Meltwater Evolution during Defrosting on Superhydrophobic Surfaces.

Micro droplet handling is very important for micro and nano fluidic devices and an intelligent bio interface. Micro droplet transfer via a high adhesive superhydrophobic surface has been reported in recent years. We demonstrated water droplet adhesion controllable superhydrophobic metal-polymer surfaces. Moreover we achieved micro droplet transfer between superhydrophobic surfaces by using different droplet adhesion properties. Water micro droplets were transferred from a low-adhesive superhydrophobic surface to a midium-adhesive superhydrophobic surface via a high-adhesive superhydrophobic surface without any mass loss. After droplet transfer, water contact angle was about 150°. Droplet handlings on the adhesive superhydrophobic surfaces will be expected for fluidic bio devices with energy saving.

Superhydrophobic surfaces with tunable water adhesion have attracted much interest in fundamental research and practical applications. In this paper, we used a simple method to fabricate superhydrophobic surfaces with tunable water adhesion. Periodic microstructures with different topographies were fabricated on copper surface via femtosecond (fs) laser irradiation. The topography of these microstructures can be controlled by simply changing the scanning speed of the laser beam. After surface chemical modification, these as-prepared surfaces showed superhydrophobicity combined with different adhesion to water. Surfaces with deep microstructures showed self-cleaning properties with extremely low water adhesion, and the water adhesion increased when the surface microstructures became flat. The changes in surface water adhesion are attributed to the transition from Cassie state to Wenzel state. We also demonstrated that these superhydrophobic surfaces with different adhesion can be used for transferring small water droplets without any loss. We demonstrate that our approach provides a novel but simple way to tune the surface adhesion of superhydrophobic metallic surfaces for good potential applications in related areas.

Engineering a robust, multifunctional superhydrophobic/oleophobic microporous aluminum surface via a two-step chemical etching process

Facile preparation of mechanically durable, self-healing and multifunctional superhydrophobic surfaces on solid wood

Durable superhydrophobic coatings were synthesized using a system of silica nanoparticles (NPs) to provide nanoscale roughness, fluorosilane to give hydrophobic chemistry, and three different polymer binders: urethane acrylate, ethyl 2-cyanoacrylate, and epoxy. Coatings composed of different binders incorporating NPs in various concentrations exhibited different superhydrophobic attributes when applied on polycarbonate (PC) and glass substrates and as a function of coating composition. It was found that the substrate surface characteristics and wettability affected the superhydrophobic characteristics of the coatings. Interfacial tension and spreading coefficient parameters (thermodynamics) of the coating components were used to predict the localization of the NPs for the different binders’ concentrations. The thermodynamic analysis of the NPs localization was in good agreement with the experimental observations. On the basis of the thermodynamic analysis and the experimental scanning electron microscopy, X-ray photoelectron spectroscopy, profilometry, and atomic force microscopy results, it was concluded that localization of the NPs on the surface was critical to provide the necessary roughness and resulting superhydrophobicity. The durability evaluated by tape testing of the epoxy formulations was the best on both glass and PC. Several coating compositions retained their superhydrophobicity after the tape test. In summary, it was concluded that thermodynamic analysis is a powerful tool to predict the roughness of the coating due to the location of NPs on the surface, and hence can be used in the design of superhydrophobic coatings.

Directional motion of water drop on ratchet-like superhydrophobic surfaces

We report experimental results characterizing the dynamics of a liquid jet impinging normally on hydrophilic, hydrophobic, and superhydrophobic surfaces spanning the Weber number (based on the jet velocity and diameter) range from 100 to 1900. The superhydrophobic surfaces are fabricated with both hydrophobically coated silicon and polydimethylsiloxane that exhibit alternating microribs and cavities. For all surfaces a transition from a thin radially moving liquid sheet occurs. This takes the form of the classical hydraulic jump for the hydrophilic surfaces but is markedly different for the hydrophobic and superhydrophobic surfaces, where the transition is significantly influenced by surface tension and a break-up into droplets is observed at high Weber number. For the superhydrophobic surfaces, the transition exhibits an elliptical shape with the major axis being aligned parallel to the ribs, concomitant with the frictional resistance being smaller in the parallel direction than in the transverse direction. However, the total projected area of the ellipse exhibits a nearly linear dependence on the jet Weber number, and was nominally invariant with varying hydrophobicity and relative size of the ribs and cavities. For the hydrophobic and superhydrophobic scenarios, the local Weber number based on the local radial velocity and local depth of the radially moving liquid sheet is observed to be of order unity at the transition location. The results also reveal that for increasing relative size of the cavities, the ratio of the ellipse axis (major-to-minor) increases.

Condensed drops usually display a Wenzel state on a superhydrophobic surface (SHS) only with microrough architecture, while Cassie drops easily appear on a surface with micro-nano hierarchical roughness. The mechanism of this is not very clear. It is important to understand how the hierarchical structure affects the states of condensation drops so that a good SHS can be designed to achieve the highly efficient dropwise condensation. In this study, the interface free energy (IFE) of a local condensate, which comes from the growth and combination of numerous initial condensation nuclei, was calculated during its shape changes from the early flat shape to a Wenzel or Cassie state. The final state of a condensed drop was determined by whether the IFE continuously decreased or a minimum value existed. The calculation results indicate that the condensation drops on the surface only with microroughness display a Wenzel state because the IFE curve of a condensed drop first decreases and then increases, existing at a minimum value corresponding to a Wenzel drop. On a surface with proper hierarchical roughness, however, the interface energy curve of a condensed drop will continuously decline until reaching a Cassie state. Therefore, a condensed drop on a hierarchical roughness surface can spontaneously change into a Cassie state. Besides, the states and apparent contact angles of condensed drops on a SHS with different structural parameters published in the literature were calculated and compared with experimental observations. The results show that the calculated condensed drop states are well-coordinated with experimental clarifications. We can conclude that micro-nano hierarchical roughness is the key structural factor for sustaining condensed drops in a Cassie state on a SHS.

A fluorine-free cerium palmitate superhydrophobic surface on an aluminum plate was fabricated via a two-step electrodeposition method. The mechanical durability, anti-corrosion performance, water repellency and drag reduction properties were tested. The results indicate that a superhydrophobic surface with a densely packed convex island shape with micro-pores and nano-scale strips was formed on the aluminum plate. Furthermore, the as-prepared surface exhibits excellent water-repelling ability with a water contact angle of 162.3° and a sliding angle of 1.5°. Owing to the protective effect of the convex island structure on the surface, the surface also shows superb mechanical durability, which is a shortcoming of ordinary electrodeposited surfaces. Moreover, compared with bare aluminum, the corrosion inhibition efficiency of the superhydrophobic surface is 99.55% and the surface drag decreases by 65.3% at a lower flow rate. Therefore, it is believed that the environmentally friendly fluorine-free superhydrophobic surface has promising potential applications in marine engineering.

Efficiently harvesting the inherent kinetic energy carried by raindrops upon impact offers considerable potential for powering microdevices. However, the conversion efficiency of the raindrop energy is low. In this paper, we present a piezoelectric energy harvesting device with a superhydrophobic and low adhesive surface for significantly improving the raindrop energy conversion efficiency. The piezoelectric energy harvesting device was prepared using a macro fiber composite (MFC) with low operating frequency, high sensitivity, and a large electromechanical coupling coefficient as a functional element. The superhydrophobic surface allowed raindrops to rebound. At the same time, the raindrops can leave the surface of the device quickly after the end of their action so as to reduce the loss of kinetic energy generated by the collision of raindrops with solid surfaces. Compared to the untreated hydrophilic device, the energy conversion efficiency of the superhydrophobic piezoelectric energy harvesting device increased 172%. Under different test conditions, the device achieved a maximum peak power density of 338 W/m3. This approach provides a promising solution for efficient raindrop energy harvesting.

The coalescence-induced jumping of droplets on superhydrophobic surfaces is useful in engineering-related applications to enhance condensation-based heat transfer, self-cleaning, and anti-icing and, thus, has attracted extensive attention in research. Some researchers have claimed that superhydrophobic surfaces with protuberant structures can yield droplets with a higher jumping velocity. While the structure of the surface influences droplet dynamics, the concomitant energy transition also needs to be considered. The effects of the geometry on the mechanism of jumping and the energy transition need to be investigated. In this paper, an improved volume-of-fluid method is verified based on experiments and then applied to simulate the jumping behaviors of droplets on superhydrophobic surfaces with cuboid protuberant structures. The effects of repulsion caused by the contributions of the surface tension and the superhydrophobicity of the protuberance are crucial to enhancing the jumping of the droplets. The forces due to them provide a thrust oriented in the direction of jumping of the droplets to increase the value of the positive energy term, while reducing the area and duration of contact between the droplet and the substrate to reduce the negative dissipation term and enhance the efficiency of energy conversion. Surprisingly, an excessively tall structure leads to a sustainable increase in the velocity of jumping of the droplets under the effects of repulsion and the Laplace pressure after piercing the liquid bridge. The work here provides guidance to optimally combine a superhydrophobic substrate with special structures to enhance the jumping of droplets.

Mechanism study of condensed drops jumping on super-hydrophobic surfaces

The effectiveness of coalescence-induced jumping of microdroplets on superhydrophobic surfaces is critical to a wide range of applications such as self-cleaning surfaces, anti-icing/frosting, water harvesting, phase-change heat transfer, and hotspot cooling. Introducing textures on the surfaces can readily enlarge the effective contact angle, while an overlarge texture spacing may unfavorably lead to droplet penetration into the gaps in droplet coalescence processes. To clarify the effect of surface textures on the droplet jumping dynamics, we simulated the coalescence of droplets on textured superhydrophobic surfaces with various surface wettability and texture spacings and theoretically derived the critical conditions of jumping and the optimal condition of maximum jumping velocity. The results show that the nonmonotonic emergence of "nonjumping"-"jumping"-"nonjumping" with decreasing solid fraction is synergistically controlled by the surface adhesion and the effective impinging pressure. At a large solid fraction, the transition from "nonjumping" to "jumping" is caused by the reduction of the dimensionless surface adhesion energy below a critical value, which is determined to be 0.035 for Oh = 0.02 and 0.01 for Oh = 0.12. At a small solid fraction, the transition from "jumping" to "nonjumping" is dominated by the reduction of the dimensionless effective impinging pressure, the critical value of which is identified to be 0.14 and is independent of Oh. Moreover, jumping velocity maximizes when wetting critically transits from the Cassie-Baxter (CB) state to the partial-wetting state, and a penetration index is proposed from the wetting theory to predict such transition, which shows good agreement with both present simulations and previous experiments. The present findings are helpful for the design of superhydrophobic surfaces that pursue robust and efficient jumping of droplets.

Electrowetting (EW) effect is defined as changing in the interfacial energy of solid–liquid interface which causes to change in solid–liquid contact angle. In EW on dielectric (EWOD) actuations, the microliter drops are usually placed between two substrates which have some advantages. By modifying the voltages applied to the electrodes, it is possible to manipulate the droplets by setting up surface tension gradients along the plate. In this paper, the free energy based lattice Boltzmann method which has been recently extended by the authors for modeling and simulation of EW phenomenon has been applied to simulate EWOD. Also here we report the results of simulation of EW on superhydrophobic surfaces and pinning problem. The results have been compared with experimental data and show good accuracy of the numerical simulation.

Extremely hydrophobic surfaces, on which water droplets sit in a spherical shape leaving air entrapped into the roughness of the solid, are often called superhydrophobic. Hierarchically rough superhydrophobic surfaces that possess submicron scale fine structures combined with micron scale structures are generally more hydrophobic, and water droplet adhesion to those surfaces is lower in comparison with surfaces possessing purely micrometric structures. In other words, usually a fine structure on a superhydrophobic surface reduces liquid-solid contact area and water droplet adhesion. Here we show that this does not apply to a high-adhesive superhydrophobic rose petal surface. Contrary to the present knowledge, the function of the fine structure on the petal surface is to build up the high adhesion to water droplets. Understanding of the specific adhesion mechanism on the rose petal gives insight into an interesting natural phenomenon of simultaneous superhydrophobicity and high water droplet adhesion, but, in addition, it contributes to more precise comprehension of wetting and adhesion mechanisms of superhydrophobic surfaces overall.