Reducing Contact Time of Droplets Impacting Superheated Hydrophobic Surfaces.

Reducing the contact time (tc ) of a droplet impacting a solid surface is crucial in various fields. Superhydrophobic (SHB) surfaces are used to reduce tc at room temperature. However, at high temperatures, SHB surfaces cannot achieve tc reduction because of the failure of the coating materials or the Leidenfrost (LF) effect. Therefore, a surface that can suppress the LF effect and reduce tc at high temperatures is required. To create such a surface, a double-reentrant groove (DRG) array surface with an overhanging structure on top of the microgrooves is developed. The overhanging structure renders the surface hydrophobic (HB). Despite its HB nature, the DRG surface's LF point (LFP) is observed at ≈530 °C, which is higher than the LFP on other HB surfaces. Moreover, a tc smaller than the inertia-capillary limit on the DRG surface is observed at between 400 and 500 °C. Accordingly, the DRG surface is currently the only HB surface for tc reduction at high temperatures. The DRG surface avoids the limitation of low LFPs observed on HB surfaces. Due to its HB properties, the DRG surface is determined to exhibit self-cleaning characteristics and can be used in various applications at high temperatures.

Elongated Bouncing and Reduced Contact Time of a Drop in the Janus State.

Fast drop bouncing is desired in numerous applications. However, it has never been realized on a superheated surface with concurrent contact boiling and the Leidenfrost effect (the so-called Janus thermal state). This is presumably because of the increased drop adhesion as a result of bubbling on a Janus surface. Nevertheless, on a hydrophilic SiN x surface with v-shaped Si microgrooves in this thermal state, an elongated bouncing for a water drop was observed and a small drop contact time of 10.36 ms was obtained. We propose that the elongated bouncing was originated from an asymmetric momentum force created by the vapor bubbles at the rim of the drop. Analytical expressions for the contact time and the asymmetric momentum force were established and the predictions qualitatively agreed with the experimental results. The obtained contact time was the lowest value that has ever been reported in the Janus thermal state. The proposed approach has promising applications in state-of-the-art technologies requiring high liquid mobility.

Ultrafast Diameter-Dependent Water Evaporation from Nanopores.

Evaporation from nanopores plays an important role in various natural and industrial processes that require efficient heat and mass transfer. The ultimate performance of nanopore-evaporation-based processes is dictated by evaporation kinetics at the liquid-vapor interface, which has yet to be experimentally studied down to the single nanopore level. Here we report unambiguous measurements of kinetically limited intense evaporation from individual hydrophilic nanopores with both hydrophilic and hydrophobic top outer surfaces at 22 °C using nanochannel-connected nanopore devices. Our results show that the evaporation fluxes of nanopores with hydrophilic outer surfaces show a strong diameter dependence with an exponent of nearly -1.5, reaching up to 11-fold of the maximum theoretical predication provided by the classical Hertz-Knudsen relation at a pore diameter of 27 nm. Differently, the evaporation fluxes of nanopores with hydrophobic outer surfaces show a different diameter dependence with an exponent of -0.66, achieving 66% of the maximum theoretical predication at a pore diameter of 28 nm. We discover that the ultrafast diameter-dependent evaporation from nanopores with hydrophilic outer surfaces mainly stems from evaporating water thin films outside of the nanopores. In contrast, the diameter-dependent evaporation from nanopores with hydrophobic outer surfaces is governed by evaporation kinetics inside the nanopores, which indicates that the evaporation coefficient varies in different nanoscale confinements, possibly due to surface-charge-induced concentration changes of hydronium ions. This study enhances our understanding of evaporation at the nanoscale and demonstrates great potential of evaporation from nanopores.

Control of Ice Formation.

Ice formation is a catastrophic problem affecting our daily life in a number of ways. At present, deicing methods are costly, inefficient, and environmentally unfriendly. Recently, the use of superhydrophobic surfaces has been suggested as a potential passive anti-icing method. However, no surface is able to repel frost formation at a very cold temperature. In this work, we demonstrated the abilities of spatial control of ice formation and confinement of the ice-stacking direction. The control and confinement were achieved by manipulating the local free energy barrier for frosting. The V-shaped microgroove patterned surface, which possessed these abilities, exhibited the best anti-icing and deicing performances among the studied surfaces. The insight of this study can be applied to alleviate the impact of icing on our daily life and in many industrial systems.

Scale effect on dropwise condensation on superhydrophobic surfaces.

Micro/nano (two-tier) structures are often employed to achieve superhydrophobicity. In condensation, utilizing such a surface is not necessarily advantageous because the macroscopically observed Cassie droplets are usually in fact partial Wenzel in condensation. The increase in contact angle through introducing microstructures on such two-tier roughened surfaces may result in an increase in droplet departure diameter and consequently deteriorate the performance. In the meantime, nanostructure roughened surfaces could potentially yield efficient shedding of liquid droplets, whereas microstructures roughened surfaces often lead to highly pinned Wenzel droplets. To attain efficient shedding of liquid droplets in condensation on a superhydrophobic surface, a Bond number (a dimensionless number for appraising dropwise condensation) and a solid-liquid fraction smaller than 0.1 and 0.3, respectively, are suggested.