Superhydrophobicity from the Inside - Langmuir (ACS Publications)


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Superhydrophobicity from the Inside Tomer Simovich, Cameron Ritchie, George Belev, and Robert Norman Lamb Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01350 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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Superhydrophobicity from the Inside Tomer Simovich a*, Cameron Ritchie a,b, George Belev c, Robert N. Lamb a,c a

School of Chemistry, University of Melbourne, Parkville, 3010, Victoria, Australia b

Bio21 Institute, University of Melbourne, Parkville 3010, Victoria, Australia c

Canadian Light Source (CLS), Saskatoon, SK, Canada

Keywords: superhydrophobic,

micro-tomography,

nanostructure, interface, air-water

interaction. ABSTRACT The nature of trapped air on submersed ultra-water-repellent interfaces has been investigated. These gaseous layers (plastrons) can last from hours to, in some examples such as the Salvinia molesta fern, months. The interface of submerged superhydrophobic surfaces with carefully controlled micro-patterned surface roughness has been probed using synchrotron based high resolution X-ray phase tomography. This technique looks in situ, through the aqueous/gas interface in three dimensions. Long term plastron stability appears to correlate with the appearance of scattered micro-droplets less than 20 µm in diameter that are sandwiched within the 30 µm thick gaseous interfacial layer. These micro-droplets are centered on defects or damaged sections within the substrate surface approximately 20-50 µm apart. Such irregularities represent heterogeneous micro/nano hierarchical structures with varying surface structures and chemistry. The stability of micro-droplets is governed by a combination of electrostatic repulsion, contact angle limitations and a saturated vapor pressure, the latter of which reduces the rate of diffusion of gas out of the air-layer thus

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increasing underwater longevity. Homogenous surfaces exhibiting purely nano or micro regularity do not support such micro-droplets and as a consequence, plastrons can disappear in less than 20 hours compared to greater than 160 hours for surfaces with scattered microdroplets. Such behavior may be a requirement for long term non wetting in any system. INTRODUCTION Non-stick, water repellent coatings are the result of a combination of chemical hydrophobicity and multiscale surface roughness. Significant interest stems from their inherent self-cleaning1, drag reduction properties2, 3, and more recently, marine antifouling4 capabilities. The fabrication and characterization of superhydrophobic coatings have been thoroughly explored in the literature, while the air-retaining abilities of submerged superhydrophobic surfaces have been given less attention due to the lack of performance standards and measurement techniques. The underwater longevity of superhydrophobic surfaces is related to the lifetime of the entrapped air layer. The stability of air layers is not linked with water-surface contact angle because two separate systems exist; a static instantaneous measurement for contact angle, and a dynamic, shifting system for air layer longevity where the shape of the entrapped air layer changes over time. The gaseous layer present between structural features of the surface topography continuously dissipates by dissolving into the surrounding liquid via diffusion. The rate of diffusion is primarily related to the gas pressure inside the air layer and the liquid-gas interfacial area. Thus, it is important to address the shape of the plastron as well as its constituents. Superhydrophobic surfaces consist of a rough topography usually involving protrusions and porous structures to reduce the solid-liquid contact area and minimize liquid adhesion. A range of surface structures may be utilized in order to achieve high superhydrophobic contact angle but many result in increased liquid-air interfacial area due to heterogeneous roughness. Hierarchical structures are known to produce ideal superhydrophobic character and low hysteresis5 but their effect on underwater longevity is poorly studied. When immersed in water, a heterogeneous nano-

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rough surface gives rise to high liquid-gas interfacial area. Although the liquid-solid contact area is small, there are many points of contact and they may vary in both height and distance leading to a ‘crumpled’ liquid-gas interface. On the other hand, a surface with homogenous, repeating structures of the same height may lead to a smoother ‘wavy’ liquid-gas interface. An important factor is that the shape of the plastron also changes with time; initially the air layer is convex due to an overpressure inside the plastron and buoyancy of the air layer underwater6, 7. As air diffuses out and the pressure equalizes, the air layer shifts to a concave shape8,

9

as the liquid-solid contact line progresses. As the plastron volume changes, the

pressure will remain constant because the loss of gas due to diffusion results in a reduction in volume. Therefore during the over-arching macro-plastron stage, the pressure in the air layer is determined by the hydrostatic pressure. The curvature of the air-liquid interface is important in determining the solvation of the gas into the water and therefore controls the lifetime of the plastron underwater. A combination of surface morphologies can improve longevity of the plastron by preventing the gradual increase in curvature throughout plastron dissipation. The current limitation surrounding the design and fabrication of better coatings involves the depletion of this air layer and nature still has the upper hand through millennia of optimization. The Salvinia fern and lettuce leaf (Pistia stratiotes) can withstand wetting for up to several days6. Due to this, the longevity of superhydrophobic coatings has been a highlight of recent literature9,

10, 11

. In this paper, we utilize synchrotron x-ray imaging to

elucidate a possible cause of increased plastron longevity on pillared microstructures and discuss what possible surface properties give rise to the micro-droplet phenomenon. EXPERIMENTAL Micro-pillared silicon substrates were prepared via photolithography using an EVG6200 through inductively coupled plasma etching at the Melbourne Centre for Nanofabrication (MCN). The surfaces were designed with a range of pillar widths and spacing with a height of ≈ 15 µm (Figure 1). Substrates were then spray coated with a silica

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nanoparticle sol-gel solution as described in Arnott et al.4 before half were coated with C4F8 vapor deposition to ensure consistent hydrophobic chemistry using an Oxford ICP PlasmaLab100, 150 sccm, 10ºC baseplate temperature for 30 seconds.

Figure 1. (A) Smooth homogenous nanoparticle coating (scale bar 5 µm) and (B) heterogeneous micro-pillared nanoparticle coating with highlighted (green) smooth silicon defect (scale bar 5 µm). 1x1 mm samples of pillared surfaces and lettuce leaves (Pistia statiotes) were fixed onto dental wax and submerged in distilled water (approx. 1 mL) for 2 hours prior to microcomputed tomography imaging in order to equilibrate any dissolved gases with the atmosphere (BMIT-ID, Canadian Light Source, Saskatchewan, Canada) (See Figure 2 for schematic). Scans were performed at atmospheric pressure and at 25ºC. The sample tube was 5mm in diameter and the water level was approximately 13 mm. For each sample a total of 600 micro-tomographic slices were acquired covering a total 3.6 mm x 2.4 mm field of view in order to image the entire surface of the water-sample interface throughout the sample rotation. Total examination time per sample was approximately 2 hours for a voxel size of 0.9 µm3 (X-ray energy = 30 keV, detector to sample distance = 160 mm, distance to source = 55 m). The system was presumed stable with minimal high energy x-ray interactions demonstrating no visible change in bubble morphology2.

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Figure 2. Schematic illustrating sample mounting and scanning procedure (straw diameter = 5mm). Reconstruction of tomographic slices was carried out using NRecon (Bruker Micro-CT) and 3-dimensional images were rendered in Amira ResolveRT (FEI, Thermo Fisher Scientific) software. Electrostatic force calculations were conducted using Mathematica® 9.0 For wetting studies samples were submerged in 10 cm of distilled water and time lapse images were recorded through a 500x USB digital microscope captured at a 45 º angle to the surface. Contact angle measurements were obtained using a Ramé-hart Inc. goniometer with 15 µL droplets. RESULTS & DISCUSSION Hierarchically rough superhydrophobic coatings exhibited longlived plastrons when submerged underwater lasting up to one week. The fabricated surfaces consisted of naturally occurring surface defects due to the spraying process; where smooth silicon pillars were exposed under a layer of nano-rough hydrophobized sol-gel. In fluorinated samples, removal of the nanoparticle coating due to damage caused the exposed silicon wafer to act as a hydrophilic pinning point both structurally (very smooth) and chemically. A closer look at the interface, where liquid, solid and gas meet, revealed a scattered array of metastable micro-droplets ranging from 6 to 20 µm in diameter in the air layer (Figure 3 & 4). They may form on surface defects in two ways; during immersion water gets attached to pinning points while the surrounding water gets pushed away by the

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hydrophobic environment, or smooth hydrophilic pinning points act as sources for capillary condensation12. Due to the long scan times the initial formation of the micro-droplets cannot be captured, samples were only imaged hours after preparation, reinforcing the presumed stability of the micro-droplets. When a larger surface structure is present, for example in Pistia stratiotes (water lettuce), micro-droplets up to 50 µm in diameter are observed to be lodged between features such as hairs. It therefore seems as the size of the micro-droplets is limited by the available area and larger droplets that form immediately coalesce with the water layer above without wetting the surface. Figure 3 illustrates two states of micro-droplet distribution on the surface; near a large surface defect and throughout a regular array of superhydrophobic pillars. For both smooth and pillared nanoparticle surfaces, the plastron is approximately 30 µm higher than the plane of the surface projection. This is due to the initial positive pressure present in the air layer created by fully submerging the sample in water7. Typically, the inter-droplet distance varies from 20-50 µm but droplets become densely packed around large defects due to more pinning points and smaller droplets were observed in areas of high micro-droplet density. It was observed that 10 µm micro-droplets exist for hours while sitting 5 µm away from each other, supporting the stability and repulsion theories explored below. Since the micro-droplets do not coalesce with the planar water interface above the plastron and remain stable during imaging for an extended period of time13, 14, 15, it is theorized that they contribute to the extended longevity of the coating’s non-wetting properties by means of both maintaining a vapor saturated environment and slight electrostatic repulsion. Water droplets have been observed using confocal microscopy16 and described as facilitating wetting, but only in a system with increasing hydrostatic pressure. As the water layer is pushed against the micro-droplets, they ultimately coalesce. If the pressure forcing the wetting line is constant, the air layer depletion solely depends on gas solubility via

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diffusion. It is possible to calculate the diffusion time of gas out of a plastron. By considering the concentrations of gases near the interface and the gas exchange coefficient:

   =  ℎ Where D is the diffusion coefficient of gas in water in m2/s, and h is the height of the water layer in m. The flux can be estimated using the Ostwald solubility coefficient:

 = ×  Where Hcp is Henry solubility defined via concentration in mol/(L atm). The gas exchange coefficient may be used in the water convection model17 which takes into account a large range of factors including temperature gradients, evaporation and condensation, pressure fluctuations and currents. Simple models in literature agree that: 

 =   × (, ) where Sc is the schmidt number  =





where v is the kinematic velocity of water (0.892 ×

10# m2s-1), and f(q,L) is some turbulence function with respect to some turbulent velocity q and length scale L. f(q,L) is difficult to parameterize accurately, especially for systems with low/no wind or currents that would otherwise dominate the function, although, the range can be estimated based on the lifetime of bubbles and plastrons on submerged surfaces. Thus a dissipation time experimentally measured to be approximately 20 hours fits well with the water convection model above, resulting in water convection in the order of m/s. This value is in line with experimental longevity results of homogenous silica nanoparticle coatings that remain dry anywhere between 10 and 20 hours.

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Figure 3. Surface projection of a submerged sample including a large horizontal surface defect (Black). Water micro-droplets (in green) are scattered throughout the pillared surface and additionally clustered around the defect The vertical white artefact is a result of x-ray reflections at the scan’s center of rotation due to the air-water interface above. The survivability of discrete micro-droplets of water inside the plastron is related to their ability to resist coalescence. With very small diameters a high Laplace pressure exists inside water micro-droplets preventing them from deforming. The spherical shape inherently limits the contact area between to micro-droplets or the water layer above, creating a kinetic activation barrier to their coalescence. Additionally, at small scales the electrostatic forces caused by surface charge even in pure water become apparent. Many years of experiments on

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the mobility of bubbles in an electric field have indicated the air/water interface retains a negative charge through the evidence of measured negative zeta potentials18, 19. Molecular simulations20 have determined the density of hydroxide ions at the interface to be one in every 1000 nm2 leading to a surface charge density of 0.021 µC cm-2. Although small, the charged surface influences the repulsion of micro-droplets from the water layer above. The repulsive force between a micro-droplet and the water layer above is dependent on the droplet size due to a surface area dependence of the droplets total charge, as well as a radius dependence on the distance the individual charges interact across. Electrostatic forces between the water layer and micro-droplets were calculated in the bispherical coordinate system using analytical calculations of a dielectric sphere with surface charge density at set separation from a dielectric plane also exhibiting a surface charge density20, 21. The repulsive force for 20 µm and 50 µm micro-droplets on the water layer above were conducted at various separations to investigate all of length-scales of interest and are presented in Table 1. Relevant calculation details are summarized from Khachatourian et al.22 in Supporting Information.

Table 1. Repulsive force of a micro-droplet on a water layer at various droplet-water layer separation distances. The dielectric constant of both the micro-droplet and water layer were 78.4, the dielectric constant of the surrounds was 1 corresponding to air, the surface charge density on the micro-droplet and water layer were 0.021 µC cm-2, and the number of terms in the Legendre polynomial expansions followed Table II in Khachatourian et al.22 Micro-droplet to water layer Force from 20 µm Force from 50 µm diameter separation (µm) diameter droplet (N) droplet (N) $ 1 9.91 × 10 1.66 × 10& 5 1.20 × 10$ 2.40 × 10# 10 4.66 × 10( 1.05 × 10# 20 1.35 × 10( 2.93 × 10$ The repulsive forces calculated are significant when compared to the gravitational force applied on a 13 mm water layer over a 10 µm x 10 µm area, a typical micro-droplet spacing:

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+, = -./01 × 12345 × 6789345 = (10 × 10 ; × 13 ) × 1

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<  × 9.8  = 1.3 × 10& > =  

It should be noted however that the repulsive force does not continue to increase with decreasing micro-droplet size in the sub-micron separation regime. Instead a decrease in repulsive force occurs, which becomes an attractive force at ≈300 nm, resulting in water layer and the micro-droplet fusion22. This significant electrostatic repulsion supports the reported longevity of micro-droplets inside submerged superhydrophobic coatings. A scattered array of micro-droplets may produce a hindering effect on the slow progression of the wetting line over hours and days of surface immersion. The pressure exerted on the water layer by microdroplets can be calculated as follows using an example case: For a 20 µm diameter sphere, with a micro-droplet to water layer spacing of 20 µm, over a typical area containing a single micro-droplet of 100 µm2,

[email protected] =

[email protected] 0.135 × 10$ [>] > = = 135 F  G = 1.33 × 10= [84] & &  BC (10 × 10 )[ ] 

Where Pmd is the pressure exerted on the water layer by the micro-droplet due to electrostatic repulsion, Fmd is the electrostatic force of a single micro-droplet on the water layer, and Asp is the surface area spacing between microdroplets. The hydrostatic pressure exerted by a 13 mm water layer (present during imaging) due to gravity on the plastron is 1.26 × 10= [84] (see Supporting Information for calculations). The pressure experienced in the plastron relative to atmospheric pressure is then:

?H − [email protected] = 1.26 × 10= [84] − 1.33 × 10= [84] = −0.07 × 10= [84] This implies that there would in fact be a lower concentration of gas inside the plastron relative to the amount solubilized in the water and as such a small net influx of gas into the plastron under these conditions is expected. If the micro-droplets are stable, this system

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would reach an equilibrium in which a plastron would be present indefinitely. Other entropic factors as well as external convection and temperature fluctuations limit the microdroplet and plastron lifetime.

Figure 4. Computed Tomography reconstructions of micro-droplets (air/water interface colored blue) stabilized on micro-pillared structures (yellow/orange) inside an air layer (black) while underwater (upper black). Bottom – Tomographic slice. The existence of micro-droplets slows the wetting progression, preventing highly curved interfaces (Figure 5). Studies have indicated a characteristic ‘onset time’ followed by rapid decay7. Poetes et al. describes a decrease in reflectivity during air layer diffusion only after a long period of relative stability. While the air-layer is relatively planar, micro-droplet

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repulsion prevents wetting progression. Once decay has taken place, prompting interface curvature, Laplace pressure coupled with hydrostatic pressure overcomes any micro-droplet repulsion.

Figure 5. Schematic of micro-droplets inside the almost planar plastron highlighting saturated water vapor and electrostatic repulsion and the effects on the plastron lifetime and tomograph of plastron on nanoparticle coated pillared sample. Live water lettuce leaves (Pistia stratiotes) were successfully imaged via synchrotron X-Ray computed micro-tomography and were shown to exhibit stabilized micro-droplets trapped between superhydrophobic hairs (Figure 6). This supports the hypothesis that micro-droplets may slow the gradual collapse of the plastron and plants have evolutionarily combated this by forming regular hydrophilic pinning points located in a surrounding superhydrophobic structure6.

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Figure 6. 3-dimensional reconstruction of X-Ray tomography scans of a submerged water lettuce leaf with micro-droplets highlighted in blue. The white structures are the dry hydrophobic hairs of the leaf and the over-arching orange hue signifies the water layer above the submerged leaf. To confirm the increased stability of these coatings, wetting studies were performed, in triplicate, on (a) pillared substrates (Figure 1B) coated in nanoparticles, (b) smooth substrates (Figure 1A) coated in nanoparticles and (c) pristine fluorinated silicon pillars without nanoparticle coating. The pillared substrate (a) possesses variable structure with imperfections present as exposed smooth pillars or inconsistent nanoparticle thickness and conversely the smooth substrate (b) consists of a consistent homogenous roughness. It is

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possible to pinpoint the transformation from a superhydrophobic state to a wetted state by examining the reflectivity of the plastron present on the surface4. The pillared surfaces (a) exhibited greater longevity and remained dry after being submerged for 160 hours. Pristine silicon pillars (c) became fully wetted within 30 minutes after immersion (dry contact angle of 156º). The consistent nanoparticle coatings (b) were wet after 20 hours and upon removal demonstrated a contact angle of 54º ± 1º while the pillared surface (a) was dry and still indicated superhydrophobicity with a contact angle of 150º ± 2.4º. CONCLUSION For the first time, the existence of water micro-droplets entrapped within plastrons has been observed experimentally. These correlate with a major stabilizing effect on the plastron longevity. The repulsive effect of very small droplets has been extensively explored in literature and supports the observation that micro-drops can exist in a meta-stable state while repelling the water layer above with significant force compared to the effect of gravity. The micro-droplets additionally maintain high plastron humidity, thermodynamically reducing gas dissolution resulting in longer plastron lifetimes. Micro-droplets require irregularities on the surface to form where they can nucleate at impurities or imperfections with reduced mobility achieved by a change in surface structure or chemistry. The formation of immobile droplets through condensation has been discussed in the literature and confirms the innate stability of micro-droplets. The micro-droplet phenomenon outlined here provides a parallel pathway to water-repellent characteristics, demonstrating that nanostructure is used not only to trap air, but to trap water droplets capable of reducing wetting of a superhydrophobic surface. This provides insight into the mechanism of superhydrophobic surfaces and may aid further research by incorporating chemical and structural irregularities into the design of superhydrophobic coatings for plastron longevity in underwater applications.

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AUTHOR INFORMATION

Corresponding Author * Corresponding Author E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources The financial support of the Australian Research Council’s Discovery Projects (Project DP120104536) is gratefully acknowledged. ACKNOWLEDGMENT The authors acknowledge Assoc. Prof. David M.L. Cooper for assistance in acquiring tomographic data. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). We acknowledge travel funding provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron and funded by the Australian Government. Research described in this paper was performed on 05ID-2 beamline25 at the BMIT facility at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research.

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21. Chaplin, M. Theory vs Experiment: What is the Surface Charge of Water? 22. Khachatourian, A.; Chan, H.-K.; Stace, A. J.; Bichoutskaia, E. Electrostatic force between a charged sphere and a planar surface: A general solution for dielectric materials. The Journal of chemical physics 2014, 140 (7), 074107.

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Figure 1. (A) Smooth homogenous nanoparticle coating (scale bar 5µm) and (B) heterogeneous micropillared nanoparticle coating with highlighted (green) smooth silicon defect (scale bar 5µm). 725x243mm (96 x 96 DPI)

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Langmuir

Figure 2. Schematic illustrating sample mounting and scanning procedure (straw diameter = 5mm). 416x216mm (96 x 96 DPI)

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Figure 3. Surface projection of a submerged sample including a large horizontal surface defect (Black). Water micro-droplets (in green) are scattered throughout the pillared surface and additionally clustered around the defect The vertical white artefact is a result of x-ray reflections at the scan’s center of rotation due to the air-water interface above. 268x245mm (96 x 96 DPI)

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Langmuir

Figure 4. Computed Tomography reconstructions of micro-droplets (air/water interface colored blue) stabilized on micro-pillared structures (yellow/orange) inside an air layer (black) while underwater (upper black). Bottom – Tomographic slice. 293x259mm (96 x 96 DPI)

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Langmuir

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Schematic of micro-droplets inside the almost planar plastron highlighting saturated water vapor and electrostatic repulsion and the effects on the plastron lifetime and tomograph of plastron on nanoparticle coated pillared sample. 229x225mm (96 x 96 DPI)

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Langmuir

Figure 6. 3-dimensional reconstruction of X-Ray tomography scans of a submerged Water Lettuce leaf with micro-droplets highlighted in blue. The white structures are the dry hydrophobic hairs of the leaf and the over-arching orange hue signifies the water layer above the submerged leaf. 164x236mm (96 x 96 DPI)

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Langmuir

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TOC Figure 445x195mm (96 x 96 DPI)

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