Non-wetting Surfaces Research Articles

Technoeconomic analysis of thermoelectric power plant condensers with nonwetting surfaces

This paper investigates the dynamic performance and economic benefits of using nonwetting tube surfaces fabricated by electrodeposition process in thermoelectric power plant condensers. Nonwetting surfaces enhance performance by reducing fouling resistance and promoting dropwise condensation of steam on the shell side. Using a thermal resistance network of a shell and tube condenser, detailed parametric studies are presented to investigate the effects of design parameters and operating parameters on the annual condenser performance and its impact on the net electric energy output of a representative 550 MW coal-fired power plant. A cost model is presented to evaluate the technoeconomic benefits of the enhanced condenser design in a unified manner, in terms of a new levelized cost of condenser (LCOC) metric. The model is coupled with a numerical optimization method to identify optimal nonwetting condenser tube designs that minimize LCOC. The results show optimal cooling water flow velocity of 2.5 m/s and tube diameter of 0.032 m (0.025 m) for nonwetting (plain) condenser tube surfaces that exhibit dropwise (filmwise) condensation. Condensers with nonwetting surfaces are shown to reduce LCOC by a factor of 2.5 and 1.7, respectively, compared to plain titanium and aluminum-brass tubes.

Gas-Phase Temperature Mapping of Evaporating Microdroplets.

Evaporation is a ubiquitous and complex phenomenon of importance to many natural and industrial systems. Evaporation occurs when molecules near the free interface overcome intermolecular attractions with the bulk liquid. As molecules escape the liquid phase, heat is removed, causing evaporative cooling. The influence of evaporative cooling on inducing a temperature difference with the surrounding atmosphere as well as within the liquid is poorly understood. Here, we develop a technique to overcome past difficulties encountered during the study of heterogeneous droplet evaporation by coupling a piezo-driven droplet generation mechanism to a controlled micro-thermocouple to probe microdroplet evaporation. The technique allowed us to probe the gas-phase temperature distribution using a micro-thermocouple (50 μm) in the vicinity of the liquid-vapor interface with high spatial (±10 μm) and temporal (±100 ms) resolution. We experimentally map the temperature gradient formed surrounding sessile water droplets having varying curvature dictated by the apparent advancing contact angle (100° ≲ θ ≲ 165°). The experiments were carried out at temperatures below and above ambient for a range of fixed droplet radii (130 μm ≲ R ≲ 330 μm). Our results provide a primary validation of the centuries-old theoretical framework underpinning heterogeneous droplet evaporation mediated by the working fluid, substrate, and gas thermophysical properties, droplet apparent contact angle, and droplet size. We show that microscale droplets residing on low-thermal-conductivity substrates such as glass absorb up to 8× more heat from the surrounding gas compared to droplets residing on high-thermal-conductivity substrates such as copper. Our work not only develops an experimental understanding of the heat transfer mechanisms governing droplet evaporation but also presents a powerful platform for the study and characterization of liquid-vapor transport at curved interfaces wetting and nonwetting advanced functional surfaces.

Diverse surface properties reveal that substratum roughness affects fungal spore binding

SummaryBinding to surfaces by fungal spores is a prerequisite to biofilm formation. The interactions of polytetrafluoroethylene (PTFE), glass, and silicon with three fungal spores, of differing shapes and sizes (Aspergillus niger 1957, Aspergillus niger 1988, and Aureobasidium pullulans), were investigated. A multifractal analysis was conducted to provide quantitative measures of density, dispersion, and clustering of spores on the surfaces. The PTFE, glass, and silicon surfaces presented a range of surface topographies and wettabilities. PTFE was the roughest and most non-wettable surface, whereas silicon was the opposite in terms of both these aspects. The A. niger species were more non-wettable than A. pullulans. Overall, A. niger 1957 attached in higher numbers to PTFE, whereas A. niger 1988 and A. pullulans bound in highest numbers to glass. The results of this work demonstrated that the overall substratum surface roughness influenced spore binding rather than the physicochemical or chemical properties of surfaces or spores.

Coalescence of an Oil Droplet with a Permeating One over a Membrane Surface: Conditions of Permeation, Recoil, and Pinning.

When a droplet lands over a nonwetting surface it forms a convex interface that makes a contact angle larger than 90°. If the droplet lands over a pore opening, an interface is also formed at the pore opening that can prevent the droplet from permeating. The conditions for permeation and pinning are very much related to a threshold critical pressure that above which the droplet will permeate. This property defines a selectivity criterion for microfiltration processes of oily water systems using membrane technology. Such a feature of the membrane gets compromised, however, due to the permeation of droplets that are relatively smaller in size or whose critical entry pressure is smaller than the applied transmembrane pressure (TMP). In this work, we investigate what happens to a droplet when it coalesces with a droplet that undergoes permeation. Two scenarios are considered: namely, (1) a droplet coalesces with a permeating one whose interface inside the pore has not broken through the pore exit and (2) a droplet coalesces with a permeating one whose interface in the pore has broken through. We show that a larger droplet (that will essentially not permeate if pinned over a membrane opening) will now permeate when the pore is filled with oil from a preceding one or recoils when the interface inside the pore of a preceding droplet has not broken through the exit of the pore. This has interesting implications for the rejection capacity of the membrane, which decreases due to the permeation of droplets that would, otherwise, not permeate. A computational fluid dynamic (CFD) study has been conducted to confirm the conclusions obtained from the theoretical study and to reproduce the fates of the combined droplet after coalescence at the surface of the membrane. Furthermore, a simplified formula for estimating the critical entry pressure is developed.

Alteration of Wettability of Copper-Copper Oxide Nanocomposites through Cu-O Bond Breaking Swayed by Ultraviolet and Electron Irradiation.

Converting a nonwetting surface to a highly wetting one, aided by ultraviolet radiation, is well explored. Here, in this work, we show just the reverse behavior of a copper-copper oxide nanocomposite surface where ultraviolet radiation turned the superhydrophilic surface to a superhydrophobic one. This observation is explained both experimentally and theoretically using first-principles density functional theory-based calculations considering the metal-oxygen (Cu-O) bond breaking and related change in surface chemistry. This observation has further been corroborated with electron irradiation on the same nanocomposite material. To the best of our knowledge, for the first time, we show that the radiation-induced breaking of the copper-oxygen bond makes the nanostructure surface energetically unfavorable for water adsorption.

Feature Size Modulation in Dewetting of Nanoparticle-Containing Ultrathin Polymer Films

Ultrathin polymer films (thinner than ∼100 nm) undergo spontaneous rupture on a non-wetting surface with the appearance of random holes due to interfacial van der Waal’s interaction or disjoining pressure. These holes grow in size and coalesce into threadlike morphology, which subsequently disintegrate in an isotropic array of nearly equal-sized droplets, the size (dD) and periodicity (λD) of which scales with the initial film thickness (hF). We show that both λD and dD of the dewetted droplets can be modulated by adding trace amounts of nanoparticles (NPs) in the films. While addition of higher proportion of NPs is known to stabilize the films against dewetting, the presence of a lower amount of NPs leads to non-monotonic variation of λD and dD with hF, with the possible creation of miniaturized dewetted features under certain conditions. We also show that λD and dD of the dewetted films depend on whether dewetting is engendered by thermal annealing of the film beyond the glass transition temperature of the constituent polymer (TG) or by exposing the film to solvent vapor (SV). Our findings reveal that both λD and dD are much larger when the film (both with and without particles) is dewetted in the SV atmosphere, arguably due to penetration of the solvent into the film matrix, resulting in an increase in the effective thickness of the film during dewetting. We show that in SV-mediated dewetting, the much faster dynamics is attributed to solvent penetration through the film and its wetting the substrate, rather than a drastic drop in viscosity. We also observe that SV exposure not only leads to much faster dewetting dynamics, but it also successfully engenders dewetting in films with higher hF, which remain stable when annealed thermally.

Non-wetting surface design with magnetic and visible-light-active properties and its application to pollutant treatment

How to deal with organic pollutants of different densities and water-soluble pollutants is a challenge. In this paper, the slurry containing photocatalyst Ag3PO4, alkylated Fe3O4 and adhesive layer polydimethylsiloxane can be loaded to various substrates (including fabric, sponge, cotton) with a simple adsorption method. The non-wetability to water and the oleophilicity to organic solvent were realized by reducing the surface tension of solid through alkyl chain. It can not only remove pollutants floating on the water surface, but also adsorb pollutants whose density is greater than water and then realize the recovery with the response of the external magnetic field. It is important that the Ag3PO4 photocatalyst component in the material can absorb light energy to degrade water-soluble pollutants and further improve water quality. The stability tests (involving ultrasonic treatment, boiling water treatment and compression experiments) show that the prepared precursor slurry shows an excellent bonding stability with various substrates. Furthermore, graphene oxide has the best photodegradation behavior when 4 mg are added. Finally, photodegradation mechanism, the conduction (2.665 eV) and valence band (0.195 eV) information of the catalyst were analyzed.

What Can Probing Liquid-Air Menisci Inside Nanopores Teach Us About Macroscopic Wetting Phenomena?

Solid surfaces with excellent nonwetting ability have drawn significant interest from interfacial scientists and engineers. While much effort was devoted to investigating macroscopic wetting phenomena on nonwetting surfaces, the otherwise microscopic wetting has received less attention, and the surface/interface properties at the microscopic scale are not well resolved and correlated with the macroscopic wetting behavior. Herein, we first characterize the nanoscopic morphology and effective stiffness of liquid-air interfaces inside nanopores (nanomenisci) on diverse nonwetting nanoporous surfaces underneath water droplets using atomic force microscopy. Detailed three-dimensional imaging of the droplet-surface contact region reveals that water only slightly penetrates into the nanopores, allowing for quantitative prediction of the macroscopic contact angle using the Cassie-Baxter model. By gradually increasing the scanning force, we observe incrementally wetting of nanopores by water, and dewetting occurs when the force is lowered again, exhibiting reversible wetting-dewetting transitions. Further, nanoindentation measurements demonstrate that the nanomenisci show apparent elastic deformation and size-dependent effective stiffness at small indenting forces. Finally, we correlate the effective stiffness of the nanomenisci with the transition from complete rebound to partial rebound for impinging droplets on nanoporous surfaces. Our study suggests that probing the physical properties of the liquid-air menisci at the nanoscale is essential to rationalize macroscopic static and dynamic wetting phenomena on structured surfaces.

Coalescence-induced self-propelled jumping of three droplets on non-wetting surfaces: Droplet arrangement effects

Coalescence-induced self-propelled droplet jumping has attracted extensive attention because of its huge potential for enhancing dropwise condensation heat transfer, anti-icing, and self-cleaning. Most previous studies focus on binary droplet jumping, with little research on the more complex and realistic multi-droplet jumping. As a result, the effect of the droplet arrangement on the multi-droplet jumping phenomenon remains unclear. In this paper, the self-propelled jumping of three droplets with different arrangements (two droplets are fixed, and the location of the third one is changed) is numerically simulated, and energy conversion efficiency is studied. Based on two different forming mechanisms, region I (the coalescence between the lateral droplets forms the central liquid bridge) and region II (the changed interface curvature of central droplets turns into the central liquid bridge under surface tension) are defined in three-droplet arrangements. The liquid bridges exhibit different dynamic behaviors in two particular regions, even the jumping velocity is determined by the moving synchronicity of liquid bridges in each region. The critical distribution angle that leads to the overall nonmonotonic change of jumping velocities ranges between 110° and 120° (0.02 ≤ Oh ≤ 0.16). Compared with the symmetry of the droplet configuration, the geometry of the droplet arrangement plays a dominate role in the nonmonotonic change. The maximum energy conversion efficiency is just over 6.5% and the minimum is just under 3%. The findings of this study not only reveal how the arrangement affects ternary droplet jumping and explain the phenomenon that cannot be explained before, but deepens our understanding of multi-droplet jumping as well.

Spatio-temporal maneuvering of impacting drops

Controlling impacting drops on nonwetting surfaces is desired in multifarious processes. Efforts have been made to solely spatially control the drop movement after the impact or solely temporally reduce liquid-solid contact via surface design. We present a fin-stripe nonwetting surface that enables spatial offset maximization and temporal contact minimization simultaneously, just via structure design without the need for external energies. The wetting stripe provides a large wettability gradient for lateral movement, while the macroscale nonwetting fin restricts the drop movement direction and confines drop spreading for contact time reduction. The fin-stripe surface can decrease the contact time by roughly 30% and provide a normalized lateral distance of roughly 20, an order of magnitude larger than the reported values. Our surface enables effective spatio-temporal maneuvering of impacting drops, essential for various applications that involve liquid transport.

Analytical model for drag reduction on liquid-infused structured non-wetting surfaces.

Liquid-infused structured non-wetting surfaces provide alternating no-slip and partial slip boundary conditions to the fluid flow, resulting in reduced friction at the interface. In this paper, an analytical model is developed for the evaluation of effective slip and, in turn, friction factor and drag reduction on liquid-infused structured non-wetting surfaces. By considering the entire range of anisotropy and heterogeneity of the surface structures as well as the full range of partial slip offered by the infusion liquid, the present model eliminates empirical fitting or correlations that are inherent in previous studies. Based on the effective slip length, drag reduction and skin friction coefficient values for Newtonian flow between two infinite parallel plates and flow in round tubes are presented. Extension of Moody charts for non-wetting surfaces and design maps of surface meso/micro/nano texturing for achieving desired drag reduction are presented for a broad range of engineering applications. The article further presents independent validation of the model across experimental and computational data from the literature and brings together several previous studies in a unified manner.

Wetting and corrosion characteristics of thermally sprayed copper-graphene nanoplatelet coatings for enhanced dropwise condensation application

Water vapour condensation is frequently used as an effective means of transferring heat using dropwise condensation on non-wetting surfaces. The rate of heat transfer can be enhanced with dropwise condensation on non-wetting hydrophobic and superhydrophobic surfaces when compared to filmwise condensation on a wetting surface. Since low surface energy materials are mostly polymeric with low durability and poor thermal conductivity, copper-graphene nanoplatelet (Cu-GNP) composite coatings are developed in this work using atmospheric plasma spray (APS) and high-velocity oxy-fuel (HVOF) as versatile and scalable surface engineering techniques. It is demonstrated that by adjusting the deposition parameters, it is possible to preserve the GNPs in the coating structure. After treating with a stearic acid solution, the coatings demonstrate superhydrophobicity and extreme water repelling behavior with a water contact angle of 164° and sliding angle of 1° Raman analysis confirmed the presence of GNP in the coating with minimal defects. It is shown that the HVOF Cu-GNP coating developed in this work can improve the corrosion resistance up to 89% when compared to the uncoated Cu surface. This coating can potentially promote dropwise condensation while offering enhanced corrosion stability.

Analysis of Laminar Convective Heat Transfer Over Structured Non-Wetting Surfaces

Structured non-wetting surfaces provide alternating no-slip and partial slip boundary conditions to the fluid flow which, in turn, affects the convective heat transfer performance over the surfaces. In this paper, an analytical model is developed for the interfacial Nusselt number, the overall Nusselt number and a thermal hydraulic performance factor for fluid flow in a cylinder patterned with structured non-wetting surfaces, for the two cases of uniform wall heat flux and uniform wall temperature. In addition, by considering the stability of the Cassie state of wettability and its transition to the Wenzel state for flow over superhydrophobic surfaces, the present model overcomes certain limitations of the previously reported studies in the literature. Based on the analytical formulations and the stability constraints, the present paper provides optimum design maps for tailoring structured non-wetting surfaces for maximizing convective heat transfer and the combined thermal-hydraulic performance in applications. Use of the design maps on example cases is also discussed. It is shown that the use of structured non-wetting surfaces is most effective for low Reynolds numbers and/or small cylinder radius.

Decoupled rolling, sliding and sticking of a viscoplastic drop on a superhydrophobic surface

Abstract

Fractal Model for Drag Reduction on Multiscale Nonwetting Rough Surfaces.

Rough surfaces in contact with a flow of fluid exhibit alternating no-slip and free shear boundary conditions at the solid-liquid and air-liquid interfaces, respectively, thereby potentially offering drag reduction benefits. The balance between the dynamic pressure in the flow and the restoring capillary pressure in the interasperity spaces determines the stability of the Cassie state of wettability and is a function of the relative extent of no-slip and free shear regions per unit surface area. In the present study, using a fractal representation of rough surface topography, an analytical model is developed to quantify the stability of the Cassie state of wettability as well as drag reduction and the friction factor for laminar flow in a rectangular channel between nonwetting multiscale rough surfaces. A systematic study is conducted to quantify the effects of fractal parameters of the surfaces and the flow Reynolds number on drag reduction and the friction factor. The studies are used to develop friction factor curves extending the classical Moody diagram to hydrophobic and superhydrophobic surfaces. On the basis of the studies, regime maps are derived for estimating the extent of drag reduction offered by hydrophobic and superhydrophobic surfaces, revealing that superhydrophobic surfaces do not always offer the best drag reduction performance. The application of the fractal model to practical topographies of nonwetting surfaces of copper, aluminum, and zinc oxide fabricated via electrodeposition and etching is also discussed.

Laminar drag reduction in microchannels with liquid infused textured surfaces

Liquid infused surfaces (LIS) achieve non-wetting properties through asperity structures containing pockets of a lubricating liquid rather than air. Although studies have demonstrated several potential applications of LIS, the effect of liquid infused texture on the flow of liquid in microchannels has not been systematically addressed. The present work focuses on the effect of liquid-infused texture geometry and the infused liquid properties on the flow characteristics of a bulk fluid in rectangular and round microchannel geometries. Considering four different textured geometries of transverse ridges, longitudinal ridges, in-lined square posts and staggered squares, a detailed parametric study is presented to assess the resulting drag reduction and friction factor, expressed in terms of the Poiseuille number, on the flow in microchannels. The effect of geometrical parameters is investigated for different asperity solid fraction (ϕs), asperity height (hs) and channel thickness (H) or tube diameter (D), while the effect of the infused fluid is studied in terms of a ratio of its viscosity relative to the viscosity of the flowing bulk fluid. Based on the results of the parametric study an extension of the conventional Moody diagram for non-wetting surfaces is developed. Further, a design of experiments study is conducted to relate the drag reduction and Poiseuille number to the various governing parameters, using which conditions that maximize drag reduction are identified.

Toward Passive Defrosting with Heterogeneous Coatings

A large amount of energy is lost in industrial active deicing. One problem is that water retention on the surface after ice melting decreases the overall heat-transfer coefficient by up to 20%. The Miljkovic group utilizes heterogeneous biphilic surfaces to enhance defrosting performance. They avoid water retention as the slush is rapidly drawn toward the hydrophilic regions before it completely melts.

On the Mechanism of the Origin of the Cavitation Effect on Electrochemically Hydrogen-Charged Palladium

The relationship between the amount of hydrogen electrochemically introduced to Pd and the volume and linear sizes of the formed PdH system, as well as the intensity of cavitation that arises after saturation of Pd with hydrogen, has been determined. The results of the study allows one to describe the mechanism of origin of cavitation on PdH in spite of the impossibility to determine the composition of atmosphere in bulk defects (microvoids, microcracks, and dislocations), which are formed upon hydrogenation of Pd. A Harvey–Knapp model has been employed, which describes well the origin of cavitation on nonwettable surfaces with fine cracks. The model is based on the hypothesis that presence of cavitation cores are present in water in the form of fine air bubbles.

Facile synthesis of nano silica-based coating on API5L-x80 steel to achieve ultra non-wetting surface and its corrosion resistance

A facile and acceptable way for arrangement of non-wetting ultra-hydrophobic silica coatings is proposed in this work. The X80 steel samples with the dimension of 10 × 10 × 2 mm were used as the base metal. Commercially silica nanoparticles were modified using alkoxide organosilane as an active modifying agent. The ultra-hydrophobic silica coatings with a water contact angle of 142° were achieved (ultra-high degree of contact angle). The prepared silica coatings exhibit ultra-hydrophobicity and excellent corrosion behaviors under 3.5 wt% NaCl immersion testing conditions, which surfaces with 2.0% and 2.5% of hydrophobic silica specimens show a low corrosion potential and current, significantly. The corrosion resistance of the samples was evaluated by the electrochemical impedance spectroscopy technique also the microstructure, and surface topography of the samples was evaluated with scanning electron microscopy and Fourier transform infrared, respectively. This coating method is expected such a convincing process technology against fabricating self-cleaning coatings for the encouraging industrial implementation.

Recent advances on regiocentric homo-epitaxial approaches in bio-mimetically derived artificial non-wettable surfaces

Abstract Natural biosurfaces shows water repellence mainly because of its unique surface roughness and peculiar chemical compositions. Herein, we have mimicked the properties of the natural, non-wettable surfaces and created artificial water repellent surfaces using perpendicularly aligned zinc oxide (ZnO) nanostructured coatings having rough surface texture. These non-wettable surfaces were prepared through simple seed assisted hydrothermal methods. In the present method, a seed layer of nano regime zinc oxide was initially prepared on clean glass slides by coating and calcination methods. The as-synthesized seed layer was utilized for the hydrothermal growth of vertically oriented nanostructures. The structural and functional features of the resultant samples were studied using XRD, FTIR, UV-Vis. spectrophotometer and Contact angle analyzer. The homo-epitaxial growth and the crystallinity of ZnO nano particles were assessed using XRD. The structural studies were performed using FTIR and the optical transparency studies of the samples were carried out using UV–Vis. spectrophotometer. Along with the photographic image analyses, the water contact angle analyses were performed to predict the non-wettable character of the resultant coatings. At low nutrient solution concentrations (10 mM) and at low seed solution concentrations (1–10 mM), hydrophobic nanomaterials surfaces with high water contact angle, ˃130°, were successfully prepared through the present hydrothermal methods that extends many practical applications in biomedical, defense and industrial sectors.