Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2022 Dec 14;38(51):16073–16083. doi: 10.1021/acs.langmuir.2c02694

Dynamic Wetting of Ionic Liquid Drops on Hydrophobic Microstructures

Ahmed Aldhaleai 1, Peichun Amy Tsai 1,*
PMCID: PMC9799069  PMID: 36516403

Abstract

graphic file with name la2c02694_0009.jpg

Ionic liquids (ILs)—salts in a liquid state—play a crucial role in various applications, such as green solvents for chemical synthesis and catalysis, lubricants, especially for micro- and nanoelectromechanical systems, and electrolytes in solar cells. These applications critically rely on unique or tunable bulk properties of ionic liquids, such as viscosity, density, and surface tension. Furthermore, their interactions with different solid surfaces of various roughness and structures may uphold other promising applications, such as combustion, cooling, and coating. However, only a few systematic studies of IL wetting and interactions with solid surfaces exist. Here, we experimentally and theoretically investigate the dynamic wetting and contact angles (CA) of water and three kinds of ionic liquid droplets on hydrophobic microstructures of surface roughness (r = 2.61) and packing fraction (ϕ = 0.47) formed by micropillars arranged in a periodic pattern. The results show that, except for water, higher-viscosity ionic liquids have greater advancing and receding contact angles with increasing contact line velocity. Water drops initially form a gas-trapping, CB wetting state, whereas all three ionic liquid drops are in a Wenzel wetting state, where liquids penetrate and completely wet the microstructures. We find that an existing model comparing the global surface energies between a CB and a Wenzel state agrees well with the observed wetting states. In addition, a molecular dynamic model well predicts the experimental data and is used to explain the observed dynamic wetting for the ILs and superhydrophobic substrate. Our results further show that energy dissipation occurs more significantly in the three-phase contact line region than in the liquid bulk.

Introduction

Sessile droplet wetting on solid substrates is an omnipresent phenomenon in numerous natural and industrial processes. Some common examples include inkjet or electrohydrodynamic jet printing,1 droplet-based microfluidics,2,3 membrane technology,4,5 and surface coating.6 A liquid drop either spreads as a thin film or partially wets on a solid substrate when deposited. In the former case, the liquid penetrates the surface cavities as a so-called homogeneous Wenzel (W) wetting state.7 In the latter, a three-phase contact line exists as the drop sits on the top of the surface structures with gas pockets trapped underneath in the so-called Cassie–Baxter (CB) or “Fakir” wetting state.8,9

The fundamentals of static wetting have been systematically studied previously and are relatively well understood.10−12 First, Thomas Young13 postulated that when a small drop is put in an ideally rigid, chemically inert, and homogeneous flat solid surface, its static or equilibrium contact angle, θe, is related to the interfacial tensions for the solid–vapor, solid–liquid, and liquid–vapor interfaces. In contrast to the flat surface case, real solid surfaces are not completely flat or homogeneous, and the static contact angle here deviates from the equilibrium one. Instead, the static contact angle falls between the advancing and the receding contact angles, denoted as θA and θR, respectively.10−12

The dynamic wetting of a partially wetting liquid resulting from the movement of the three-phase contact line, however, has been investigated to a lesser extent and remains only partially understood.14 The contact line movement in the dynamic wetting could result from the unbalanced surface tension exerted on the contact line15−17 or by applying an external force on the drop.18 The dynamic contact angle measurements due to a moving contact line depend on the contact line velocity. Previous studies have proposed two models, each of them considering different dissipation regimes of the excess free energy during the advancing or receding contact line. The first model is called the viscous or hydrodynamic dissipation (HD) model19,20 and ascribes energy dissipation to viscous hydrodynamic friction in the bulk liquid in motion. Second, the molecular kinetic (MK)21,22 model attributes energy dissipation to the molecular friction at the three-phase contact line. While each model discusses only one dissipation regime, in reality, the contact line spreading process can be dominated by both dissipation mechanisms.23−25 Recently, both models have been shown to describe the dynamic wetting of ionic liquids14 on hydrophilic solid surfaces, such as gold and glass,18 or hydrophobic glass slides treated with Teflon AF1600.22 However, the physical parameters of both models have not been examined for ionic liquid drops on hydrophobic microstructures of regular pillars, yielding superhydrophobic surfaces for water droplets, nor have they been correlated to the physicochemical properties of the wetting ionic liquid.

Ionic liquids (ILs) are a subset of organic molten salts with low melting points (Tm) below 373 K. The chemical structure of ionic liquids comprises two main parts: long-chain or shorter-chain organic cations and inorganic anions.26,27 Due to their distinctive characteristics, including low vapor pressure,18,22 high thermal electrochemical stability,18,26 relatively high electrical conductivity,28 nonvolatile and nonflammable liquids,29 and high liquid viscosity range,30 ionic liquids have attracted significant attention concerning practical industrial applications. For example, ionic liquids are utilized as green solvents for chemical synthesis and catalysis,31−33 lubricants,34 especially for micro- and nanoelectromechanical systems (MEMS and NEMS) since the lubricants for MEMS and NEMS are required to be electrically conductive,18 fuel processing and production,35 in electrowetting on dielectric (EWOD)-based microfluidic devices,36 and in electrolytes in solar cells. Therefore, the study of IL drop wetting and interaction with solid surfaces is crucial for the wide range of applications discussed.37,38

To date, the spreading and wetting properties of ionic liquids on solid surfaces have been experimentally reported, including glass slides treated with Teflon AF1600;22 gold and glass;18 polar and nonpolar organic monolayers supported on Si wafers;39 AISI 52100 steel; CrN, TiN, and ZrN substrates;40 muscovite mica discs;41 coal samples;29 steel AISI 52100; aluminum 6082 T6; tungsten carbide; 6% cobalt; cast iron BS1452 grade 240 and bronze PB1 BS 1400 discs;30 and hydrophobic (PTFE and UHMWPE) and hydrophilic (BK7 glass),42 superhydrophobic (OTFE, SiPFAPosts, SiMeSiCl3, SiMe3SiCl/SiCl4), and smooth (SiPFA, SiMe2SiCl2), PET surfaces.43 Most of the studies focus on hydrophilic surfaces, whereas ionic liquid wetting on hydrophobic surfaces has rarely been investigated.22,42,43 Furthermore, the effect of the hydrophobic microstructures of regular roughness (e.g., pillars) on the occurring wetting, for example, CB or W, states of ionic liquids has not been systematically studied.

In this study, we investigate the wetting state and the contact angle (CA) of an IL droplet on a superhydrophobic substrate formed by regular hydrophobic micropillars. The dynamic wetting states and CA measurements are performed for water and three ionic liquids. We compare our experimental results with two theoretical models that describe different energy dissipation mechanisms during drop wetting and spreading. Our results show energy dissipates in bulk liquid and at the three-phase contact line. Finally, the molecular kinetic model parameters were extracted and related to the molecular dimensions of the ionic liquids. We find that the molecular dynamic model predicts the experimental data well and is used to elucidate the observed dynamic wetting for these liquids and substrates. These results show that the contribution of the three-phase contact line is more significant for the energy dissipation than the bulk liquid for the ionic liquids and surfaces studied.

Experimental Section

Materials

Three ionic liquids (ILs) used in the experiments: 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4), 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4), and 1-Hexyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (HMIM NTf2) were supplied from Sigma-Aldrich, Canada. The first two ionic liquids consist of an imidazolium-based cation, whose alkyl side chain length was systematically varied to be either BMIM or EMIM, while the anion side was fixed to be BF4 for a fair comparison. BMIM BF4 and EMIM BF4 were then compared with another IL of a different anion side chain HMIM NTf2 to systematically investigate the effect of different cation and anion chain lengths on the static and dynamic wetting on hydrophobic microstructures. The IL’s physical properties are given in Table 1, while the chemical structures and a microscopic image of the hydrophobic microstructures are shown in Figure 1.

Table 1. Key Physical Properties of Water and ILsa.

liquid empirical formula MW ρ γLG γLGL μ d
    (g/mol) (g/cm3) (mN/m) (mN/m) (mPa·s) (nm)
Milli-Q water H2O 18.02 0.99 72.0 ± 1.1 72.5 ± 0.1 0.95 0.31
EMIM BF4 C6H11BF4N2 197.97 1.29 53.3 ± 1.1 53.9 37 ± 2 0.66
HMIM NTf2 C12H19F6N3O4S2 447.42 1.38 31.8 ± 1.0 31.3 80 ± 3 0.81
BMIM BF4 C8H15BF4N2 226.02 1.21 42.7 ± 0.7 43.8 112 ± 4 0.68
Open in a new tab
a

MW is the molecular weight (obtained from Sigma-Aldrich); ρ is the liquid density (obtained from Sigma Aldrich); γLG is the liquid–air interfacial tension, measured using pendant drop method; γLGL is obtained from the literature;22,39 μ is the viscosity, measured using an Anton Paar MCR 302 rheometer; and Inline graphic(18) is the molecular diameter of an ion pair (assuming cubic packing)of the studied liquids. Here, NA is Avogadro’s constant.

Figure 1.

Figure 1

Open in a new tab

Chemical structures of the ionic liquids used: (a) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4), (b) 1-hexyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (HMIM NTf2), and (c) 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4). (d) Microscopic image of the hydrophobic microstructures formed by cylindrical pillars placed in a hexagonal pattern, yielding surface roughness, r = 2.61, and packing fraction, ϕ = 0.47.

All of the ILs had a purity ≥98%. Since contaminants, especially water vapor, may impact their wetting properties, the ILs were vacuum dried using a vacuum pump (MZ2CNT, Vacuubrand, Germany) at an ultimate vacuum of around 7 mbar prior to the experiments. The water content in the ILs was measured before and after exposure to the surrounding atmosphere for ≈10 min, roughly our experimental period, using a colorimetric Karl Fischer titrator (Mitsubishi CA-02 Moisture Meter). In brief, each sample is injected into the electrolysis cell, which contains a pyridine-methanol mixture, iodide (I–), and sulfur dioxide. Iodine is produced at the anode by electrolysis of iodide, which reacts stoichiometrically with the water in the sample. Each mg of water is equivalent to 10.71 coulombs of electricity. The moisture meter subsequently determines the amount of water in the sample (in mg) according to the amount of iodide electrolyzed. The Karl Fischer titrator measurements show the initial (afterward) water content to be 680 ± 9 ppm (708 ± 4 ppm), 1010 ± 120 ppm (1063 ± 125 ppm), and 902 ± 136 ppm (911 ± 141 ppm) for BMIM BF4, EMIM BF4, and HMIM NTf2, respectively.

Polydimethylsiloxane (PDMS) (Sylgard184, Part A) with a curing agent (methylhydrosiloxane with Pt catalyst, Part B) was provided by Dow Corning Corporation. Microscope glass slides (GS, 26 mm × 76 mm × 1 mm) were purchased from Bio Nuclear Diagnostics, Inc. and used as transparent support for the microstructured PDMS. Acetone and ethanol were obtained from Sigma-Aldrich, Canada, and used to clean the glass slides before use. All of the reagents were used as received without any further treatment. The water used in the wetting experiment was ultra-pure Milli-Q water, PURELAB Ultra (resistivity: 18.2 MΩ·cm).

Sample Preparation

To determine an accurate wetting state on the studied surface, transparent polydimethylsiloxane (PDMS) microstructures were prepared using a replica molding process.44 In brief, the hydrophobic microtextures are composed of a hexagonal array of parallel cylindrical pillars (of diameter D = 5.5 μm, height H = 4.8 μm, and pitch P = 7.7 μm) (see Figure 1). The packing fraction, ϕ, is the ratio of the liquid–solid surface area to the total areas and calculated with Inline graphic. The surface roughness (r) is the ratio of the total to the projected surface area on a 2D plane, that is, Inline graphic. The glass slide used as the solid support substrate was ultrasonically cleaned with acetone and ethanol, rinsed with Milli-Q water several times, and finally dried using a nitrogen gun. The as-prepared hydrophobic PDMS microstructures were then mounted on the clean glass slide prior to each experiment.

Surface Tension Measurements

The surface tension of water and the ionic liquids were measured using the pendant drop method at room temperature (22 °C).45 For each liquid, four to five drops were gently released using a 10 μL micropipette, and the side-view snapshots were captured using a CCD camera (Thorlabs DCC3240C) coupled with a long-range magnifying lens (Navitar 12×). The surface tension was then determined by fitting the profile of the formed drop with the Young–Laplace equation using ImageJ software, with γLV used as a fitting parameter.46 The surface tension values of water and ionic liquids measured in this study and reported in the literature are listed in Table 1.

Viscosity Measurements

The viscosities of the ionic liquids were measured using a modular compact rheometer (MCR 302, Anton Paar) with a particle imaging velocimetry (C-LTD 70/PIV) cell. A flow curve test was performed using the concentric cylinder geometry of the equipment. The water viscosity is adapted from a previous paper.39 The values for the water and IL viscosities are reported in Table 1.

Wetting and Contact Angle Measurements

For each liquid, five drops of 10 μL were gently deposited using a micropipette on the freshly made hydrophobic microstructures and flat PDMS surfaces. All experiments were performed under ambient temperature (22 °C) at 1 atm and relative humidity of 16 ± 1%. Details regarding the experimental setup and image analysis can be found in our previous papers reporting the initial wetting states of water and surfactant-laden drops,47−49 whereas we focus on the static and dynamic wetting of ionic liquids here.

In brief, two synchronized cameras were used to record the side and bottom views of the droplets upon deposition at 1 fps (frame per second). One CCD camera (Thorlabs DCC3240C) coupled with a long-range magnifying lens (Navitar 12×) was used to record the side view, and a color camera (Axiocam 105) integrated into an inverted microscope (Zeiss, with a 5× objective) was used to capture the bottom view. Advancing (θA) and receding (θR) contact angles were also measured using the sessile drop method, by slowly increasing and decreasing the droplet volume using a syringe pump at a rate of 5 μL/min, with a set volume of 5 μL.

The contact angles were measured by drawing numerically a tangent at the edge of each droplet,22 using Matlab code. Drop-wetting states were determined through the bottom-view snapshots, which show a bright (dark) color for a CB (Wenzel) wetting state (see Figure 2). The base radius at the three-phase contact line, RB, and the dynamic contact angle (either θA or θR) were measured as a function of time, t, from the Matlab analysis. The contact line velocity (UCL) during the advancing or receding wetting process was estimated by taking the derivative of RB(t) with respect to t numerically.

Figure 2.

Figure 2

Open in a new tab

Side- and bottom-view snapshots of water and different ionic liquid drops of different viscosities on the hydrophobic microstructures (of r = 2.61 and ϕ = 0.47) showing a transition from Cassie–Baxter (CB) to Wenzel (W) wetting by increasing the liquid viscosity from low (e.g., water) to high (such as ILs). The bottom-view snapshots reveal the wetting state accurately: a bright (dark) image indicating a CB (Wenzel) state.

Theoretical

Surface Energy Comparison

To theoretically predict the stable wetting states for water and ionic liquids on the studied surface, we used a model based on the comparison between the surface energies for a CB and a Wenzel drop on hydrophobic microstructures. The total surface energy for a CB or a Wenzel drop on the hydrophobic microtextures, denoted by ECB and EW, respectively, can be expressed by9,44,50,51

graphic file with name la2c02694_m003.jpg 1

where Sb is the drop’s base surface area, γSG and γLG are the solid–gas and liquid–gas interfacial tensions, respectively, r is the surface roughness, ϕ is the packing fraction, and Scap is the spherical cap surface area of the water drop completely in contact with the surrounding air.

The difference between the CB and Wenzel energies, ΔE = ECB – EW, enables us to predict the critical contact angle θ* when equating the two energies ECB = EW. Using the above equations, one can arrive at the physical criterion of the critical contact angle θ* that delineates the surface parameters for a stable CB vs Wenzel state, by equating ECB = EW(9,44,50,51)

graphic file with name la2c02694_m004.jpg 2

This model suggests that a CB droplet is thermodynamically more stable when ECB < EW, which corresponds to a larger contact angle, θ > θ*. In addition, a stable Wenzel occurs when EW has lower energy compared to that of a CB wetting mode, that is, EW < ECB, by tuning the surface parameters of r and ϕ.52 More discussion of our experimental data is given in Figure 3 and the Results Section of wetting states.

Figure 3.

Figure 3

Open in a new tab

Phase diagram based on the energetic argument50,51,53,54 shows a stable CB state region (in the upper yellow region) and a stable W phase (in the lower region, e.g., blue, green, brown, and pink) for different Young angles (θF) on a flat surface. The critical CB–W separation lines based on eq 2 are plotted for different Young contact angles on PDMS flat surfaces, for water θF = 109.9° (Gray dashed), BMIM BF4 (Officegreen dotted) θF = 101.6°, EMIM BF4 (Brown dotted) θF = 85.3°, and HMIM NTf2 (Magenta chain) θF = 64.7°. At a high surface roughness (r) and a high solid fraction (ϕ), the initial water drop shape always exhibits CB states (denoted by the open symbols black rectangle, □), whereas a Wenzel state (denoted by the filled regions, e.g., blue, green, brown, and pink) is more likely observed experimentally at low r and ϕ for the three ionic liquids studied.

Dynamic Contact Angle Models

When deposing a liquid drop on a solid surface, it spreads spontaneously to reach the equilibrium or the static contact angle θS. Two main parameters control the spreading/wetting dynamics: (i) the contact line velocity (UCL) at which the liquid moves across the solid and (ii) the dynamic contact angles in terms of θA and θR. We utilize two approaches to investigate the velocity-dependent dynamic contact angle: the hydrodynamic theory19,20 and the molecular kinetic theory.14,21,25 The former approach emphasizes the energy dissipation due to viscous flow in the bulk liquid inside the drop, whereas the latter discards dissipation due to viscous flow and pivots instead on that occurring at the moving contact line due to the interactions between the molecules of the solid substrate and the liquid phase.

Hydrodynamic (HD) Model

To study the dependence of the dynamic contact angle on the contact line velocity, we follow a hydrodynamic model proposed by Cox19 and Voinov.20 This hydrodynamic model assumes that the excess free energy in the system resulting from the deviation of the drop shape and contact angle from its equilibrium static values are dissipated by the bulk viscous flow in the moving liquid and does not consider the flow near the contact line.18,19

To solve the dynamic contact angle according to the hydrodynamic model, Cox19 introduces L to be some characteristic macroscopic length, for example, drop size, and UCL to be the characteristic velocity for the flow occurring when the liquid (of known viscosity, μL, and density, ρL) displaces the gas/air (of viscosity, μG, and density, ρG). During this process, slip between the liquid and the solid surface must occur at distances of order LS from the contact line to eliminate the singularity of the nonintegrable stress. In addition, the hydrodynamic theory assumes that the liquid–gas surface tension, γLG, is sufficiently large so that interfacial tension effects dominate viscous effects. In other words, the capillary numbers for the liquid and gas phases are small. In its simplest form, the relation describing the dependence of the dynamic contact angle (θ) on the contact line velocity (UCL) is given by Cox19 and Voinov20 as

graphic file with name la2c02694_m005.jpg 3

Here, θ is the dynamic contact angle (either θA or θR), θS is the static contact angle, the plus (minus) sign denotes for the advancing (receding) contact angle, μ is the liquid’s viscosity, UCL is the contact line velocity, γLG is the liquid–gas surface tension, L is a characteristic length, for example, the droplet size, LS is the slip length, which is expected to be of the order of the molecular dimension.18,19

This modified equation relates the dynamic contact angle (θ) of a drop on a solid surface with UCL and is used to fit our experimental data of θA or θR on the hydrophobic microstructures to compute θS and the logarithmic ratio of the two length scales, ln(L/LS). We then fitted our experimental data to eq 3 using Matlab curve fitting tool; the fitting was based on 16 data points for each liquid.

Molecular Kinetic (MK) Model

While the hydrodynamic model concerns only the excess energy dissipation in the bulk viscous flow, the molecular kinetic (MK) model focuses on the effect of the intermolecular interactions at the contact line wetting dynamics. The MK model postulates that liquid’s viscosity dissipated through the interactions between the molecules of the liquid and the solid phases at the contact line. The first MK is given by Blake and Haynes;21 they proposed that the movement of the contact line of a liquid drop is determined by the processes of adsorption and desorption of liquid molecules at the contact line. During the addition or withdrawal of liquid, the contact line is forced to move across the solid surface. As a result, the excess free energy per unit area, γLG(cos θS – cos θ), is out of balance, and hence the adsorption–desorption equilibrium characterized by the frequency k0 is disrupted.18 To solve the velocity-dependent dynamic contact angle, we follow the Blake and Haynes’ approach21 by coupling and solving the absolute reaction rate theory, the pressure drop across the moving interface equation, and the work of adhesion equation simultaneously, and finally arrive at the final form of the MK theory. In its simplest form, the final relation of the MK model relating the dynamic contact angle to UCL is expressed as14,18,21,22

graphic file with name la2c02694_m006.jpg 4

where k0 is the equilibrium frequency, λ is the molecular displacement distance (of microscopic length scale), kB is the Boltzmann constant, and T is the absolute temperature. According to the MK model, UCL is governed by intermolecular equilibrium frequency, k0, and the average distance for each adsorption–desorption displacement that occurs along the contact line length, λ.14

If the Inline graphic, meaning that the argument of the sine hyperbolic is negligible, eq 5 simplifies to

graphic file with name la2c02694_m008.jpg 5

where Inline graphic is the contact line friction coefficient, has the physical dimensions of shear viscosity, and can be compared to the bulk viscosity of the liquid, but here focuses on the liquid near the contact line region.18,22

Recently, for the dynamic wetting, Blake55 proposed a relation between the equilibrium frequency (k0) and the work of adhesion (WA), specifically the activation free energy of wetting per molecule emerging from the liquid–solid interactions, ΔGS*

graphic file with name la2c02694_m010.jpg 6

where νL is the molecular volume for simple liquids and predicted to correspond to the molecular volume for the studied liquids.22 In addition, in this case, the work of adhesion between the liquid and the solid interfaces is assumed to be equated with the specific activation free energy of wetting per unit area, ΔgS* = ΔGS/λ2,55 and is given by

graphic file with name la2c02694_m011.jpg 7

We then fitted our experimental data to eq 4 using Matlab curve fitting tool to predict the fitting parameters, namely θS, k0, and λ. The fitting was based on 16 data points for each liquid.

Results and Discussion

Wetting States

Figure 2 shows the side- and bottom-view snapshots of water and ionic liquid drops on the hydrophobic microstructures. From the bottom-view images, two distinct wetting states are observed being either Cassie–Baxter (CB) or Wenzel (W). To determine the wetting states of each liquid on the studied surface, a bright central area with air trapped beneath the drop is observed for CB water drops, whereas a dark area at the center is detected when the liquid entirely wets the microstructures in a Wenzel state. This color contrast (bright vs dark) between the partial CB and complete Wenzel wetting arises from the different refractive indices of air and liquid. Revealed in Figure 2 is that water drops are in the CB wetting state, while with five independent experiments, all of the three ionic liquids’ (BMIM BF4, HMIM NTf2, and EMIM BF4) drops are in the Wenzel wetting state.

To interpret the experimentally observed wetting states (being a CB or Wenzel state), we use a model based on the comparison of the global surface energies, ECB and EW, accounting for the total surface energy for a CB or Wenzel drop on the microstructures. The physical parameters of this model are discussed in the theoretical section, and the physical criterion of the critical contact angle θ*, for example, eq 2 is plotted in Figure 3. Based on the global energy model, a stable CB state (shown by the upper yellow region in Figure 3) occurs with a high roughness satisfying r > (ϕ – 1)/cos θF + ϕ. We measured the contact angles of water and BMIM BF4, the EMIM BF4, and HMIM NTf2 ionic liquids droplet on the flat PDMS surface to be θF = 109.9, 101.6, 85.3, and 64.7°, respectively.

In Figure 3, we plot the critical criteria using eq 2 with different θF values measured. Based on the thermodynamic model, a stable CB drop occurs in the upper region above the critical criterion (i.e., higher r). In contrast, a Wenzel state is more favorable for low-roughness surfaces (i.e., the blue, green, brown, and pink regions for different Young angles, θF) since EW < ECB. In a good agreement, we always observed a CB state of the initial water drop on our microstructures r = 2.61 and ϕ = 0.47, as shown (by the open symbols black rectangle, □) in Figure 3, which is located in the upper stable CB state region (above the critical criterion boundary for the Young angle of interest, e.g., θF = 109.9°). Differently, for the three ionic liquids, we consistently observed a Wenzel state on the hydrophobic microstructures, as shown (by the open symbols black rectangle, □) in Figure 3, which is located in the lower regime of the stable W state (below the critical criterion boundary for the Young angles of interests, e.g., θF = 101.6°, θF = 85.3°, and θF = 64.7°).

Dynamic Advancing and Receding Contact Angles

Dynamic wetting properties of water and the three ionic liquid (BMIM BF4, HMIM NTf2, and EMIM BF4) drops on the hydrophobic microstructures are investigated through the advancing (θA) and receding (θR) contact angle measurements as a function of the advancing and receding time, shown in Figure 4a,b, respectively. Five drops were deposited and analyzed for each liquid to ensure reproducibility; the average values with standard deviations are reported in Figure 4. On the one hand, Figure 4a shows that the contact angle increases with adding liquid until the advancing contact angle is reached for all liquids, for example, water θA = 146.2 ± 2.1°, BMIM BF4 θA = 145.7 ± 1.8°, HMIM NTf2 θA = 140.8 ± 2.7°, and EMIM BF4 θA = 138.2 ± 2.9°. On the other hand, the contact angle then decreased with liquid withdrawal until the (initial, nearly steady) receding contact angle is reached, for example, water θR = 142.7 ± 1.7°, BMIM BF4 θR = 142.6 ± 2.9°, HMIM NTf2 θR = 127.9 ± 1.2°, and EMIM BF4 θR = 126.9 ± 1.3°. The latter two ILs reveal a greater CA hysteresis of θA – θR ≳ 10°.

Figure 4.

Figure 4

Open in a new tab

(a) Advancing and (b) receding contact angles measured for water and three ionic liquid drops on the studied hydrophobic microstructures, r = 2.61, ϕ = 0.47, versus dimensionless wetting time, t/tf, using a sessile drop method by adding (or withdrawing) liquid to measure the advancing (or receding) CAs. Here, time, t, is normalized with the total advancing (or receding) time, tf. The error bars show the standard deviations obtained from five independent experiments.

As shown in Table 1, as expected, water has the largest contact angle due to its high surface tension, γLG, and low viscosity, μ, among the studied liquids. In contrast, EMIM BF4 has higher surface tension and lower viscosity than both BMIM BF4 and HMIM NTf2, but still, it has the lowest contact angle among all the four liquids. There is no clear trend in the dynamic contact angle with increasing or decreasing both the surface tension and the viscosity. To predict these experimentally observed contact angles and to have more insights into the wetting behavior of the ionic liquids on the hydrophobic microstructures, we apply two models, each considering a specific dissipation regime of the excess free energy during the advancing or receding contact line. The fitting of our experimental data to those models is discussed below.

Hydrodynamic (HD) Model

Since the viscosities of ILs are much higher than that of water, the hydrodynamic model, which focuses on the viscous friction in the bulk liquid, is used first to fit the experimental data and to describe the observed wetting behavior of water and ILs. Figure 5 shows the experimental and theoretical predictions of the dynamic contact angles on the hydrophobic microstructures. For convenience, the data were plotted in a cubic relation (θ3vs UCL) to be easily compared with the linear relation predicted by the HD model, eq 3. However, the experimental data reveal more deviation from the linearly predicted fitting between θ3 and UCL by the HD model, shown in Figure 5.

Figure 5.

Figure 5

Open in a new tab

The measured and fitted cubic-scale of (a) the advancing and (b) receding contact angles, θ3, for water and ionic liquids droplets on the hydrophobic microstructures vs the contact line velocity UCL. Different symbols denote water (Blue triangle), BMIM BF4 (□), HMIM NTf2 (Magenta triangledown), and EMIM BF4 (Red circle) drops on the studied surface. The error bars represent the standard deviations of five drops for each liquid. Theoretical fitting based on the hydrodynamic model (HD) of the cubic-scale advancing and receding contact angles, given by eq 3, are also plotted for a comparison (solid lines). The HD fitting parameters are given in Table 2.

The deviation between the experimental data and the fitted is more clear in Table 2, which questioned the applicability of the HD model for the dynamic wetting of water and ILs on the MS surface. As shown in Table 2, one indicator that the HD theoretical model may be applicable for the dynamic wetting of water and ILs on the studied surface is the plausible correlation between the measured and the fitted static contact angles with a maximum error estimation of 4%. However, the greater value of fitted θSR than θS also questions the applicability of the HD model since physically the experimental value of θA is always larger than θR. In addition to the static contact angle, the characteristic slip length is another fitting parameter that could assist asessing the applicability of the HD model, shown in Table 2. From Table 2, the values for L/LS are between 2 and 8 in the advancing case and 6 and 11 in the receding case. By considering L is the capillary length estimated to be Inline graphic, via the fitting results, the estimated values of the characteristic slip length LS are in the range of 10–8–10–6 m, much larger (by 10–1000×) than the molecular size of the IL ion pair. Therefore, due to the overestimated characteristic slip length LS, the HD model is likely not applicable to describe the dynamic wetting of water and ILs on the hydrophobic microstructures. This observation also agrees with the predicted slip length of six similar ionic liquids on Teflon AF1600 surfaces.22

Table 2. Comparison between the Fitting Parameters (θS, ln(L/LS) ) Extracted from Fitting the Eexperimental Data with the HD Model, eq 3, and the Experimentally Measured θSa18.

liquid θS (deg) θSA (deg) ln(L/LS) LSA θSR (deg) ln(L/LS) LSR
  measured fitted   (m) fitted   (m)
Milli-Q water 145 ± 2 140 8.34 ± 0.66 7.44 × 10–7 148 11.22 ± 1.56 7.23 × 10–8
EMIM BF4 130 ± 1 134 7.35 ± 0.79 1.61 × 10–6 132 8.11 ± 0.96 8.20 × 10–7
HMIM NTf2 139 ± 3 136 4.85 ± 0.46 1.28 × 10–5 134 6.59 ± 0.68 2.44 × 10–6
BMIM BF4 143 ± 2 139 2.79 ± 0.17 1.17 × 10–4 149 11.53 ± 2.18 6.17 × 10–8
Open in a new tab
a

The data on the middle (right) column show the fitting of eq 3 to the experimental results of θA (θR) in Figure 5a,b. To calculate LS, we used the capillary length, Inline graphic as the characteristic length, L.18

Figure 6 and Table 2 show that the dynamical values of θA,R3 – θSvsCa = μUCL/γLV, the capillary number defined as the ratio of viscous to interfacial forces, reveal two distinct trends. High-viscosity ionic liquids (BMIM BF4 and HMIM NTf2) have relatively high Ca (≈0.003–0.048) and a less deviation between the fitted LS from the advancing and receding cases. In contrast, water and low-viscosity ionic liquid (EMIM BF4) have relatively lower Ca (≈4 × 10–6–4 × 10–4) and a larger deviation between the advancing and receding fitted LS by one order of magnitude difference. The former fair agreement suggests that the HD model [eq 3] works better for high-Ca liquid wetting, whereas the latter low-Ca liquids show a greater discrepancy from the HD results. These two different trends of the data of θA,R3–θSvsCa (shown in Figure 6) may imply Ca-dependent wetting processes, whereby the HD (MD) theory is more applicable for relatively high (low)-Ca regime, consistent with the previous finding by Perrin et al.56

Figure 6.

Figure 6

Open in a new tab

The values of (a) θA3–θS and (b) θR3–θS for water and ionic liquid drops on the studied hydrophobic microstructures vs the capillary number, Ca = μUCL/γLV. The error bars show the standard deviations obtained from five independent experiments.

Molecular Kinetic (MK) Model

The advancing and receding contact angles data are then plotted in Figure 7 in terms of cos θA or cos θRvs UCL, convenient for a comparison with the MK model given by eq 5. Figure 7 shows that the MK model could predict the experimental data of the dynamic contact angles for water and ionic liquids on the hydrophobic microstructures, and the extracted fitting parameters are summarized in Tables 3 and 4 for the advancing and receding contact line, respectively.

Figure 7.

Figure 7

Open in a new tab

Measured and fitted data presented as the cosine of advancing cos (θA) in (a) and the cosine of the receding cos (θR) in (b) contact angles for water and ionic liquid droplets on the hydrophobic microstructures (of r = 2.61, ϕ = 0.47) vs the contact line velocity UCL. Different symbols denote water (Blue triangle), BMIM BF4 (□), HMIM NTf2 (Magenta triangledown), and EMIM BF4 (Red circle) drops on the studied surface. The error bars represent the standard deviations of five drops for each liquid. Theoretical fitting by the molecular kinetic (MK) model eq 5 are also plotted for a comparison (solid lines). The MK fitting parameters for the advancing (θA) and receding (θR) contact lines are given in Tables 3 and 4, respectively.

Table 3. Comparison between the Fitting Parameters (θS, λ, k0) Extracted from Fitting the Experimental data of cos(θA) in Figure 7a with the MK model [eq 5] and the experimentally measured static contact angle θSa.

liquid θSA (deg) θSA (deg) λ k0 ζ WA
  fitted measured (nm) (MHz) (Pa·s) (mJ/m2)
Milli-Q water 147 ± 1 145 ± 2 0.52 ± 0.01 5.64 ± 0.35 5.17 ± 0.62 11.85 ± 0.39
EMIM BF4 129 ± 1 130 ± 1 0.68 ± 0.01 3.93 ± 0.28 3.32 ± 0.38 19.65 ± 0.43
HMIM NTf2 140 ± 1 139 ± 3 0.79 ± 0.02 1.73 ± 0.16 4.84 ± 0.82 7.33 ± 0.43
BMIM BF4 145 ± 1 143 ± 2 0.67 ± 0.02 0.84 ± 0.05 16.28 ± 2.43 7.91 ± 0.34
Open in a new tab
a

ζ and WA are calculated using Inline graphic and eq 7,7 respectively.

Table 4. Comparison between the Fitting Parameters (θS, λ, k0) Extracted from Fitting the Experimental Data of cos(θR) in Figure 7b with the MK Model, eq 5 and the Experimentally Measured Static Contact Angle θSa.

liquid θSA (deg) θSA (deg) λ k0 ζ WA
  fitted measured (nm) (MHz) (Pa·s) (mJ/m2)
Milli-Q water 146 ± 1 145 ± 2 0.32 ± 0.01 12.24 ± 4.04 11.26 ± 4.95 12.14 ± 0.74
EMIM BF4 131 ± 3 130 ± 1 0.68 ± 0.01 6.35 ± 2.61 2.35 ± 1.15 18.59 ± 1.95
HMIM NTf2 140 ± 3 139 ± 3 0.79 ± 0.01 3.72 ± 1.16 2.40 ± 0.88 7.36 ± 1.18
BMIM BF4 144 ± 5 143 ± 2 0.68 ± 0.02 1.85 ± 0.83 8.44 ± 4.89 8.37 ± 2.05
Open in a new tab
a

ζ and WA are calculated using Inline graphic and eq 7, respectively.

Tables 3 and 4 show that the fitted and measured θS are in good agreement with a maximum error estimation of about 1%. The fitting parameters for the MK model, k0 and λ, show a better correlation with the liquid physical properties, for example, liquid viscosity. Intriguingly, the equilibrium frequency, k0, at the three-phase contact line decreases with increasing liquid viscosity, in agreement with eq 5 and with two previous studies investigating the dynamic wetting of ILs on a fluoropolymer surface (Teflon AF1600)22 and gold and glass substrates.18 This implies that for liquids with low viscosity, the interaction between neighboring IL molecules is small, and thus, it is relatively easy for the molecules to move from one adsorption point on the surface to another, leading to a higher k0 at the contact line. As expected, water (with the lowest viscosity) has the highest equilibrium frequency, while BMIM BF4 has the lowest k0 (due to its largest viscosity) among the liquid samples.

In addition, the molecular displacement distance, λ, appears to be independent of the liquid’s physical properties or the type of ILs used. From Tables 3 to 4, the fitted values of λ vary from 0.32 to 0.79 nm, which is close and has the same order of magnitude as the water and IL ion-pair diameters (between 0.31 and 0.81 nm, Table 1). The agreement between the experimental data and the MK model parameters here suggests that the energy dissipation is significant at the three-phase contact line. Similar observations were reported by Li et al.22 and Delcheva et al.18 when studying the dynamic wetting of similar ILs on hydrophobic Teflon and hydrophilic gold and glass substrates, respectively.

According to the MK model, there is another parameter that combines both k0 and λ to explain the observed contact angle dependence on the contact line velocity. This parameter is the contact line friction, ζ, and is expected to increase with both liquid viscosity and the work of adhesion according to eq 6. From our experimental data, water, as a low-viscous fluid, has the largest contact angles as a function of UCL due to its high ζ ≈11 Pa·s, which is more clear from the receding contact line in Table 4. For high-viscous liquids, BMIM BF4 has the largest contact angles as a function of UCL among all the ILs, which could be explained by it is distinctly higher contact line friction, ζ ≈ 16 (±2) and 8 (±4) Pa·s in Tables 3 and 4, respectively. HMIM NTf2 and EMIM BF4 liquids have similar contact line friction values (within standard deviation error) and, as a result, have similar contact angles for a given wetting velocity. Therefore, the contact line friction (ζ) appears to be the ultimate determinant of the dynamic wetting behavior for water and the three particular ILs on our hydrophobic microstructures.

We now further discuss a number of key findings regarding our dynamic wetting data and its comparison with previous literature data. First, the HD model seems to be inadequate to interpret the velocity-dependent dynamic contact angle, partially due to the nonlinearity of most of the data sets when plotted as a cubic relation (θ3vs UCL) and due to the nonphysicality of the fitted slip lengths (LS). The latter should be of a length scale similar to the ion-pair size of the ILs, ∼10–9 m. Previously, Li et al.22 reported the dynamic advancing contact angles of similar ionic liquids on a fluoropolymer Teflon AF1600 substrate to be in the range of ∼80 to 160°. In their work, LS seemed to vary from being too large ∼10–6 m (in the high-velocity regime ∼0.02 to 0.18 m/s) to reasonable ∼10–8 m (in the low-velocity regime ∼0 to 0.02 m/s). Unlike these findings, Delcheva et al.18 obtained a small LS that is by 1–2 orders of magnitude smaller than the molecular size of the ILs when studying advancing dynamic wetting of similar ILs on flat glass and gold surfaces of ∼10 to 80°. Our fitted value of LS agrees with the former work ∼10–6 to 10–8 m, while the latter work underestimates the value of LS.

Second, the MK model is shown to better predict the dynamic wetting in terms of the advancing contact angle of ILs on hydrophobic microstructures (our work), fluoropolymer Teflon AF1600 substrate,22 and gold and glass surfaces.18 One crucial observation that requires further investigation is that θR of the various ILs show two different trends in both Figures 5b and 7b. One possible explanation for the observed differences in the θR is associated with water molecule content in the ILs. All ionic liquids can easily absorb water from the atmosphere. This fact needs to be taken into consideration when conducting any experiments. Even with careful sample preparation, hundreds of ppm of water molecules are still present in ILs,18 especially during the θR measurements after exposure to the surrounding atmosphere for 5–8 min when measuring θA.

Conclusions

In summary, we systematically measured the dynamic wetting and contact angles of the droplets of water and three ionic liquids on hydrophobic micropillars of surface roughness (r = 2.61) and packing fraction (ϕ = 0.47). Water drops initially form a gas-trapping, CB wetting state for drop wetting. In contrast, all the ionic liquid drops wet in a completely wetting Wenzel state, where liquids penetrate the microstructures due to minor surface tension. To explain these distinct wetting states, we used a model comparing the global surface energies, ECB and EW, which account for CB and Wenzel drops on the hydrophobic microstructures. Both the observed and the predicted wetting states agree, revealing the validity of the global surface energy model to predict the wetting states for both low-viscous (e.g., water) and high-viscous fluids (e.g., ionic liquids).

The dynamic contact angles are measured and evaluated in terms of the advancing and receding contact angles as a function of contact line velocity, UCL. The results show that, except for water, higher-viscosity ionic liquids have greater advancing and receding contact angles with increasing UCL. We find that the hydrodynamic model (attributing the viscous dissipation in the bulk liquid) inadequately describes the dynamic wetting of water and ionic liquid droplets on the hydrophobic microstructures because of the overestimated slip length LS. The value of LS is expected to be of the order of the molecular size, but the fitted one is much larger 10–8–10–6 m. In contrast, the molecular dynamic model well predicted the experimental data and is used to explain the observed dynamic wetting of the IL on the hydrophobic substrates. Generally, energy is dissipated in the bulk liquid and at the three-phase contact line during dynamic wetting. Our results, however, show that the contribution of the three-phase contact line is more significant for the IL liquids and hydrophobic microstructures. Despite using the hydrophobic microstructures, which are superhydrophobic for water drops yielding a high contact angle, the IL drops completely wet the substrates. To realize the promising applications of low-friction IL lubricant, future studies are needed on the dynamic wetting of ILs on various superhydrophobic or super-repellent surfaces. Those studies must consider the complexity of IL structure and the influence of varying anion and cation chains on the wetting phenomena.

Acknowledgments

A.A. and P.A.T. thankfully acknowledge the funding support from the Alberta Innovates (AI) and the Natural Sciences and Engineering Research Council of Canada (NSERC), particularly NSERC Discovery grant (RGPIN-2020-05511). P.A.T. holds a Canada Research Chair in Fluids and Interfaces (CRC TIER2 233147). This research was undertaken, in part, thanks to funding from the Canada Research Chairs (CRC) Program.

The authors declare no competing financial interest.

References

  1. Park J. U.; Hardy M.; Kang S. J.; Barton K.; Adair K.; Mukhopadhyay D. K.; Lee C. Y.; Strano M. S.; Alleyne A. G.; Georgiadis J. G.; Ferreira P. M.; Rogers J. A. High-resolution electrohydrodynamic jet printing. Nat. Mater. 2007, 6, 782–789. 10.1038/nmat1974. [DOI] [PubMed] [Google Scholar]
  2. Baret J.-C. Surfactants in droplet-based microfluidics. Lab Chip 2012, 12, 422–433. 10.1039/C1LC20582J. [DOI] [PubMed] [Google Scholar]
  3. Xu J. H.; Dong P.; Zhao H.; Tostado C.; Luo G. The dynamic effects of surfactants on droplet formation in coaxial microfluidic devices. Langmuir 2012, 28, 9250–9258. 10.1021/la301363d. [DOI] [PubMed] [Google Scholar]
  4. Koh J. J.; Lim G. J.; Zhou X.; Zhang X.; Ding J.; He C. 3D-Printed Anti-Fouling Cellulose Mesh for Highly Efficient Oil/Water Separation Applications. ACS Appl. Mater. Interfaces 2019, 11, 13787–13795. 10.1021/acsami.9b01753. [DOI] [PubMed] [Google Scholar]
  5. Lv J.; Gong Z.; He Z.; Yang J.; Chen Y.; Tang C.; Liu Y.; Fan M.; Lau W.-M. 3D printing of a mechanically durable superhydrophobic porous membrane for oil-water separation. J. Mater. Chem. A 2017, 5, 12435–12444. 10.1039/C7TA02202F. [DOI] [Google Scholar]
  6. Lomga J.; Varshney P.; Nanda D.; Satapathy M.; Mohapatra S.; Kumar A. Fabrication of durable and regenerable superhydrophobic coatings with excellent self-cleaning and anti-fogging properties for aluminium surfaces. J. Alloys Compd. 2017, 702, 161–170. 10.1016/j.jallcom.2017.01.243. [DOI] [Google Scholar]
  7. Wenzel R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 988–994. 10.1021/ie50320a024. [DOI] [Google Scholar]
  8. Cassie A. B. D.; Baxter S. Wettability of porous surfaces. J. Chem. Soc. Faraday Trans. 1944, 40, 546–551. 10.1039/tf9444000546. [DOI] [Google Scholar]
  9. Quéré D. Non-sticking drops. Rep. Prog. Phys. 2005, 68, 2495–2532. 10.1088/0034-4885/68/11/R01. [DOI] [Google Scholar]
  10. De Gennes P. G. Statics and Dynamics: Wetting. Rev. Mod. Phys. 1985, 57, 827–863. 10.1103/RevModPhys.57.827. [DOI] [Google Scholar]
  11. McHale G.; Orme B. V.; Wells G. G.; Ledesma-Aguilar R. Apparent contact angles on lubricant-impregnated surfaces/slips: From superhydrophobicity to electrowetting. Langmuir 2019, 35, 4197–4204. 10.1021/acs.langmuir.8b04136. [DOI] [PubMed] [Google Scholar]
  12. Lu J. Y.; Ge Q.; Li H.; Raza A.; Zhang T. Direct prediction of calcite surface wettability with first-principles quantum simulation. J. Phys. Chem. Lett. 2017, 8, 5309–5316. 10.1021/acs.jpclett.7b02270. [DOI] [PubMed] [Google Scholar]
  13. Young T. Phil. Trans. R. Soc. Lond. 1802, 95, 65. 10.1098/rstl.1805.0005. [DOI] [Google Scholar]
  14. Blake T. D. The physics of moving wetting lines. J. Colloid Interface Sci. 2006, 299, 1–13. 10.1016/j.jcis.2006.03.051. [DOI] [PubMed] [Google Scholar]
  15. Shikhmurzaev Y. D. The moving contact line on a smooth solid surface. Int. J. Multiphase Flow 1993, 19, 589–610. 10.1016/0301-9322(93)90090-H. [DOI] [Google Scholar]
  16. Chengara A.; Nikolov A.; Wasan D. Surface tension gradient driven spreading of trisiloxane surfactant solution on hydrophobic solid. Colloids Surf., A 2002, 206, 31–39. 10.1016/S0927-7757(02)00060-2. [DOI] [Google Scholar]
  17. Suda H.; Yamada S. Force measurements for the movement of a water drop on a surface with a surface tension gradient. Langmuir 2003, 19, 529–531. 10.1021/la0264163. [DOI] [Google Scholar]
  18. Delcheva I.; Beattie D.; Ralston J.; Krasowska M. Dynamic wetting of imidazolium-based ionic liquids on gold and glass. Phys. Chem. Chem. Phys. 2018, 20, 2084–2093. 10.1039/C7CP06404G. [DOI] [PubMed] [Google Scholar]
  19. Cox R. The dynamics of the spreading of liquids on a solid surface. Part 1. Viscous flow. J. Fluid Mech. 1986, 168, 169–194. 10.1017/S0022112086000332. [DOI] [Google Scholar]
  20. Voinov O. V. Hydrodynamics of wetting. Fluid Dyn. 1977, 11, 714–721. 10.1007/BF01012963. [DOI] [Google Scholar]
  21. Blake T.; Haynes J. Kinetics of liquidliquid displacement. J. Colloid Interface Sci. 1969, 30, 421–423. 10.1016/0021-9797(69)90411-1. [DOI] [Google Scholar]
  22. Li H.; Sedev R.; Ralston J. Dynamic wetting of a fluoropolymer surface by ionic liquids. Phys. Chem. Chem. Phys. 2011, 13, 3952–3959. 10.1039/c0cp02035d. [DOI] [PubMed] [Google Scholar]
  23. De Gennes B.-W. F.; Pierre-Gilles; Quéré D.. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves, Springer, 2004; Vol. 315. [Google Scholar]
  24. Ralston J.; Popescu M.; Sedev R. Dynamics of wetting from an experimental point of view. Annu. Rev. Mater. Res. 2008, 38, 23–43. 10.1146/annurev.matsci.38.060407.130231. [DOI] [Google Scholar]
  25. De Ruijter M. J.; Blake T.; De Coninck J. Dynamic wetting studied by molecular modeling simulations of droplet spreading. Langmuir 1999, 15, 7836–7847. 10.1021/la990171l. [DOI] [Google Scholar]
  26. Delcheva I.; Ralston J.; Beattie D. A.; Krasowska M. Static and dynamic wetting behaviour of ionic liquids. Adv. Colloid Interface Sci. 2015, 222, 162–171. 10.1016/j.cis.2014.07.003. [DOI] [PubMed] [Google Scholar]
  27. Hayes R.; Warr G. G.; Atkin R. Structure and nanostructure in ionic liquids. Chem. Rev. 2015, 115, 6357–6426. 10.1021/cr500411q. [DOI] [PubMed] [Google Scholar]
  28. Bhattacharjee S.; Chakraborty D.; Khan S. Wetting behavior of aqueous 1-alkyl-3-methylimidazolium tetrafluoroborate {[Cn MIM][BF4](n= 2, 4, 6)} on graphite surface. Chem. Eng. Sci. 2021, 229, 116078 10.1016/j.ces.2020.116078. [DOI] [Google Scholar]
  29. Guanhua N.; Hui W.; Baisheng N.; Yan W.; Haoran D.; Shouqing L.; Gang W. Research of wetting selectivity and wetting effect of imidazole ionic liquids on coal. Fuel 2021, 286, 119331. 10.1016/j.fuel.2020.119331. [DOI] [Google Scholar]
  30. Blanco D.; Rivera N.; Oulego P.; Díaz M.; González R.; Battez A. H. Novel fatty acid anion-based ionic liquids: Contact angle, surface tension, polarity fraction and spreading parameter. J. Mol. Liq. 2019, 288, 110995. 10.1016/j.molliq.2019.110995. [DOI] [Google Scholar]
  31. Wasserscheid P.; Keim W. Ionic liquidsew olutionsfor transition metal catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772–3789. . [DOI] [PubMed] [Google Scholar]
  32. Shi D.-Q.; Zhou Y.; Rong S.-F. Ionic liquid,[bmim] Br, as an efficient promoting medium for synthesis of 3-acetoacetylcoumarin derivatives without the use of any catalyst. Synth. Commun. 2009, 39, 3500–3508. 10.1080/00397910902779306. [DOI] [Google Scholar]
  33. Hallett J. P.; Welton T. Room-temperature ionic liquids: solvents for synthesis and catalysis. 2. Chem. Rev. 2011, 111, 3508–3576. 10.1021/cr1003248. [DOI] [PubMed] [Google Scholar]
  34. Ye C.; Liu W.; Chen Y.; Yu L. Room-temperature ionic liquids: a novel versatile lubricant. Chem. Commun. 2001, 2244–2245. 10.1039/b106935g. [DOI] [PubMed] [Google Scholar]
  35. Sun X.; Luo H.; Dai S. Ionic liquids-based extraction: a promising strategy for the advanced nuclear fuel cycle. Chem. Rev. 2012, 112, 2100–2128. 10.1021/cr200193x. [DOI] [PubMed] [Google Scholar]
  36. Nanayakkara Y. S.; Moon H.; Payagala T.; Wijeratne A. B.; Crank J. A.; Sharma P. S.; Armstrong D. W. A fundamental study on electrowetting by traditional and multifunctional ionic liquids: possible use in electrowetting on dielectric-based microfluidic applications. Anal. Chem. 2008, 80, 7690–7698. 10.1021/ac8009802. [DOI] [PubMed] [Google Scholar]
  37. Kawano R.; Matsui H.; Matsuyama C.; Sato A.; Susan M. A. B. H.; Tanabe N.; Watanabe M. High performance dye-sensitized solar cells using ionic liquids as their electrolytes. J. Photochem. Photobiol. A 2004, 164, 87–92. 10.1016/j.jphotochem.2003.12.019. [DOI] [Google Scholar]
  38. Abate A.; Hollman D. J.; Teuscher J.; Pathak S.; Avolio R.; D’Errico G.; Vitiello G.; Fantacci S.; Snaith H. J. Protic ionic liquids as p-dopant for organic hole transporting materials and their application in high efficiency hybrid solar cells. J. Am. Chem. Soc. 2013, 135, 13538–13548. 10.1021/ja406230f. [DOI] [PubMed] [Google Scholar]
  39. Batchelor T.; Cunder J.; Fadeev A. Y. Wetting study of imidazolium ionic liquids. J. Colloid Interface Sci. 2009, 330, 415–420. 10.1016/j.jcis.2008.10.019. [DOI] [PubMed] [Google Scholar]
  40. Blanco D.; Bartolomé M.; Ramajo B.; Viesca J.; González R.; Hernadez Battez A. Wetting properties of seven phosphonium cation-based ionic liquids. Ind. Eng. Chem. Res. 2016, 55, 9594–9602. 10.1021/acs.iecr.6b00821. [DOI] [Google Scholar]
  41. Wang Z.; Li H.; Atkin R.; Priest C. Influence of water on the interfacial nanostructure and wetting of [Rmim][NTf2] ionic liquids at mica surfaces. Langmuir 2016, 32, 8818–8825. 10.1021/acs.langmuir.6b01790. [DOI] [PubMed] [Google Scholar]
  42. Restolho J.; Mata J. L.; Saramago B. On the interfacial behavior of ionic liquids: Surface tensions and contact angles. J. Colloid Interface Sci. 2009, 340, 82–86. 10.1016/j.jcis.2009.08.013. [DOI] [PubMed] [Google Scholar]
  43. Gao L.; McCarthy T. J. Ionic liquids are useful contact angle probe fluids. J. Am. Chem. Soc. 2007, 129, 3804–3805. 10.1021/ja070169d. [DOI] [PubMed] [Google Scholar]
  44. Tsai P.; Lammertink R. G.; Wessling M.; Lohse D. Evaporation-triggered wetting transition for water droplets upon hydrophobic microstructures. Phys. Rev. Lett. 2010, 104, 2–3. 10.1103/PhysRevLett.104.116102. [DOI] [PubMed] [Google Scholar]
  45. Berry J. D.; Neeson M. J.; Dagastine R. R.; Chan D. Y.; Tabor R. F. Measurement of surface and interfacial tension using pendant drop tensiometry. J. Colloid Interface Sci. 2015, 454, 226–237. 10.1016/j.jcis.2015.05.012. [DOI] [PubMed] [Google Scholar]
  46. Schneider C. A.; Rasband W. S.; Eliceiri K. W. NIH Image to ImageJ: 25 years of image analysis. Nat. Methods 2012, 9, 671–675. 10.1038/nmeth.2089. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Aldhaleai A.; Khan F.; Thundat T.; Tsai P. A. Evaporation dynamics of water droplets on superhydrophobic nanograss surfaces. Int. J. Heat Mass Transfer. 2020, 160, 120149. 10.1016/j.ijheatmasstransfer.2020.120149. [DOI] [Google Scholar]
  48. Aldhaleai A.; Tsai P. A. Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces. Langmuir 2020, 36, 4308–4316. 10.1021/acs.langmuir.0c00288. [DOI] [PubMed] [Google Scholar]
  49. Aldhaleai A.; Tsai P. A. Evaporation Dynamics of Surfactant-Laden Droplets on a Superhydrophobic Surface: Influence of Surfactant Concentration. Langmuir 2022, 38, 593–601. 10.1021/acs.langmuir.1c03097. [DOI] [PubMed] [Google Scholar]
  50. Bussonnière A.; Bigdeli M. B.; Chueh D.-Y.; Liu Q.; Chen P.; Tsai P. A. Universal wetting transition of an evaporating water droplet on hydrophobic micro-and nano-structures. Soft Matter 2017, 13, 978–984. 10.1039/C6SM02287A. [DOI] [PubMed] [Google Scholar]
  51. Aldhaleai A.; Tsai P. A. Fabrication of transparent and microstructured superhydrophobic substrates using additive manufacturing. Langmuir 2021, 37, 348–356. 10.1021/acs.langmuir.0c02945. [DOI] [PubMed] [Google Scholar]
  52. Shardt N.; Bozorg Bigdeli M.; Elliott J.; Tsai P. How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures. J. Phys. Chem. Lett. 2019, 10, 7510–7515. 10.1021/acs.jpclett.9b02802. [DOI] [PubMed] [Google Scholar]
  53. Bico J.; Tordeux C.; Quéré D. Rough wetting. Europhys. Lett. 2001, 55, 214–220. 10.1209/epl/i2001-00402-x. [DOI] [Google Scholar]
  54. Ishino C.; Okumura K.; Quéré D. Wetting transitions on rough surfaces. Europhys. Lett. 2004, 68, 419–425. 10.1209/epl/i2004-10206-6. [DOI] [Google Scholar]
  55. Blake T.; De Coninck J. The influence of solid-liquid interactions on dynamic wetting. Adv. Colloid Interface Sci. 2002, 96, 21–36. 10.1016/S0001-8686(01)00073-2. [DOI] [PubMed] [Google Scholar]
  56. Perrin H.; Lhermerout R.; Davitt K.; Rolley E.; Andreotti B. Defects at the nanoscale impact contact line motion at all scales. Phys. Rev. Lett. 2016, 116, 184502 10.1103/PhysRevLett.116.184502. [DOI] [PubMed] [Google Scholar]

Articles from Langmuir are provided here courtesy of American Chemical Society

RESOURCES