Textured Superhydrophobic Surfaces Research Articles

Enhanced cascaded lattice Boltzmann model for multiphase flow simulations at large density ratio

In recent years, the pseudopotential multiphase lattice Boltzmann method (LBM) has been widely applied in the numerical simulations of complex multiphase flow. However, numerical stability at high Weber (We) and Reynolds (Re) numbers seriously restricts its application. In the present work, the pseudopotential multiphase LBM is developed by introducing the entropic stabilizers to the cascaded collision operator, and thus, the numerical stability is effectively enhanced. The relaxation rates for different order moments are all determined by fluid viscosities and the maximum entropy condition instead of being user-defined. Furthermore, the surface tension and contact angle can be adjusted in a wide range, which extends the application of the pseudopotential multiphase model. The accuracy and capability of the present model to simulate the complex multiphase flow are validated by reproducing the droplet impact on the flat or textured superhydrophobic surfaces. In addition, a droplet impact on the superhydrophobic surface at We=1280 and Re=18000 is simulated, in which the Weber and Reynolds numbers have significantly increased. This strongly demonstrates that the present model can enhance the numerical stability in the simulation of large density ratio multiphase flows at simultaneously high Weber and Reynolds numbers, which means the present model can further extend the pseudopotential multiphase LBM to more realistic scientific and technical applications.

Bouncing modes and heat transfer of impacting droplets on textured superhydrophobic surfaces

Superhydrophobic modifications could effectively minimize heat exchange at the interfaces of impacting droplets and solid surfaces. Previous studies have lacked numerical explorations regarding the effect of bouncing modes on the heat transfer characteristics during droplets impacting on textured superhydrophobic surfaces with micro-pillars. To address this issue, a multiple distribution function phase-field lattice Boltzmann model is developed to numerically study dynamic behaviors and heat transfer during droplet impact. Comparisons between the simulations and previous experimental results validate the computation model. Subsequently, the dynamic behaviors of impacting droplets and the effects on the heat transfer were studied using the proposed model. The effects of the textured surface structural parameters on the dynamics and heat transfer are discussed in detail. The numerical results indicate four possible bouncing modes of the impacting droplets: Cassie bouncing, partially penetrated bouncing, pancake bouncing and Wenzel bouncing. These modes depend on the surface energy stored in the penetrating droplet in the microstructures cavities of the surface. Moreover, the synergistic effects of contact time and contact area affect the heat transfer performance. Further, the developed theoretical model to predict the total transferred heat is based on the identified droplet dynamics. Finally, the effects of roughness parameters on total transferred heat are studied, and the design principles of textured superhydrophobic surfaces for heat transfer suppression are given for two application scenarios. The results demonstrate that the control of microstructures would be crucial for the dynamics and heat transfer of impacting droplets on textured superhydrophobic surfaces.

Water droplet impact on perfluoropolyether-based liquid-like surfaces

Water droplet impact on water-repellent surfaces is of relevance to many important engineering applications. Although the droplet impact behavior on water-repellent surfaces has been extensively investigated, in particular, textured superhydrophobic surfaces, that on water-repellent liquid-like surfaces has been barely reported. In this study, we experimentally investigated the oblique impact of water droplets on a surface with a liquid-like perfluoropolyether coating using a high-speed camera. The direct observation showed that all the impinging water droplets released from various heights exhibited similar outcomes, including an inelastic collision without rebound as well as a continuous sliding over the liquid-like surface. The liquid-like perfluoropolyether molecular chain on the surface was suggested to behave like a high viscosity oil layer that could cause more intense energy dissipation during the impact process. The crucial impact parameters of the maximum tangential droplet extension, the spreading time and the sliding length were further analyzed quantitatively by the methodology widely adopted in the exploration of the impact behavior in previous studies. It was indicated that the process of the impinging droplet spreading on the liquid-like surface was more likely dominated by the competition between the inertial and capillary forces although the increased viscous dissipation arising from the liquid-like layer significantly suppressed the rebound of the impinging droplet. Compared to superhydrophobic surfaces, the liquid-like surface could generate a stronger friction force for the slide motion of the impinging water droplet. The triboelectric effect arising from the water-solid contact was proven to inhibit the retraction of the impinging droplet with relatively higher impact velocities. This study is expected to improve the fundamental understanding of the droplet impact process and facilitate us to make more rational designs of liquid-like surfaces for relevant applications.

Adverse impact of macro-textured superhydrophobicity on contact time reduction at high Weber numbers

Macro textured superhydrophobic surfaces are reported to be highly effective in contact time reduction and thus a potential candidate for anti-icing applications. However, we find that the macro textured superhydrophobic cylindrical surfaces have higher contact time compared to its counterpart without any macro texturing at high impact Weber numbers (We)for the various range of curvature ratios of the cylinder and aspect ratios of the macro texturing. The asymmetric spreading is aided by the preferential jetting and flow redirection results in a stable lamella in the azimuthal direction resulting in the contact time enhancement for intermediate We number regimes. The retraction velocity is also adversely influenced by the presence of ribs in these We number regimes thus resulting in enhanced residence time. However, the dynamics at the high We regimes are governed by the hole nucleation and film rupturing. The presence of ribs is found to reduce the nucleation inception and nucleation density resulting in higher contact time compared to the non-ribbed superhydrophobic surfaces. An outcome based map was developed based on the systematic experimental observations over a wide spectrum of parametric variation. Detailed analysis is presented to explain the counter-intuitive observations.

Effects of Normalized Periodic Unit Length of Superhydrophobic Transverse Grooves on Thermal Developing Flow in Tube

The flow dynamics and heat transfer in fully-developed flow region changes significantly with the size of the water-repellent microstructures. It is noted that the hydrophobic coated surface could reduce the wall friction, thereby increasing the mass flow rate through the tube. This leads to the interest in employing this salient surface for heat transfer application. Numerical study is carried out on thermal entrance flow under the influence of textured superhydrophobic surface with transverse grooves and ribs. In this study, the effect of normalized periodic unit length is investigated. Under constant heat transfer rate condition, it is revealed that patterning the water repellent surface along the tube wall can be beneficial for heat transfer application, in particularly in the thermal developing region, with the predicted water temperature to be consistently lower than that of smooth walls.

A robust and anti-UV layered textured superhydrophobic surface based on water-glass interface enhancement

Bioinspired superhydrophobic surfaces (SHS) have potential application prospects in many fields, but weak mechanical stability of the microstructures has become the main bottleneck. In this work, the water-glass strengthened the interfacial force between the inorganic SiO 2 and ZnO particles and the substrate through chemical reaction rather than physical embedding, which obtains a robust superhydrophobic surface. Based on surface characterization and analysis, the mechanism of the water-glass that played a role of enhancing the interface between the particles and the substrate in the coating was explored and proposed. Furthermore, the effect of the treatment temperature on the surface structure of the water-based paint during the spraying process was studied. Through simulating different environments, the anti-acid and -alkali resistance and anti-aging ability were tested in the full pH range or under strong UV irradiation, showing an excellent performance. Different mechanical stability tests confirmed that the silicate network structure enhances the binding force between the particles, improving the robustness of the superhydrophobic surface. This work may provide a new insight for the construction of superhydrophobic surfaces with enhanced interfaces.

Simultaneous Effect of Droplet Temperature and Surface Wettability on Single Drop Impact Dynamics

In this paper, the influence of the liquid droplet temperature on thermo–hydrodynamics of a single droplet impinging on surfaces having different hydrophobicities is experimentally investigated. Variation in the liquid temperature typically results in alteration of properties such as the density, the viscosity, the surface tension, and the enthalpy, consequently, the droplet dynamics gets to be modified. Employing high-speed imaging technique, the morphology and spreading pattern are investigated for water droplet collision on hydrophilic, hydrophobic and super-hydrophobic surfaces. The droplet deformation is monitored qualitatively and quantitatively for drops in the temperature range from 5 to 85°C and the Weber number between 14.5 and 160. It is observed that with an increase in the liquid temperature the spreading factor increases owing to the combined effect of reduction in the density, the surface tension, the viscosity and the contact angle of the solid surface. The differences in extension of droplets under the extreme temperatures for hydrophilic, hydrophobic and super-hydrophobic surfaces are noted to be 62.7, 27.76, and 20.52%, respectively. At the low temperature, the surface tension force dominates and the Cassie–Baxter state prevails on a textured super-hydrophobic surface and the droplets bounce off. In contrast at elevated temperatures, the liquid–solid interface ruptures and liquid penetrates into the cavities and results in the Wenzel state. Furthermore, the drop which exhibits multiple bounces in the low temperature regime is found sticking on a super-hydrophobic substrate at the high droplet temperature irrespective of the Weber number.

Nanosecond laser fabrication of superhydrophobic surface on 316L stainless steel and corrosion protection application

Herein, we developed an facile, cost-effective and efficient method to fabricate microstructures of different scales on 316 L stainless steel substrate surface by laser modification. By changing the laser parameters, such as various laser fluence (2.69, 3.96, 6.28, 8.14,9.55 J/cm2) during laser irradiation, different sizes of micro-cracks and brain-like microstructures can be obtained. The experimental results indicate that a static contact angle of 160 ± 5° and the sliding angle of 3 ± 0.5°, can be achieved at a scanning interval of 30.0 μm and laser fluence of 8.14 J/cm2, which has the best outstanding wettability. In addition, the microstructures and chemical compositions on the surface were examined by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The corresponding corrosion resistance properties was evaluated by the means of potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). The laser textured superhydrophobic surface was found to enhance the corrosion resistance dramatically. Also, after the laser modification, excellent self-cleaning performance was demonstrated based on tests with quartz sand particles. Particularly, we extensively discussed how the structures on the 316 L stainless steel were formed after the ultraviolet nanosecond laser irradiation and the interaction mechanism of the laser and substrate. This study not only provides important insights into the formation mechanism of laser textured surfaces, but also guides practical industrial applications as this approach is simple, low-cost and convenient for fabrication of large-area superhydrophobic surfaces on various metal materials.

Multistage textured superhydrophobic polytetrafluoroethylene surface prepared by fabric embossing and thermal annealing

A novel method combined with fabric embossing (embossing with fabric template) and thermal annealing is proposed to prepare multistage textured superhydrophobic polytetrafluoroethylene (PTFE) surface. Multistage (four-stage) textures of microscale Y-grooves (≈175 µm), F-grooves (≈10 µm), warts (≈3 µm), and nanoscale dendrites (≈380 nm) were obtained. The multistage textures endowed the PTFE surface with better superhydrophobicity, comparing with the two-stage textures with Y-grooves and F-grooves or the two-stage textures with warts and dendrites. Optimum superhydrophobicity is obtained at thermal annealing temperatures 345 °C and 360 °C.

Influence of texture shape and arrangement on thermo-hydraulic performance of the textured microchannels

Textured superhydrophobic surfaces (TSS) are purported to reduce flow friction in microchannels due to velocity slip at liquid-gas interface. At the same time, the liquid-gas interface inhibits heat transfer in textured microchannels due to the low thermal conductivity of entrapped gas phase. Despite significant understanding on fluid flow and thermal transport on the TSS, the interplay of texture shape and arrangement on thermo-hydraulic performance has not been investigated in detail hitherto. To this end, we have numerically investigated the pressure-driven flow through textured microchannels with an aim to enhance the thermo-hydraulic performance. The effective slip length and temperature jump length were estimated as a function of flow and geometry parameters for three types of micropillar shapes viz., square, triangular and herringbone, decorated in microchannels in regular and staggered manner. Scaling relations for the effective slip length and temperature jump length have been shown to be valid for triangular and herringbone shaped micropillars at different flow and geometry related parameters. Herringbone shaped micropillars exhibit more flow friction and allow a significant heat transfer in microchannels within the parameter range investigated, followed by triangular and square shaped micropillars. Although the arrangement of textures in microchannels was found to affect the flow friction substantially, its effect on heat transfer was found to be marginal. Subsequently, the overall thermo-hydraulic performance was observed to be superior in regularly arranged herringbone shaped micropillars, at moderate to high constriction ratios (a ratio of texture pitch to half channel height) and high Peclet numbers over the other texture shapes. The results presented in this work would serve as a useful guide to attain maximum thermo-hydraulic performance in textured microchannels.

Micro-nano textured superhydrophobic 5083 aluminum alloy as a barrier against marine corrosion and sulfate-reducing bacteria adhesion

Abstract Corrosion attack and undesired bio-adhesion are thorny issues threatening and restricting the service performance and security of the widely used structural 5083 aluminum alloys (AA5083) materials in maritime fields. In this paper, we employed a facile ammonia etching approach followed by 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTES) modification to fabricate micro-nano textured superhydrophobic AA5083 surface as an effective layer for corrosion inhibition and bio-adhesion suppression. The dual-scale surface morphology and roughness were revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) were employed to depict the chemical components of the micro-nano textured AA5083 surface. X-ray photoelectron spectroscopy (XPS) was employed to confirm the presence of functional groups of fluorinated carbon with low surface energy. The self-cleaning ability and corrosion resistance of the fabricated surfaces were investigated, suggesting an excellent water-proofing and anti-corrosion performance. Moreover, sulfate-reducing bacteria (SRB) was chosen as a typical bacteria to evaluate and demonstrate the anti-adhesion property of the as-fabricated superhydrophobic AA5083 surface. The present study therefore demonstrates that achieving superhydrophobicity on AA5083 substrate is expected to possess quite potential applications in marine corrosion and bio-fouling fields.

Cassie-to-Wenzel transition of droplet on the superhydrophobic surface caused by light induced evaporation

The droplet-based open microfluidics, which manipulates the liquid droplets in an open space to accomplish various functions, has attracted ever-increasing attentions all over the world. In recent, the incorporation of the optics into the droplet-based open microfluidics has resulted in new methods for optically manipulating the droplets. Among them, the photothermal effect has shown its distinct potentials in controlling the droplet by the light-caused phase change with unique merits of flexibility, precision, remarkable efficiency and non-contact state. In this work, we studied the droplet evaporation on the textured superhydrophobic surface actuated by the photothermal effect of a focused infrared laser with the wavelength of 1550 nm. The transition of the droplet from the Cassie state to the Wenzel state accompanying with the obvious extension of the triple-phase contact line was observed. It was confirmed that this Cassie-to-Wenzel transition was actuated by the continuous condensation in the microstructures of the superhydrophobic surface and coalescence between the condensed droplets and the original one. The effects of the laser power and the initial droplet volume on the Cassie-to-Wenzel transition and the extension of the triple-phase contact line were investigated. It was found that the final migration distance of the triple-phase contact line was gradually increased with the initial droplet volume. With the increase of the laser power, the extension process was accelerated as a result of the intensified light-caused evaporation. Noticeably, it was demonstrated that the dimensionless final migration distance under different conditions were similar.

Spectral Analysis of the Slip-Length Model for Turbulence over Textured Superhydrophobic Surfaces

We assess the applicability of slip-length models to represent textured superhydrophobic surfaces. From the results of direct numerical simulations, and by considering the slip length from a spectral perspective, we discriminate between the apparent boundary conditions experienced by different lengthscales in the overlying turbulent flow. In particular, we focus on the slip lengths experienced by lengthscales relevant to the near wall turbulent dynamics. Our results indicate that the apparent failure of homogeneous slip-length models is not the direct effect of the texture size becoming comparable to the size of eddies in the flow. The texture-induced signal scatters to the entire wavenumber space, affecting the perceived slip length across all lengthscales, even those much larger than the texture. We propose that the failure is caused by the intensity of the texture-induced flow, rather than its wavelength, becoming comparable to the background turbulence.

Dropwise Condensation on Superhydrophobic Microporous Wick Structures

Previous research in dropwise condensation (DWC) on rough microtextured superhydrophobic surfaces has demonstrated evidence of high heat transfer enhancement compared to smooth hydrophobic surfaces. In this study, we experimentally investigate the use of microporous sintered copper powder on copper substrates coated with a thiol-based self-assembled monolayer to attain enhanced DWC for steam in a custom condensation chamber. Although microtextured superhydrophobic surfaces have shown advantageous droplet growth dynamics, precise heat transfer measurements are underdeveloped at high heat flux. Sintered copper powder diameters from 4 μm to 119 μm were used to investigate particle size effects on heat transfer. As powder diameter decreased, competing physical factors led to improved thermal performance. At consistent operating conditions, we experimentally demonstrated a 23% improvement in the local condensation heat transfer coefficient for a superhydrophobic 4 μm diameter microporous copper powder surface compared to a smooth hydrophobic copper surface. For the smallest powders observed, this improvement is primarily attributed to the reduction in contact angle hysteresis as evidenced by the decrease in departing droplet size. Interestingly, the contact angle hysteresis of sessile water droplets measured in air is in contradiction with the departing droplet size observations made during condensation of saturated steam. It is evident that the specific design of textured superhydrophobic surfaces has profound implications for enhanced condensation in high heat flux applications.

Delay in the Freezing of Supercooled Water Drops on Superhydrophobic Surfaces of Silicone Rubber at Negative Temperatures

A method is proposed for fabricating textured superhydrophobic surfaces of silicone rubber with mechanical resistance toward liquid or freezing aqueous solutions. The anti-icing characteristics of silicone rubber samples that differ in the wetting characteristics and mechanical stability of their micro- and nanotextures are derived by analyzing the delays in the freezing of supercooled sessile water drops deposited on the sample surface. The longest delay in freezings are observed for sessile water drops on superhydrophobic surfaces prepared by laser texturing with subsequent application of a layer of a hydrophobic agent to consolidate the textural elements. Delay in freezings can be as long as tens of hours on such surfaces at T = −18°C. The prepared superhydrophobic surfaces exhibit greater anti-icing ability with respect to aqueous salt solutions than to deionized water.

Slow viscous flow near a superhydrophobic surface with trapped gas bubbles

AbstractThe Boundary Element Method (BEM) is used to solve the problem of Stokes flow of a viscous fluid over a periodic striped texture of a superhydrophobic surface (SHS), partially filled with frictionless gas bubbles. The shape of the bubble surfaces and the position of the meniscus pinning points relative to the cavity walls are taken into account in the study. Two kinds of flows important for practical applications are considered: a pressure‐driven flow in a thin channel with a bottom superhydrophobic wall and a shear‐driven flow over a periodic texture. We study the flow pattern in the fluid over a single cavity containing a bubble with a curved phase interface shifted into the cavity. A parametric numerical study of the averaged slip length of the SHS is performed as a function of the geometric parameters of the texture. It is shown that the curvature of the phase interface and/or its shift into the cavity both result in the decrease in the average slip length. It is demonstrated that the BEM can be an efficient tool for studying Stokes flows over textured superhydrophobic surfaces with different geometries of microcavities and phase interfaces. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Design and Fabrication of the Lyophobic Slippery Surface and Its Application in Anti-Icing

A suspension that can be sprayed onto substrates was developed to form a superhydrophobic/oleophilic surface. Lyophobic slippery surfaces were prepared by infusing perfluorinated lubricants into the superhydrophobic coating to repel almost all liquids with low surface tension values, including hexane, kerosene, and diesel oil, showing a transition between superoleophilicity and lyophobicity. In addition, the traveling speeds of liquids appeared to be negatively correlated with the kinematic viscosity. In the anti-icing tests, the droplet was pinned after contacting a 0 °C textured superhydrophobic surface for a few seconds because of the meniscus caused by the condensation of atmospheric humidity; by contrast, on the lyophobic slippery surface, a water droplet could easily slide even at −20 °C, demonstrating superior icing resistance.

Comment on “Nanosecond laser textured superhydrophobic metallic surfaces and their chemical sensing applications” by Duong V. Ta, Andrew Dunn, Thomas J. Wasley, Robert W. Kay, Jonathan Stringer, Patrick J. Smith, Colm Connaughton, Jonathan D. Shephard (Appl. Surf. Sci. 357 (2015) 248–254)

Nowadays the problem of design of durable ecologically friendly superhydrophobic surfaces is of great importance for science and technology. A recent paper in Applied Surface Science reports the method of fabricating the superhydrophobic metallic surfaces by infrared nanosecond laser surface texturing without using hydrophobic agents. Since this method of surface texturing can be considered as one of the most suitable for various industrial applications, the nature of superhydrophobic state of surfaces produced by laser texturing in the abovementioned paper deserves to be analyzed in detail. Authors of the commented paper attributed the change in wettability to the partial deoxidation of CuO into Cu2O on the surface during storage in atmosphere. However, such interpretation of the results contradicts to the basic notions in the theory of wetting and to more accurate and detailed data. In our Comment we discuss these contradictions point by point.

Stokes flow over a cavity on a superhydrophobic surface containing a gas bubble

Within the approximation of Stokes hydrodynamics, several problems of a steady-state flow over a two-dimensional cavity containing a gas bubble are solved using the method of boundary integral equations. In contrast to previous publications, the method developed makes it possible to study the situation in which the cavity is only partially filled with gas, and the edges of a curved phase interface do not coincide with the cavity corners. Using periodic boundary conditions for the velocity, the flows with pure-shear and parabolic velocity profiles, and also the flow over a group of cavities were considered. The aim of the study was to calculate the effective (average) slip velocity over a microcavity, as applied to flows near textured superhydrophobic surfaces. A parametric numerical study of the effective velocity slip as a function of the radius of curvature of the interface and the position of the interface relative to the cavity boundaries was performed. The accuracy of the method is validated by the calculations of a number of limiting flows over a cavity, for which a quantitative agreement with the results known in the literature is demonstrated.