractive forces) from the graphene nanopillar and the underlying goldsupported graphene monolayer substrate (in the free body in the inset corresponding to the snapshot for t = 4 ps, we also identify these forces, denoted as Fa,BL and Fa,ML, representing the attraction forces from gold-supported bilayer and monolayer graphene, respectively). For the drop configuration, where this resultant horizontal force becomes equal and opposite to the horizontal spreading forces resulting from FLV and FSL, the contact line will get pinned. Here we observe that both the left and the right contact lines get pinned

scales for the drop are τc = ρR3/γ ≈ 40 picoseconds and τv = ρR2/η ≈ 25 ps (using radius R = 5 nm, density ρ = 1000 kg/ m3, surface tension γ = 0.07 N/m, and dynamic viscosity η = 0.001 Pa·s); therefore, we run our simulations for time (1000 ps) that is much larger than these two time scales that typically dictate the drop spreading dynamics.34,54 Further, previous similar studies on drop dynamics on graphene layers have considered equilibrium at such total simulation times (1000 ps).27,28 Consequently, we can safely assume that there will be no further change in the drop equilibrium behavior allowing us to infer that the drop equilibrates with such pinned contact lines, enforcing a large contact angle of ∼130°, with the drop being in the Wenzel state. Finally, we also carry out a separate set of simulations (results not shown) for N = 3000 water molecules that ensure the attainment of receding contact angle (see ref 28 for the definition); qualitatively, the dynamics are similar to that described in Figure 3, although the attainment of the pinned state occurs slightly faster (t ∼ 60 ps) given that N is smaller. Drop Dynamics Bilayer Graphene Nanopillar on GoldSupported Monolayer Graphene: Hydrophobic Wenzel State. In Figure 4 we show the snapshots from the simulation of the drop dynamics on bilayer graphene nanopillars on goldsupported graphene monolayer (also see Supporting Information, Video S2). The simulations are performed using 4500 water molecules to ensure attainment of an advancing contact angle. Here too, very much like Figure 3, we find that both contact lines are pinned almost simultaneously at approximately t = 30 ps and, as expected, on the graphene nanopillars (for reasons already described above). However, a key difference with respect to the case depicted in Figure 3 is how fast the pinning occurs in the present case (pinning in the case depicted in Figure 3 sets in at t = 83 ps). This obviously points to a much larger pinning force, which stalls a much larger spreading force. This spreading force is larger as it corresponds to a larger instantaneous contact angle and hence a larger difference between the instantaneous and the equilibrium contact angles. In the present case, the pinning force results from the difference of the unequal attractive forces (pinning force is the difference of the horizontal component of these attractive forces) from the graphene nanopillar and the gold-supported graphene monolayer. The attractive force is more from the nanopillars (shown by blue arrows) as compared to the force from the goldsupported graphene monolayer (shown by green arrows), 10014



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CONCLUSIONS Large hydrophobic behavior through controlled arrangements of graphene provides a vast new window of exciting applications in the ever-expanding uses of graphene. This paper reports a means by which hydrophobic surfaces with large contact angles can be developed by using a surface-arrayed graphene nanopillars. The fascinating graphene property of wetting translucency interplays with the roughness effect introduced by these nanopillars to trigger a roughness-induced chemical heterogeneity. In fact, one can vary this nonuniformity and, most remarkably, witness a variation in the chemical heterogeneity. This chemical heterogeneity ensures a wettability-jump-induced stable contact line pinning, which is responsible for the development of this hydrophobic behavior. Most importantly, this is a remarkable example where a particular physical property of graphene (wetting translucency) is utilized for the first time to trigger graphene-based hydrophobicity in both Wenzel and CB states. We anticipate that through our study we bring to the fore a new perspective of hydrophobization of 2-D materials and future research may explore similar properties and behavior of nongraphene 2-D materials like hexagonal boron nitride, molybdenum disulfide, and so on.

ensuring the generation of the pinning force (shown by yellow arrows) in a direction that always opposes the spreading force. Here the graphene nanopillars are “two graphene layers” closer to the drop contact line as compared to the gold-supported graphene monolayer. Consequently, the difference between the attractive forces is larger in the present case as compared to the case depicted in Figure 3 (where the graphene nanopillars are “one graphene layer” closer to the drop contact line as compared to the gold-supported graphene monolayer), ensuring a larger pinning force for the present case. However, unlike the previous case (case depicted in Figure 3), the spreading does not completely stop after this early pinning event. After some finite duration post this pinning, the right contact line via molecular motion first releases some molecules to wet the adjacent underlying gold-supported graphene monolayer. This, in turn, reduces the pinning force, thereby allowing the contact line to become depinned and wet the adjacent pillar (see the snapshots corresponding to t = 70 and 95 ps). However, this depinned contact line again becomes pinned at t = 180 ps. We carry out simulations for much longer time but witness no further depinning. Hence, the drop equilibrates with both of the contact lines pinned enforcing a large hydrophobic contact angle (∼138°); however, unlike the monolayer graphene nanopillar on underlying gold-supported graphene monolayer, the contact line motion occurs via the pinning-depinning mechanism. Of course, here too, this entire pinning process is occurring on account of the graphene wetting-translucency-induced wettability jump across the nanopillars for reasons already described for the previous case. Figures 3 and 4 establish that the graphene nanopillars indeed provide a new route to achieve remarkable hydrophobicity with the drop being in the fully wetted state caused by the wettability-jump-induced pinning effects. Trilayer Graphene Nanopillar on Gold-Supported Monolayer Graphene: Hydrophobic CB State. In Figure 5, we show the snapshots from the simulation of the drop for the case where the drop attains CB state at equilibrium (also see Supporting Information, Video S3): we consider three layers of graphene nanopillar on gold-supported graphene monolayer. The simulations are carried for 4500 water molecules, which ensure attainment of an advancing contact angle. Here too, the right left contact line gets pinned after the drop hits the pillars (case shown here for t = 13 ps). Here the pinning sets in even earlier than the cases studied in Figures 3 and 4, since the pinning force is larger resulting from the difference of the attractive forces from nanopillars and the goldsupported graphene monolayer with the nanopillars now being “three graphene layers” closer to the drop contact line. On the other hand, the right contact line is not pinned, but rather slides along the pillar (a snapshot is shown for t = 75 ps); note that unlike the previous cases (shown in Figures 3 and 4), this sliding is occurring without wetting the pillar except its top, thereby ensuring that the drop maintains its CB configuration throughout. Obviously, the gold-supported graphene monolayer being “three graphene layers” away from the drop imparts a very weak attractive force on the contact line enforcing this strictly CB state. Eventually, this left contact line also gets pinned (see t = 99 ps). This is the equilibrium configuration of the drop with both the contact lines pinned. Simulation beyond 99 ps reveals no further depinning, which eventually results in CB contact angle (∼138°), which is comparable (very similar) to the angle witnessed even for n = 2 (Wenzel state).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02222. Video S1 showing the dynamics of nanodrop spreading on nanopillared gold-supported graphene (AVI). Video S2 showing the dynamics of nanodrop spreading on nanopillared gold-supported graphene (AVI). Video S3 showing the dynamics of nanodrop spreading on nanopillared gold-supported graphene (AVI).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shayandev Sinha: 0000-0001-9476-8974 Siddhartha Das: 0000-0002-1705-721X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support received by Mr. Yanbin Wang from the CECD Fellows program. REFERENCES

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