Smooth Hydrophilic Surfaces Research Articles

Redox-Induced Lipid Vesicle Fusion Onto Electroactive Self-Assembled Monolayers

Supported lipid bilayers (SLBs) are popular model systems to study cell membrane functionalities and various biomolecular interaction forces. A key advantage of SLBs on metallic surfaces, is the application of spectroscopic, electrochemical, and surface plasmon resonance techniques for biomolecular recognition studies. The most common method for forming SLBs is vesicle fusion, which involves the adsorption, deformation, and rupture of small unilamellar vesicles (SUVs) of lipids from aqueous suspension to the substrate surface.1 The formation of continuous single bilayers by vesicle fusion can be problematic due to the many experimental parameters influencing vesicle rupture and bilayer spreading (i.e., lipid and vesicle properties, physicochemical characteristics of the surface, temperature, and solvent environment)1. Vesicle fusion typically works only with smooth hydrophilic surfaces, such as glass, silica, and mica. Although approaches have been developed to form bilayers on technologically relevant surfaces such gold (e.g., surface functionalization with hydrophilic organic films,2 solvent-assisted lipid bilayer formation3 or the addition of a vesicle-destabilizing agent4), there remains a need for active strategies that are fast, versatile, and scalable.We present a redox-induced approach for the formation of single bilayers on gold functionalized with electroactive self-assembled monolayers (SAMs) of ferrocenylalkanethiolates. The electrochemical oxidation of the SAM-bound ferrocene (Fc) to ferrocenium (Fc+) involves coupled electron transfer and ion pairing reactions. Counteranions from solution pair with the ferroceniums to stabilize the oxidized cations and neutralize the excess positive charge at the SAM/aqueous interface. The surface-confined redox reaction triggers the formation of single bilayer membranes from SUVs of anionic or zwitterionic phospholipids onto gold surfaces modified with the electroactive SAM. The ion pairing association of the charged lipid head groups with the electrogenerated ferroceniums drives the assembly of the phospholipids on the SAM surface to produce solid-supported bilayers of high surface coverage (≳ 90%) from gel- or fluid-phase forming phospholipids within minutes at room temperature. The redox-mediated strategy reported in this work is a conceptual advance in the preparation of solid-supported lipid bilayers. It is fast, insensitive to the phase state of the phospholipid in the vesicle precursor, and not limited to hydrophilic surfaces. References Richter, R. P.; Bérat, R.; Brisson, A. R., Formation of Solid-Supported Lipid Bilayers: An Integrated View. Langmuir 2006, 22 (8), 3497-3505.Silin, V. I.; Wider, H.; Woodward, J. T.; Valincius, G.; Offenhausser, A.; Plant, A. L., The Role of Surface Energy on the Formation of Hybrid Bilayer Membranes. J. Am. Chem. Soc. 2002, 124, 14676-14683.Ferhan, A. R.; Yoon, B. K.; Park, S.; Sut, T. N.; Chin, H.; Park, J. H.; Jackman, J. A.; Cho, N.-J., Solvent-Assisted Preparation of Supported Lipid Bilayers. Nat. Protoc. 2019, 14 (7), 2091-2118.Cho, N.-J.; Cho, S.-J.; Cheong, K. H.; Glenn, J. S.; Frank, C. W., Employing an Amphipathic Viral Peptide to Create a Lipid Bilayer on Au and TiO2. J. Am. Chem. Soc. 2007, 129 (33), 10050-10051. Figure 1

Fabrication of Transparent Silica/PEG Smooth Thin Coatings on Polymeric Films for Antifogging Applications.

Fog accumulation on surfaces typically has a negative effect by reducing their transparency and efficiency. Applications such as plastic packaging, agricultural films, and particularly many optical devices suffer from these negative effects. One way to prevent fogging is to coat the substrate with an antifogging coating having a smooth surface and hydrophilic surface chemical groups. This causes the fog water droplets that come into contact with the substrate to completely flatten across its surface, thus retaining transparency. These coatings are mostly relegated to laboratory research due to their insufficient stability and costly synthetic processes. We proposed the use of organically modified silica particles consisting of a mixture of tetraethyl orthosilicate and methacryloxypropyltriethoxysilane, which were grown in situ in the presence of a corona-activated polyethylene film, thus providing a thin siloxane coating containing activated double bonds. An additional coating of poly(ethylene glycol) diacrylate was then spread on the coated film and polymerized via UV curing. The in situ process and UV curing anchored the coating to the substrate through covalent bonds, which provided additional stability. This coating exhibited low surface roughness and contact angle, which resulted in excellent antifogging properties when exposed to a hot-fog test. Furthermore, the antifogging coating retained its properties after 10 hot-fog cycles, indicating the high coating stability. Additionally, the coating was found durable to immersion in aqueous pH levels 1–13 and detergent solutions as well as to tape test applications and sand test. This coating was compared to a commercially available antifogging spray, which was used to coat a polyethylene film. This resulted in excellent initial antifogging properties, which decreased after exposure to durability tests. The results of the in situ coating process indicate its potential uses for industrial applications.

Experimental Study of Pool Boiling on Heaters with Nanomodified Surfaces under Saturation

A technology has been developed for the synthesis of arrays of oriented microropes and cocoon-like microstructures consisting of silicon oxide nanowires on a copper substrate with barrier layers of silicon dioxide, molybdenum, and tungsten. In experiments, a copper surface coated with a tungsten layer showed the best stability and technological applicability. Experiments were performed to study pool boiling heat transfer on copper heaters with nanomodified surfaces under saturation conditions. A comparison of the data on heat transfer enhancement on smooth and nanomodified hydrophilic and hydrophobic copper surfaces was made. Relatively high heat transfer coefficients were observed on copper surfaces with a tungsten barrier layer initially coated with microropes and micrococoons. The experiments showed that microropes from nanowires were not resistant to boiling; micrococoons from nanowires were more stable. Superhydrophobic surfaces were produced by hot-wire chemical vapor deposition (HWCVD) of fluoropolymer coatings on micrococoons. In the boiling experiments, the greatest heat transfer enhancement was obtained on such surfaces. The fluoropolymer coating was shown to lose its continuity. As a result, a random biphilic coating was formed and intense bubble boiling occurred in local hydrophobic areas of the fluoropolymer, resulting in heat transfer enhancement.

CONDENSATION PHASE CHANGE BEHAVIORS ON A ROUGH SURFACE CHARACTERIZED BY FRACTAL CANTOR

Understanding the fundamental mechanisms of vapor condensation on rough surfaces is crucial to a wide range of industrial applications. A hybrid thermal lattice Boltzmann model of the condensation heat transfer process on downward-facing rough surfaces characterized by the Cantor fractal is developed and numerically analyzed to investigate the condensation phase change behaviors on rough hydrophobic and hydrophilic surfaces. The dynamic behaviors of vapor condensation, including the evolutions of vapor–liquid interface, heat flux, condensate mass, and temperature distribution, on the hydrophilic and hydrophobic rough surfaces are presented and compared with corresponding smooth surfaces. The results indicate that the rough surface preferred a filmwise condensation under hydrophilic conditions but a hybrid dropwise–filmwise condensation under hydrophobic conditions. On the rough hydrophobic surface, the liquid film can rapidly adsorb droplets, maintaining a high-efficiency dropwise condensation. The absorption of droplets accelerates the liquid film growth and detachment process on the rough hydrophobic surface, which reduces the time-averaged thermal resistance of the filmwise region. These two behaviors together enhance condensation heat transfer on the downward-facing rough hydrophobic surface. Besides, stable dropwise condensation could also be formed on smooth hydrophilic surfaces and has better heat transfer performance than corresponding hydrophobic surfaces under the same heat transfer condition.

Icing delay of sessile water droplets on superhydrophobic titanium alloy surfaces

Superhydrophobic surfaces have been widely considered and expected to play an important role in promoting anti-icing performance. In this research, superhydrophobic surfaces based on titanium alloy were fabricated by combining laser processing and further chemical modification. Surface morphology and wettability were characterized with scanning electron microscopy and contact angle measurement, respectively. Icing behaviors of sessile water droplets on smooth hydrophilic, smooth hydrophobic and superhydrophobic surfaces, maintained at subzero temperatures, were investigated under a humidity condition on a self-built apparatus. The effects of surface wettability, morphology and temperature on droplet icing were analyzed. Results show that droplet icing is remarkably delayed on the superhydrophobic titanium alloy surfaces through the temperature range. Decreasing surface temperature, the extended time for the cooling stage and the freezing stage in sequence dominates the total icing delay. It was also observed that the icing delay of water droplets is more obvious on the pitted superhydrophobic surface than on the grooved one, due to the larger liquid-air interface fraction. This study provides an effective approach for fabricating superhydrophobic surfaces to resist icing on titanium alloy.

Investigation of pool boiling heat transfer on hydrophilic-hydrophobic mixed surface with micro-pillars using LBM

Surface modification technology by controlling the surface wettability or applying micro/nano-structures to enhance the boiling heat transfer performance has attracted a great deal of interest in recent years. Abundant experiments were performed to investigate the boiling processes on the modified surfaces, and lots of experimental data as well as reliable conclusions were obtained. With the advantages of saving costs and time, the numerical method has been a new reliable way to investigate the bubble dynamics and heat transfer during pool boiling processes on the modified heated surfaces. Pseudopotential LB model is capable of simulating the entire boiling processes including the bubble nucleation, and this model has been successfully applied to simulate the pool boiling processes on both the smooth surfaces and the hydrophilic-hydrophobic mixed surfaces with micro-pillars. However, the numerical simulations of the boiling processes on the mixed surfaces were still rare, and the influence of geometrical parameters of pillar structures of the mixed surface on boiling heat transfer performance was still unclear yet. Moreover, some of the conclusions in existing literature were inconsistent with those in others. Thus it's necessary to carry out more numerical and experimental researches to solve these problems. In this study, the MRT pseudopotential model coupled with phase-change model was applied to simulate the pool boiling processes on the hydrophilic-hydrophobic mixed surface textured with micro-pillars. Under different wall superheats, the bubble dynamics and heat transfer during boiling processes on the mixed surface with micro-pillars were compared to those on the smooth hydrophilic surface and the hydrophilic-hydrophobic mixed surface without micro-pillars. The heat transfer enhancement mechanism of the mixed surfaces was revealed and the influence of geometrical parameters of pillars, including pillar width and pillar number, on bubble dynamics and heat transfer performance during pool boiling processes was investigated in detail.

Active effect of super-hydrophobicity on droplet nucleate boiling

It is well known that a surface with microstructures can be used to promote droplet nucleate boiling. However, the effect of the microstructure wettability on the droplet nucleate boiling is unknown. In this article, a test surface consists of two regions, one square region with microstructures is fabricated in the central area of the test surface, and another region keeps smooth. The droplets can maintain a similar shape before boiling so that the influence of droplet morphological changes on the boiling of the droplets is avoided. Surfaces with more hydrophilic and super-hydrophobic microstructures are called heterogeneous hydrophilic and heterogeneous hydrophobic surfaces, respectively. The result shows that the super-hydrophobic microstructure can promote the droplet to enter the nucleate boiling state at a lower superheat. The reason is that when the bubbles break up at the surface of the droplet, the super-hydrophobic substrate can attract part of the vapor to be trapped under the droplet. This part of the trapped bubble will continue to grow up and break up, leading the droplet entering the nucleate boiling state. Compared with the smooth hydrophilic surface, the heat transfer rate of the droplet on the heterogeneous hydrophobic surface can increase 381.25% for a superheat of 20 K.

The Effect of Surface Roughness on the Contact Line and Splashing Dynamics of Impacting Droplets

Whether a droplet splashes upon impact onto a solid is known to depend not only on the fluid properties and its speed, but also on the substrate characteristics. Past research has shown that splashing is heavily influenced by the substrate roughness. Indeed, in this manuscript, we demonstrate that splashing is ruled by the surface roughness, the splashing ratio, and the dynamic contact angle. Experiments consist of water and ethanol droplets impacting onto solid substrates with varying degrees of roughness. High speed imaging is used to extract the dynamic contact angle as a function of the spreading speed for these impacting droplets. During the spreading phase, the dynamic contact angle achieves an asymptotic maximum value, which depends on the substrate roughness and the liquid properties. We found that this maximum dynamic contact angle, together with the liquid properties, the ratio of the peak to peak roughness and the surface feature mean width, determines the splashing to no-splashing threshold. In addition, these parameters consistently differentiate the splashing behaviour of impacts onto smooth hydrophilic, hydrophobic and superhydrophobic surfaces.

Self-Cleaning Mechanism: Why Nanotexture and Hydrophobicity Matter.

Dust particles can adhere to surfaces, thereby decreasing the efficiency of diverse processes, such as light absorption by solar panels. It is well known that superhydrophobicity reduces the friction between water droplets and the surface, thus allowing water drops to slide/roll and detach (clean) particles from surfaces. However, the forces that attach and detach particles from surfaces during the self-cleaning mechanism and the effect of nanotextures on these forces are not fully understood. To shed light on these forces and the effect of nanotexture on them, we prepared four Si-based samples (relevant to solar panels): (1) smooth or (2) nanotextured hydrophilic surfaces and (3) smooth or (4) nanotextured hydrophobic surfaces. In agreement with previous publications, it is shown that the efficiency of particle removal increases with hydrophobicity. Furthermore, nanotexture enhances the hydrophobicity, whereby particle removal is further increased. Specifically, hydrophilic particle removal increased from ∼41%, from hydrophilic smooth Si wafers to 98% from superhydrophobic Si-based nanotextured surfaces. However, the reason for the increased particle removal is not low friction between the droplets and the superhydrophobic surfaces; it is the reduction of the adhesion force between the particle and the surface and the altered geometry of the water-particle-air line tension acting on the particles on superhydrophobic surfaces, which increases the force that can detach particles from the surfaces. The experimental methods we used and the criterion for particle removal we derived can be implemented to engineer self-cleaning surfaces using other surfaces and dust particles, exhibiting different chemistries and/or textures.

Dropwise condensation heat transfer on superhydrophilic-hydrophobic network hybrid surface

Superhydrophilic-hydrophobic (SHPi-HPo) network hybrid surface was designed to investigate the condensation heat transfer using stainless steel as substrate material. The SHPi-HPo surface was comprised of superhydrophilic network grooves and hydrophobic regions. Hydrophobic (HPo) surface was prepared with fluorocarbon coating using polytetrafluoroethylene (PTFE) as the matrix resin and micro-nano silicon dioxide (SiO2) as additive to control surface roughness. Three kinds of SHPi-HPo surfaces were tested, having a grid spacing of 1.5, 2.5 and 3.5 mm and named as SHPi-HPo-1, SHPi-HPo-2 and SHPi-HPo-3, respectively. To study the effects of the wall subcooling, steam mass flux, cooling water temperature, cooling water mass flow rate and grid spacing, a series of experiments were conducted and a high speed camera was used to visualize the condensation process. The results show that SHPi-HPo surface can well control condensate droplet diameters and its condensation heat transfer performance is better than that of smooth hydrophilic (HPi) and HPo surfaces. This is attributed to SHPi-HPo surface sucking away droplets in time and limiting the growth of large condensate droplets through the superhydrophilic grooves. At wall subcooling ΔTw = 6.3 K, the heat transfer coefficient of SHPi-HPo-2 surface is 2.7 and 3.4 times that of HPi and HPo surfaces, respectively. For SHPi-HPo surface, there is optimum grid spacing between superhydrophilic grooves to enhance condensation heat transfer. Among three SHPi-HPo surfaces, the heat transfer coefficient of SHPi-HPo-2 surface has the best condensation heat transfer performance, about 0–10% higher than that of SHPi-HPo-1 surface, and at ΔTw = 9 K, the heat transfer coefficient is 1.7 times that of SHPi-HPo-3 surface.

Quantitative Description of the Vesicle Fusion Mechanism on Solid Surfaces and the Role of Cholesterol

Supported lipid bilayers on solid surfaces have promising potential for diverse applications, such as separation processes, biosensors, drug delivery, and more. However, the self-assembly of supported lipid bilayers via vesicle fusion—the commonly used preparation method for these lipid bilayers—is not fully understood. It is often found that lipid bilayers are patchy or exhibit holes/defects, which may hinder their applicability. Moreover, it is not fully understood whether these holes are transient, kinetically trapped, or thermodynamically stable (long-lasting). Here, we derived equations to quantitatively describe the mechanism of vesicle fusion on atomically smooth hydrophilic surfaces. The derived equations determine whether defectless lipid bilayers are thermodynamically stable/favorable and qualitatively predict the self-assembly rate. It is shown that vesicle fusion is governed by van der Waals and double layer interactions, as well as undulation repulsion between the lipid bilayers and the solid...

Minimum heat flux (MHF) behavior with different surface characteristics including structured surfaces and different surface energies

In a series of quenching experiments with smooth/structure hydrophilic/hydrophobic surfaces, the effects of surface wettability and surface structures on the minimum heat flux (MHF) behavior are observed. For a hydrophilic structure surface, early transition from film boiling to transition boiling at the MHF point occurs at the highest temperature (324 °C) in this experimental study. A smooth hydrophilic surface has 274 °C as the MHF temperature while a hydrophobic smooth surface shows the MHF behavior at lower temperature (208.8 °C). In contrast, there is no observable film destabilization with an observable MHF behavior on a hydrophobic structured surface during the quenching process. The test results are described by considering liquid-solid intermittent contact with an oscillating film interface in this study. For the intermittent contact, an equilibrium condition is proposed to describe the boiling transition with surface characteristics.

Wettability of laser textured surface: a parametric study based on numerical simulation and experimentation

PurposeDynamic spreading and wetting on the rough surfaces is complicated, which directly affects the fluxion and phrase transition properties of the fluid. This paper aims to enhance our knowledge of the mechanism of micro-texture lubrication from interface wettability and provide some guidance for the practical manufacturing of the surfaces with special wettability and better lubrication characteristics.Design/methodology/approachThe effect of surface topography on the wetting behavior of both smooth and rough hydrophilic surfaces was investigated using a combination of experimental and simulation approaches. Four types of patterns with different topographies were designed and fabricated through laser surface texturing. The samples were measured with a non-contact three-Dimensional (3D) optical profiler and were parameterized based on ISO 25178. Quantitative research on the relevancy between the topography characteristic and wettability was conducted with several 3D topography parameters.FindingsResults show that for the surfaces with isotropic textures, topography with a small skewness (Ssk) and a large kurtosis (Sku) exhibits better wettability and spreading behavior. For the surfaces with anisotropic textures (smaller texture aspect ratio, Str), dominant textures (such as long groove, rectangle) play a significant guiding role in promoting spreading. In addition, the moving mechanism of the triple contact line and anisotropic spreading were also studied using a computational fluid dynamics simulation. The simulation results have a good adherence with the experimental results.Originality/valueMost of the surface characterization methods at present remain at a level that is related to geometric description, and the topography parameters are limited to 2D roughness parameters. So in present study, the relevancy between wettability and 3D surface topography parameters is explored. The authors believe that the current work provides a new viewpoint to the relevancy between surface topography and wettability.

Observation of droplet freezing on various smooth hydrophilic surfaces by ESEM

Observation of droplet freezing on various smooth hydrophilic surfaces by ESEM

Lattice Boltzmann Simulations of Macro/Microscale Effects on Saturated Pool Boiling Curves for Heated Horizontal Surfaces

Results of lattice Boltzmann (LB) simulations of macroscale effects (heating modes, heater size, and saturation temperature) as well as microscale effects (wettability and roughness) on saturated pool boiling from superheated horizontal surfaces are summarized in this paper. These effects on pool boiling curves from natural convection through nucleate boiling to critical heat flux (CHF) and from transition boiling to film boiling are illustrated. It is found that macroscale effects have negligible influence on nucleate boiling heat transfer, and Rohsenow's correlation equation fits well with the simulated nucleate boiling heat transfer on smooth hydrophilic and hydrophobic horizontal surfaces. Both macroscale and microscale effects have important influence on critical heat flux and transition boiling heat transfer.

Exceptional Anti-Icing Performance of Self-Impregnating Slippery Surfaces.

A heat exchange interface at subzero temperature in a water vapor environment exhibits high probability of frost formation due to freezing condensation, a factor that markedly decreases the heat transfer efficacy due to the considerable thermal resistance of ice. Here we report a novel strategy to delay ice nucleation on these types of solid-water vapor interfaces. With a process-driven mechanism, a self-generated liquid intervening layer immiscible to water is deposited on a textured superhydrophobic surface and acts as a barrier between the water vapor and the solid substrate. This liquid layer imparts remarkable slippery conditions resulting in high mobility of condensing water droplets. A large increase of the ensuing ice coverage time is shown compared to the cases of standard smooth hydrophilic or textured superhydrophobic surfaces. During deicing of these self-impregnating surfaces we show an impressive tendency of ice fragments to skate expediting defrosting. Robustness of such surfaces is also demonstrated by operating them under subcooling for at least 490 h without a marked degradation. This is attributed to the presence of the liquid intervening layer, which protects the substrate from hydrolyzation, enhancing longevity and sustaining heat transfer efficiency.

Sliding viscoelastic drops on slippery surfaces

We study the sliding of drops of constant-viscosity dilute elastic liquids (Boger fluids) on various surfaces caused by sudden surface inclination. For smooth or roughened hydrophilic surfaces, such as glass or acrylic, there is essentially no difference between these elastic liquids and a Newtonian comparator fluid (with identical shear viscosity, surface tension, and static contact angle). In contrast for embossed polytetrafluoroethylene superhydrophobic surfaces, profound differences are observed: the elastic drops slide at a significantly reduced rate and complex branch-like patterns are left on the surface by the drop's wake including, on various scales, beads-on-a-string like phenomena. Microscopy images indicate that the strong viscoelastic effect is caused by stretching filaments of fluid from isolated islands, residing at pinning sites on the surface pillars, of the order ∼30 μm in size. On this scale, the local strain rates are sufficient to extend the polymer chains, locally increasing the extensional viscosity of the solution, retarding the drop and leaving behind striking branch-like structures on much larger scales.

A two‐stage surface treatment for the long‐term stability of hydrophilic SU‐8

The use of SU‐8 photoresist as a structuring material for portable capillary‐flow cytometry devices has been restricted by the near‐hydrophobic nature of the SU‐8 surface. In this work, we evaluate the use of chemical and plasma treatments to render the SU‐8 surface hydrophilic and characterise the resulting surface utilising a combination of techniques including contact angle goniometry, atomic force microscopy and X‐ray photoelectron spectroscopy. In particular, for low‐power plasma treatments, we find that the chemistry of the plasma used to modify the SU‐8 surface and the incorporation of O2 on that modified surface are paramount for improved surface wettability, whilst plasma‐induced surface roughness is not a necessary requirement. We demonstrate a technique to obtain a hydrophilic SU‐8 surface with contact angle as low as 7° whilst controlling and significantly reducing the level of surface roughness generated via the applied plasma. An additional chemical treatment step is found to be essential to stabilise the activated SU‐8 surface, and incubation of the samples with ethanolamine is demonstrated as an effective second‐stage treatment. Application of the optimised two‐stage surface treatment to cross‐linked SU‐8 is shown to result in a smooth hydrophilic surface that remains stable for over 3 months. Copyright © 2015 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd.

On jet impingement and thin film breakup on a horizontal superhydrophobic surface

When a vertical laminar jet impinges on a horizontal surface, it will spread out in a thin film. If the surface is hydrophobic and a downstream depth is not maintained, the film will radially expand until it breaks up into filaments or droplets. We present the first analysis and model that describes the location of this transition for both isotropic and anisotropic structured superhydrophobic (SH) surfaces. All surfaces explored are hydrophobic or SH, where the SH surfaces exhibit an apparent slip at the plane of the surface due to a shear free condition above the air filled cavities between the structures. The influence of apparent slip on the entire flow field is significant and yields behavior that deviates notably from classical behavior for a smooth hydrophilic surface where a hydraulic jump would form. Instead, break up into droplets occurs where the jet’s outward radial momentum is balanced by the inward surface tension force of the advancing film. For hydrophobic surfaces, or SH surfaces with random micropatterning, the apparent slip on the surface is uniform in all directions and droplet breakup occurs in a circular pattern. When alternating rib/cavity microstructures are used to create the SH surface, the apparent slip varies as a function of the azimuthal coordinate, and thus, the breakup location is elliptically shaped. The thin film dynamics are modeled by a radial momentum analysis for a given jet Weber number and specified slip length and the location of breakup for multiple surfaces over a range of jet Weber numbers and realistic slip length values is quantified. The results of the analysis show that the breakup radius increases with increasing Weber number and slip length. The eccentricity of the breakup ellipse for the rib/cavity SH structures increases with increasing Weber number and slip length as well. A generalized model that allows prediction of the transition (break-up) location as a function of all influencing parameters is presented. Model results are compared to experimental measurements with very good agreement.

Insights into the superhydrophobicity of metallic surfaces prepared by electrodeposition involving spontaneous adsorption of airborne hydrocarbons

Electrochemical fabrication of micro/nanostructured metallic surfaces with superhydrophobicity has recently aroused great attention. However, the origin still remains unclear why smooth hydrophilic metal surfaces become superhydrophobic by making micro/nanostructures without additional surface modifications. In this work, several superhydrophobic micro/nanostructured metal surfaces were prepared by a facile one-step electrodeposition process, including non-noble and noble metals such as copper, nickel, cadmium, zinc, gold, and palladium with (e.g. Cu) or without (e.g. Au) surface oxide films. We demonstrated by SEM and XPS that both hierarchical micro/nanostructures and spontaneous adsorption of airborne hydrocarbons endowed these surfaces with excellent superhydrophobicity. We revealed by XPS that the adsorption of airborne hydrocarbons at the Ar+-etched clean Au surface was rather quick, such that organic contamination can hardly be prevented in practical operation of surface wetting investigation. We also confirmed by XPS that ultraviolet-O3 treatment of the superhydrophobic metal surfaces did not remove the adsorbed hydrocarbons completely, but mainly oxidized them into hydrophilic oxygen-containing organic substances. We hope our findings here shed new light on deeper understanding of superhydrophobicity for micro/nanostructured metal surfaces with and without surface oxide films.