Sprayable Superoleophobic Coatings for Pumpless Handling of Low‐Surface‐Tension Liquids

Limited attention has been paid to repellency and wettability‐confined transport of low‐surface‐tension liquids, such as oils, alcohols, and hydrocarbons. This is because repellency becomes very difficult for fluids with surface tension below 30 mN m−1. This situation is encountered in many engineering applications using organic liquids and thus, oleophobic surfaces are of high technological importance. In this work, a nanocomposite coating comprising of fluorinated silica micro‐ and nanoparticles (filler), a copolymer (binder), and traces of fluorinated polyhedral oligomeric silsesquioxane (additive, lowest surface‐energy crystalline material reported to date) is spray‐deposited on a surface pretextured by laser etching, a technique that requires no lithographic processing. The approach results in surfaces that can repel liquid hydrocarbons with surface tension as low as 21.14 mN m−1 and also hinder the spreading of heptane (19.74 mN m−1) at room temperature. The repellency of several organic liquids with surface tensions in the range of 19.7 – 27 mN m−1 is experimentally investigated and compared with water (baseline case). To study the effect of fluid properties, comparisons are performed between the distances traveled and the velocities of microliter droplets transported pumplessly on wedge‐shaped wettability‐patterned tracks that confine the liquids by the superoleophobic background surrounding the wettable tracks.


Introduction
Liquid repellency and fluid management are important in a wide range of applications, like drag reduction, [1][2][3][4][5] anti-fogging, [6,7] self-cleaning, [8][9][10][11] non-wetting fabrics, [12][13][14][15] anti-icing, [16][17][18][19] oil-water separation, [20][21][22] anti-corrosion coatings, [23][24][25][26] etc. Environmental pollution, especially by the spillage and spreading of low-surface-tension liquids, is a major problem today.Fluid management techniques are crucial in preventing environmental DOI: 10.1002/admi.202300924pollution.Inspired by naturally occurring structural features, like those of the lotus leaf or desert animals and plants, researchers have successfully replicated the complex topography of these hierarchical natural structures and coated them with low-surface-energy materials to produce repellent surfaces.[29][30][31][32] The surfaces are generally prepared by inducing surface roughness using various techniques, like chemical etching, laser etching, lithography, etc. to impart uniform roughness before coating it with a lowsurface-energy material.There are numerous prior works [33][34][35][36][37] that fabricated surfaces focused on high-surface-tension liquids, especially water ( = 72 mN m −1 ).However, when the fluid surface tension drops below 30 mN m −1 , repellency becomes extremely difficult.Numerous studies have shown that the lotus leaf surface chemistry and surface roughness can successfully repel water, but low-surface-tension liquids, like hexadecane ( = 27.5 mN m −1 ) spread rapidly on the surface and exhibit a contact angle close to 0°. [28] Engineering surfaces that are extremely repellent even to low surface tensions (≈20 mN m −1 ) remain challenging due to their required low interfacial energies.Such surfaces, despite their rarity, have a huge potential in a multitude of industrial applications.Recognizing the role of surface parameters, i.e., surface roughness and surface energy in conjunction with reentrant surface texture, these factors become crucial for creating repellent surfaces for low-surface-tension liquids.][40][41] To develop superoleophobic (extreme repellency to oils and hydrocarbons) surfaces, the need exists to induce floating liquid states, known as Cassie-Baxter [42] wetting.However, there is a limit to which each surface can successfully repel a liquid, and for surface tensions lower than that limit, Wenzel [43] (i.e., penetrating) states are observed.
In recent work, researchers have prepared a siloxane-silane coated surface using chemical vapor deposition (CVD) to enhance the dropwise condensation of low-surface-tension liquids. [44]Although the coating successfully reduced the contact angle hysteresis and enabled dropwise condensation, the surfaces remained oleophilic (contact angles well below 90°).To fabricate surfaces that can successfully repel low-surface-tension liquids, researchers have shown that specially designed re-entrant structures are required to prevent the liquid from spreading. [28,45,46]There are prior works that have employed complex micro-fabrication techniques, like lithography, [45,47,48] which are costly and not scalable.[51][52][53][54] FD-POSS [(1H,1H,2H,2Hheptadecafluorodecyl) 8 Si 8 O 12 ], a crystalline material with a solidair surface energy of 10 mN m −1 has been developed at the Air Force Research Laboratory. [55]This compound is known as fluoroalkyl-functionalized polyhedral oligomeric silsesquioxane (POSS).POSS molecules belong to a class of hybrid organicinorganic compounds that consist of a cage-like structure made of silicon and oxygen atoms.Each POSS cage contains several Si-O-Si units, and the cage can be functionalized with various organic groups to modify its properties.In the present case, the POSS molecule has fluoroalkyl functionality on the periphery (corners of the cage) functionalized with fluorinecontaining alkyl groups.This modification can make the POSS molecule more compatible with blended polymers, which may contain fluorine or other polar functional groups. [56]In this work, the compound used, FD-POSS, is soluble in fluorinated solvents. [55]FD-POSS has been used in prior works as a component in coating formulations on solids, [57,58] meshes, [59,60] and fabrics [61][62][63][64][65] alike.Also, other studies have used harmful solvents, like Asahiklin AK-225, [53,66,67] which have been banned commercially.In an earlier study from our group, a coating using small quantities of FD-POSS was formulated and was shown to stop the spreading of liquids with surface tension as low as 23.39 mN m −1 (decane). [68]That coating formulation used fluoroalkylsilane-modified silica nanoparticles (FS-nm) only as filler.Although in that study the lowest surface-tension liquid (decane) exhibited a high static contact angle, the fluid did wick into the superoleophobic regions during the liquid transport experiments.
In this work, we present a simple technique to fabricate a superoleophobic coating that can be sprayed onto a rough surface to repel liquids of extremely low surface tension.These coatings could successfully repel high surface-tension liquids like water (superhydrophobic) as well as low surface-tension organic fluids/oils, like kerosene and octane.In addition, these coated surfaces could successfully inhibit the spreading of even lower surface-tension liquids, such as heptane ( = 19.74mN m −1 ), highlighting their robust repellent capabilities.This coating formulation will be referred to as fluorinated superoleophobic coating.The formulation of the coating is composed of fluoroalkylsilane-modified silica nanoparticles (FSnm), fluoroalkylsilane-modified silica microparticles (FS-μm), very small quantities of FD-POSS (FD), and a perfluoroalkyl methacrylate copolymer (PMC).The fluorinated silica particles, FS-μm and FS-nm, are used to impart surface roughness and reentrant surface texture to the substrate, PMC acts as a polymeric binder, and the FD-POSS as a trace filler that enhances the su-peroleophobicity of the coating.This fluorinated superoleophobic coating can be applied by spray on any rough surface to render it superoleophobic.
To demonstrate a microfluidic application, the present coating formulation is used to study the pumpless liquid transport of low-surface-tension liquids using wettability-patterned surfaces.The physical principle behind the pumpless liquid transport on a wedge-shaped wettable track laid in superphobic surroundings is well studied. [69]We use low-surface-tension liquids and compare their transport characteristics against high-surface-tension liquids, like water, which are commonly studied.The surfaces are prepared by creating wettability-patterned wedge-shaped tracks selectively applied via laser ablation/etching to create them in the coated superoleophobic background.Droplet transport events are studied using these wedge-shaped tracks.We measure the distance traveled and the corresponding transport velocities of the different liquids, especially those with low surface tension, which are more difficult to contain.Various liquids are chosen to explore the effect of fluid properties, like surface tension (), and viscosity ().Water is used as a baseline and typical lowsurface-tension liquids, like kerosene, decane, octane, and heptane are also studied.Single drops of these liquids are deposited and get transported along the wettability-patterned wedge-shaped track.The liquid transport velocities on these tracks decline with decreasing surface tension and/or increasing viscosity.The recorded velocities are used to understand the effect of liquid properties on droplet transport when everything else is kept constant.The novelty of this work lies in the finding of a simple coating that can completely repel liquids with surface tension down to 21.14 mN m −1 and also successfully arrest the spreading of liquids with even lower surface tension (19.74 mN m −1 ).The insights from this study should facilitate the design of engineering devices that can advance applications like lab-on-chip diagnostics, aerospace propulsion, and oil-water separation to prevent environmental pollution.

Formulation and Repellency of the Superoleophobic Coatings
Fluorinated silica particles -both micro-and nano-sized, were used as filler materials along with a fluoro acrylic copolymer solution, i.e., perfluoroalkyl methacrylate copolymer (PMC), commercial name-Capstone ST100, which acted as a binder, and FD-POSS.Figure 1 shows the sample coating preparation steps.The preparation of each chemical component and the coating formulation procedure are shown in Figure 1a-d.The methodology is explained in detail in the "Experimental Section" of the paper.The different coating formulations selected for experimentation were designated using a design of experiments (DOE) procedure (JMP software) that varied the ratios of the coating ingredients until the best-performing coating was attained.To reduce the complexity of the coating formulation procedure and the parameters analyzed in this research, the solid content of all dispersions that were spray-coated on the surfaces was fixed at 4.57%.The solid/solution (wt.) % of the sprayed dispersion was kept low to ensure sprayability (no nozzle clogging).If M FS − nm and M FS − μm denote the respective masses of the FS nano-and micro-particles, and M PMC the mass of PMC in the coating formulation, we define the silica mass fraction as In all tested formulations, the content of fluorinated silica microand nano-particles was equal, i.e., M FS − μm = M FS − nm .The mass fraction Y in the coating that best repelled the liquids of interest was 0.93.FD-POSS was the component with the lowest surface energy and contributed the most to the liquid repellency of the coatings.If M FD denotes the mass of FD-POSS in the dispersion, the corresponding FD:silica mass fraction was also varied and was found to be 1/10 for optimal repellency.The optimum coating formulation (see "Experimental Section" for details) was then spray-coated onto the laseretched aluminum substrates.These coated surfaces could successfully repel octane (21.14 mN m −1 ) and also hinder the spreading of heptane (19.74 mN m −1 ).We will delve into a detailed Table 1.Properties of test liquids at 25°C. [70]quid examination of the repellency of the coated surfaces in the subsequent section.
The properties of the test liquids are listed in Table 1.The capillary length for each fluid, defined as l cap = (/g) 1/2 , where  is the density of the liquid and g is the gravitational acceleration, is also listed in Table 1.Droplet sizes below l cap are not influenced by gravity.The repellency towards each liquid was evaluated by contact angle measurements of droplets that had diameters below the capillary length of each fluid.Contact angles were determined using a custom-made goniometer and were the average of two measurements on each of three different substrates (six readings) prepared in the same way.Figure 2 shows the contact angles of the test liquids on the best-performing fluorinated superoleophobic coating, along with the respective images of the beaded droplets.A higher contact angle indicates lower wetting and better repellency.As seen from the bar graph, the contact angle (CA) declines with decreasing surface tension to reduce the interfacial energy between liquid and solid, as expected.The static contact angles  were 167.2°, 164.9°, 146.9°, 144.3°, and 141.1°for water, kerosene, decane, octane, and heptane, respectively.The advancing and receding contact angles were also measured using the same goniometer setup and were averaged from three separate samples (six separate measurements).Superpho-  bicity or repellency of a surface is often defined by high static contact angle and low contact angle hysteresis i.e., <10°.Table 2 shows all the static and dynamic contact angles along with the corresponding contact angle hysteresis (CAH).As seen there, CAH <6 °for all liquids except heptane.Thus, the coated surfaces could successfully repel all liquids with surface tension down to 21.14 mN m −1 .But in the case of heptane, although the droplets exhibited a very high static and advancing contact angle, the liquid was completely pinned to the surface.The receding contact angle for the heptane droplet on the POSS-coated surfaces could not be measured, thereby exhibiting a very high contact angle hysteresis.This is due to the extremely small droplet volume and low surface tension of heptane, facilitating the partial entry into the pores and crevices, while adhering to the surface textures.
In other words, the present surfaces hindered the spreading of heptane (see heptane droplet beading and exhibiting high contact angle in the top right image of Figure 2).This best-performing superoleophobic coating formulation was subsequently used to lay out wettability-confined tracks for rapidly transporting low-surface-tension liquids with an eye on microfluidic applications.Both wettable and non-wettable spatial domains were needed for this demonstration, which were characterized as described below.

Surface Characterization
Wettability patterning on aluminum substrates coated with the present superoleophobic coatings was carried out by selectively laser-ablating sections to make the wedge-shaped superoleophilic tracks, as shown in Figure 3a,b.The dimensions of the wedgeshaped pattern affect the transport velocities of the droplets as the liquid spreads from the narrow to the wide end of the track, driven by Laplace pressure gradients developed from the shape assumed by the droplet on the non-uniform width track.Thus, to study the effect of liquid properties on the transport velocities, the track pattern was fixed and kept the same for all tests, see Figure 3b.The wedge track dimensions are also shown in Figure 3b, where the narrowest track width  = 500 μm, the wedge angle  = 4°, and the total track length L = 30 mm.
The sample surfaces were characterized physically and chemically to illuminate the fluid transport mechanisms promoted by the wettability-confined tracks.The chemical properties of the superoleophobic and superoleophilic areas were examined using X-ray photoelectron spectroscopy (XPS), while the physical surface attributes were evaluated using optical profilometry and scanning electron microscopy (SEM), as shown in Figure 3c-e.The XPS characterization was done on smaller samples (5×5 mm) due to the instrument's sample size constraints.Characterization was performed both on the laser-ablated and the coated sections, separately.
Fluorinated compounds have low surface energies. [71]Thus, for a surface to repel low-surface-tension liquids, it must contain an abundance of fluorine.Figure 3c depicts the XPS survey spectra of the coated regions of the substrates, which exhibit a high concentration of fluorine, as compared to other elements.The F1s peak (688 eV) confirms the presence of fluorine in these regions.But in the laser-ablated region, the F1s peak is nearly nonexistent when compared to the coated section (Figure 3c).This is because laser ablation removes the fluorinated coating and exposes the underlying aluminum, as seen from the pronounced Al peaks in the XPS spectrum.The elemental compositions of both the superoleophobic and the superoleophilic regions are shown in Table 3, and further confirm the hypothesis that the fluorine is almost completely removed by laser etching, while it exists at high concentration in the coated regions.According to the data, low concentrations of Carbon (14.24%) and Fluorine (4.48%) were left after the laser ablation step.This is due to the thermal degradation of the FD-POSS and FS during the laseretching step. [68]The presence of C and F in small quantities did not affect the oleophilicity of these regions, as all liquids spread completely on the surface with very low apparent contact angles.
The contact angles of all liquids used herein could not be measured in the superoleophilic regions due to their very low values (below 5°).
As mentioned already, the low surface energy of the fluorinated compound (FD-POSS) plays a vital role in the repellency of the coatings.But apart from the low surface energy, surface roughness and texture are also vital factors in designating the repellency of the surface, especially when low-surfacetension liquids -like octane and heptane-are tested. [27,72]Initially, we added just the nano-sized fluorinated silica (FS-nm) to impart nanoscale roughness to the surface.However, the liquids with surface tension below 23 mN m −1 (octane and heptane) would creep in between the roughness features and wet the entire substrate.After adding the micron-sized fluorinated silica particles (FS-μm) to the composition, the nano-and micronsized FS arrange themselves to prevent the liquid from entering into the crevices by forming hierarchical re-entrant structures at the surface.Thus, to understand the physical characteristic differences of both the laser-ablated superoleophilic and coated superoleophobic regions, SEM and profilometry were conducted on these surfaces.Figure 3d,e shows the morphological and topographical features of the coated and laser-etched regions.Figure 3d-1,2 shows the surface roughness of the coated regions, i.e. the FS particles bound by the PMC, while Figure 3e-1,2 shows the surface after it has been laser-ablated.The SEMs show the physical attributes of the coated and laser-ablated regions of the surface.The coated sections (Figure 3d-1) feature nanocomposite FS particle clusters, while the laser-ablated sections (Figure 3e-1) contain no FS particles, which have been removed by the laser, thus exposing the underlying laser-etched rough Al surface.Surface roughness affects the wettability of both (coated and laserablated) regions.The average surface roughness, as measured by the optical profilometer and quantified using R a , is approximately 13.96 μm for the coated regions, and 15.95 μm for the laserablated regions.Although quantitatively the surface roughness values are not far apart, the surface roughness of the superoleophobic domains is imparted by the micron-and nano-sized FS particles, while the roughness of the superoleophilic sections is decided by the characteristic roughness imposed by the laserablation step.The height differential between the two regions (the laser removes material, thus leaving the exposed area under the coating) was measured using an optical profilometer (Bruker-Nano Contour GT-K).This difference varied between 80-130 μm (refer to Supporting Information for details), imposing a small potential-energy differential for the liquids to overcome when moving in the track.
In the following, we demonstrate the functionality of the present patterned surfaces by placing liquid microdroplets on wettability-confined tracks.

Capillary-Driven Fluid Transport in the Wedge-Shaped Tracks
Liquid transport on the wedge-shaped superoleophilic tracks was studied for the different low-surface-tension liquids and then compared with water.Transport rates depend on multiple factors, [69,[73][74][75] such as the wettability contrast between the wettable track and the surrounding repellent region, the liquid properties (surface tension and viscosity), [74] the surface roughness, [75] and the track geometry. [69]When the different liquid drops are deposited on the narrower side of the track, each droplet is restricted by the track boundary, being confined within the superoleophilic area bounded by the superoleophobic background.Ghosh et al. [69] showed that the liquid dispensed at the narrower end of the track gets transported to the wider end due to the driving capillary (Laplace) pressure differential generated due to the confined liquid's shape in the wedge-shaped track.
In the present experiments, all factors affecting the liquid transport on the wedge-shaped tracks, namely, the track geometry and the substrate were kept fixed except for the liquid properties.We tested five different liquids with varying surface tension and viscosity (see Table 1, where all liquids and their corresponding properties are listed) to study the effect on liquid transport along a superoleophilic wedge-shaped track surrounded by a superoleophobic background.As expected, higher surface tension  and lower viscosity  lead to higher transport velocity, as seen in Figure 4.The liquid transport velocities were calculated from the highspeed videos captured, as explained in the "Experimental Section" of the paper.The leading edge of the liquid was tracked as it progressed from the narrow to the wider end of the superoleophilic track from the instant the liquid drop was dispensed gently at the narrower end (start of the track).Figure 5a shows the distance traveled by the liquid over a period of 2 s.The instantaneous liquid-front velocities in the wedge-shaped track are plotted in Figure 5b.As seen there, the maximum transport velocities for the initially dry wedge-shaped track are 204.9,137.8, 181.7, 200.9, and 192.2 mm s −1 for water, kerosene, decane, octane, and heptane, respectively.High surface tension (driving force) and low viscosity (resistive force) result in higher transport velocities.The data for the highest surface-tension liquid (water) was used as a baseline.For the low surface-tension organic liquids, the transport speeds changed significantly.Kerosene has a lower surface tension and higher viscosity than water (up to 2.7×), which results in lower transport speed when the Laplace pressure is strongest (early).Decane and water have similar viscosities, but due to the lower surface tension of decane, its transport velocity is lower than water's.Interestingly, in the case of heptane and octane, although both have much lower surface tension than water, the lower viscosity of both these liquids in comparison with water balances the low driving surface tension forces.Thus, both heptane and octane travel with maximum transport velocities almost equal to water's in the early stages.
The present results are in agreement with Sen et al., [74] who reported that the liquid transport velocity in wettability-confined wedge tracks scales as where x is the distance traveled by the spreading liquid front, and h is the height of the spreading liquid in the track.The spreading behavior of these low-surface-tension liquids is not consistent throughout due to the differences in the volume of a water droplet and that of each low-surface-tension liquid.The former is higher than the volume of a single low-surface tension liquid droplet used in these experiments (almost double, see Table 1).As a droplet begins to spread, the height of the low-surface-tension liquids declines much faster than water due to the corresponding smaller droplet volume, resulting in lower droplet transport speed (V ∼ h 3 ) as the spreading progresses along the wedgeshaped track.As a result, although the transport velocities of octane and heptane are initially comparable to water's, they decline much faster than water in the subsequent stages of spreading due to the rapid reduction of the liquid height (h) along the track.
It is important to note that liquid transport was studied here only on dry tracks and not on liquid pre-suffused tracks.The latter was avoided, as the tracking of the liquid front on the wettable path is not clear in an imaging sense.Nonetheless, the liquid transport velocity was higher on liquid pre-suffused tracks due to the slippage on the pre-existing liquid film and the reduction of the solid-liquid frictional forces that resist fluid advancement.

Conclusion
We have developed a simple and facile approach to sprayable superoleophobic (and naturally, superhydrophobic) coatings that can repel organic liquids with surface tension as low as 21.14 mN m −1 and also effectively arrest the spreading of heptane (19.74 mN m −1 ), a record insofar as approaches that use commercially available solvents are concerned.The superoleophobic coating is formed by combining appropriate amounts of nanoand micron-sized fluorinated silica powders, an extremely lowsurface-energy fluorinated compound (fluorinated polyhedral oligomeric silsesquioxane, FD-POSS), and a polymeric binder, perfluoroalkyl methacrylate copolymer (PMC).Liquid repellency is optimized by varying the fluorinated ingredients until the bestperforming formulation is found.The repellency of these coatings improved with higher FD-POSS content and reached an optimum amount.In a demonstration of functionality, these coated superoleophobic surfaces were used to facilitate droplet transport dynamics on open surfaces exemplifying microfluidic handling applications.Laser etching was performed on coated rough aluminum substrates to create wettability-patterned superoleophilic wedge-shaped tracks laid in a superoleophobic background.The dependence of droplet transport dynamics on varying liquid properties, i.e., surface tension and viscosity, was further investigated on these wettability-patterned tracks.Water was used as the baseline fluid, and comparisons were made against four low-surface tension organic liquids, namely, kerosene, decane, octane, and heptane.The maximum velocities of single droplets on initially dry tracks reached 205, 138, 182, 201, and 192 mm s −1 for water, kerosene, decane, octane, and heptane, respectively.As expected, higher liquid surface tension and lower viscosity led to higher transport velocity along the track.
This study advances the understanding and science of extreme liquid repellency and liquid microvolume management through surface chemistry modification.The work has the potential to facilitate further improvements towards the production of high-performing, scalable coatings that could repel and/or hinder the spreading of even lower surface-tension liquids, like refrigerants, whose surface tension may drop well below 10 mN m −1 .Such a development will be immensely beneficial for the industry.

Experimental Section
Materials:: Aluminum (Al) plates (mirror-finish Al 6061) were supplied by McMaster-Carr.The chemicals used in the experiments were sourced from different suppliers: anhydrous ethanol (200 proof) from Decon Laboratories, Decofluoropentane (VXF, a.k.a.Vertrel XF) from TMC Industries, Ammonium hydroxide (28-30% NH 3 in water) from Alfa Aesar, PMC copolymer dispersion (20% in water, commercial name-Capstone ST-100) from DuPont, 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (FAS, 97%), silicon dioxide nanoparticles (SiO 2 , 5-15 nm), and silicon dioxide microparticles (SiO 2 , 0.5-10 μm) from Sigma-Aldrich.The FD-POSS compound was synthesized and supplied by the Air Force Research Laboratory, Aerospace Systems Directorate (Rocket Propulsion Division, Edwards AFB). [55]reparation of Fluoroalkylsilane-Functionalized Silica Particles:: The fluoroalkylsilane-functionalized silica nanoparticles (FS-nm) and silica microparticles (FS-μm) were used to impart surface roughness.The FS-nm added a layer of roughness to the FS-m, which prevented low-surfacetension liquids -like octane and heptane-from seeping into the coating surface features.The silica micro and nano-particles were functionalized using a similar method as reported in Refs.[68, 76] (see Figure 1a).Both the micron-and nano-sized SiO 2 powders were prepared in separate vials following the same procedure.Ethanol (40 g) was added to the SiO 2 powder (0.67 g) [μm or nm sized particles] and mildly stirred (100 rpm) using a magnetic stirrer plate for 5 min.Ammonium hydroxide (3.5 g) was slowly mixed in with the Ethanol-SiO 2 dispersion and stirred continuously (300 rpm) for 5 min.Lastly, 1H,1H,2H,2H-Perfluorodecyltriethoxysilane i.e., FAS, 97% (0.35 g) was added and the solution was stirred continuously at 500 rpm for 1 h.Once the solution was stirred and mixed, the excess solvent i.e., ethanol/ammonium hydroxide was evaporated off in a chemical hood by gently heating the mixture at 35°C.The final residue obtained after evaporation was ultimately either the fluoroalkylsilane-functionalized silica nano-(FS-nm) or micro-(FS-μm) powder.
Preparation of PMC in Vertrel-XF Solution:: PMC was provided in a water-based solution (20% wt.).Fluorinated compounds are usually compatible with fluorinated solvents.FS and FD-POSS are incompatible with water-based solvents, so to ensure compatibility, the PMC was redispersed in VXF, a fluorinated solvent.The preparation of PMC in VXF is shown in Figure 1b.3.13 g of aqueous PMC (20% wt.) was poured into a polyurethane bottle (Thermo Scientific) and heated mildly at 35°C until all the residual water evaporated and only a hard, dry PMC film was left in the bottle.An appropriate amount of VXF (124.38 g) was added to the container to make a 0.5% wt.solution of PMC in VXF.The solution was then bath-sonicated for approximately 90 min until the PMC was completely dissolved in VXF to make a homogeneous solution.
Preparation of the Coating Dispersion:: After the FS-nm (filler, component 1 of Figure 1), FS-μm (filler, component 2 of Figure 1), and PMC-VXF solution (binder, component 3 of Figure 1) were prepared, appropriate amounts of FD-POSS (additive, component 4 of Figure 1) were combined with the appropriate amounts of FS-nm, FS-μm, and the PMC-VXF (see Figure 1d).9 g of PMC-VXF solution was added to the dry fluorinated silica particles, FS-μm (0.3 g) and FS-nm (0.3 g), and then additional VXF (5.75 g) was also added to make a formulation that could be easily sprayed.FD-POSS (60 mg) was then added to the mixture and after all the components of the coating had been added, the mixture was bath-sonicated (8891 Ultrasonic Cleaner, 2.5 gallons; Cole-Parmer) for 15 min to produce a homogeneous coating.
Sample Fabrication:: Aluminum samples (3×1 inch) were used for the experiments.The Al plates were first cleaned and rinsed with both ethanol (200 proof, Decon Labs), and DI water, and then dried with nitrogen.To make the surface rough, a Yb laser (90% power, 5 kHz pulse frequency, 200 mm s −1 rastering speed EMS300, Tykma Electrox) was used to etch away the top of the metal and impart roughness to the surface (see Figure 1e).The laser-etched Al plates were then again cleaned and rinsed with ethanol and DI water, and dried to remove metal dust and residue.The coating dispersion was then sprayed onto two 3"×1" rough laser-etched aluminum substrates laid side-by-side on a 3"×2" area using an airbrush sprayer (TS#3L, siphon fed; Paasche) from a distance of ≈10 cm and pressure 30 psi (see Figure 1f).Typically, 15.55 g was sprayed onto two 3″ × 1″ samples.All samples were dried at ambient room temperature for ≈4 h before performing any contact angle experiments.
For the velocity measurement and to make the wedge-shaped pattern (i.e., render those areas superoleophilic), laser etching was performed on the coated surfaces.A geometric pattern of the wedge-shaped track was uploaded to the laser marking system (EMS400; TYKMA) as a vector image and then the Yb laser (90% power, 10 kHz pulse frequency, 200 mm s −1 rastering speed EMS300, Tykma Electrox) was used to selectively etch away the coatings to render those areas superoleophilic (see Figure 3a).The wedge-shaped track inscribed on the fluorinated superoleophobic coating had L = 30 mm,  = 4°, and  = 500 μm (see Figure 3b).The laser etching was done in both directions, i.e., the beam raster direction was first done orthogonal to the track and then along the wedge-shaped track.This was done to prevent any preferred directionality in the liquid transport.
Surface Characterization:: Sessile-droplet and dynamic contact angles quantified the repellency of the fluorinated superoleophobic coated surfaces to the different liquids.An in-house goniometer was used to measure both the sessile droplet and the dynamic contact angles following the same method as in Ref. [77, 78].The liquid droplets for all contact angle measurements were kept below the capillary length.Optical profilometry was done using the Bruker-Nano Contour GT-K Optical Profilometer (10x objective) to map the topography of the coated and the laser-ablated surfaces.The roughness values were measured using the Vision 64 software.Scanning electron microscopy was performed on both the coated and the laser-ablated regions using the JEOL JSM-IT500HR SEM (5 kV accelerating voltage).Prior to SEM imaging, the samples were coated with a thin layer of gold (5-10 nm) using the Technics Hummer Model V Sputter Coater to prevent the accumulation of static electric charges.X-ray Photon Spectroscopy (XPS) was carried out by a Kratos Axis-165 instrument equipped with a monochromated Al K source and a charge neutralizing coil.The spot size was 700 × 1000 μm.The XPS testing was done on smaller samples (5 mm × 5 mm) due to instrument size constraints.
Imaging of Liquid Transport:: Transport of the liquid droplets was imaged using a high-speed camera (Phantom M310; Vision Research) at 500 frames per second (fps).The setup was illuminated using a cold light source (Rosco Litepad Axiom, 6 × 6'', Daylight).The liquid front was tracked every 2 ms using the ImageJ software, the Phantom Camera software, and an in-house MATLAB script.For each run, a single drop of liquid was dispensed onto the narrow end of the wedge-shaped track and the location of the leading edge of the liquid was tracked as it advanced through the superoleophilic wedge-shaped track.The liquid was dispensed through a syringe, whose tip (ID 100 μm) was strategically placed over the narrower side of the track.The liquid was dispensed very slowly and contacted the track at a very low velocity, minimizing the impact of inertial forces as compared to the capillary force.Subsequently, the liquid was transported pumplessly along the wedge-shaped track due to the dominant driving capillary force.All liquids were dispensed using glass syringes and chemical-resistant tubing manually, using the same syringe tip size.For each run, three experiments were completed, and data was averaged to determine the location of the liquid front.For each experimental run, a new patterned surface was used.Also, the components of the experiments used, i.e., tubing, glass syringe, and syringe tip were replaced before a new liquid was used to avoid cross-mixing and contamination of the liquids.

Figure 1 .
Figure 1.Preparation steps of the fluorinated superoleophobic coated substrates: a) Fluorination of silica micro-and nano-particles, b) dispersion of PMC in VXF, c) schematic of the FD-POSS molecule, d) combining all the components together in a closed container by sonicating at room temperature to prepare the coating formulation, e) procedure to create rough Al by laser-etching the mirror-finished Al, f) spray deposition of the coating formulation to fabricate the superoleophobic surfaces.

Figure 2 .
Figure 2. Sessile droplet contact angles for different liquids on the coated superoleophobic substrates.The scale bar in the top right image (heptane) denotes 1mm and applies to all images.

Figure 3 .
Figure 3. Preparation and characterization of wettability-patterned (superoleophobic-superoleophilic) substrates: a) Preparation of the wettabilitypatterned superoleophilic track surrounded by superoleophobic background via selective laser ablation of the wedge-shaped design, b) schematic of patterned sample featuring a wedge-shaped track with dimensions L = 30 mm,  = 4°,  = 500 μm, c) XPS survey spectra of coated (superoleophobic, in red) and the laser-etched (superoleophilic, in blue) domains of the samples.The inset shows a magnified scan of the C1s peak.Physical characterization of the coated (d) and the laser-etched (e) portions of the substrates: 1) scanning electron micrographs, 2) optical profilometry maps.The average surface roughness (R a ) for each case is listed at the top of the optical profile maps (d-2, e-2).

Figure 4 .
Figure 4. Liquid-front tracking (red arrow) as each fluid progresses on the wedge-shaped wettable track surrounded by the superoleophobic coating developed in the present work.Sequences for five different liquids are shown over a duration of 2 s, starting from the instant (time zero) when a droplet of the corresponding liquid was gently placed at the left (narrow) end of the track.The scale bars in all cases denote 5 cm.

Figure 5 .
Figure 5. Liquid spreading curves of five different liquids on the wettability-patterned wedge-shaped tracks: a) Distance traveled by the spreading liquid front (x) versus time, b) Liquid-front transport velocity (V) versus time.

Table 2 .
Static contact angles, dynamic contact angles and contact angle hysteresis for all test liquids on the best-performing superoleophobic coating.

Table 3 .
Elemental composition (%) of various elements as determined from XPS.