Direct Observation of Gas Meniscus Formation on a Superhydrophobic Surface

Effects of Gas Layer Thickness on Capillary Interactions at Superhydrophobic Surfaces

Strongly attractive forces act between superhydrophobic surfaces across water due to the formation of a bridging gas capillary. Upon separation, the attraction can range up to tens of micrometers as the gas capillary grows, while gas molecules accumulate in the capillary. We argue that most of these molecules come from the pre-existing gaseous layer found at and within the superhydrophobic coating. In this study, we investigate how the capillary size and the resulting capillary forces are affected by the thickness of the gaseous layer. To this end, we prepared superhydrophobic coatings with different thicknesses by utilizing different numbers of coating cycles of a liquid flame spraying technique. Laser scanning confocal microscopy confirmed an increase in gas layer thickness with an increasing number of coating cycles. Force measurements between such coatings and a hydrophobic colloidal probe revealed attractive forces caused by bridging gas capillaries, and both the capillary size and the range of attraction increased with increasing thickness of the pre-existing gas layer. Hence, our data suggest that the amount of available gas at and in the superhydrophobic coating determines the force range and capillary growth.

Effects of liquid surface tension on gas capillaries and capillary forces at superamphiphobic surfaces

The formation of a bridging gas capillary between superhydrophobic surfaces in water gives rise to strongly attractive interactions ranging up to several micrometers on separation. However, most liquids used in materials research are oil-based or contain surfactants. Superamphiphobic surfaces repel both water and low-surface-tension liquids. To control the interactions between a superamphiphobic surface and a particle, it needs to be resolved whether and how gas capillaries form in non-polar and low-surface-tension liquids. Such insight will aid advanced functional materials development. Here, we combine laser scanning confocal imaging and colloidal probe atomic force microscopy to elucidate the interaction between a superamphiphobic surface and a hydrophobic microparticle in three liquids with different surface tensions: water (73 mN m^-1), ethylene glycol (48 mN m^-1) and hexadecane (27 mN m^-1). We show that bridging gas capillaries are formed in all three liquids. Force-distance curves between the superamphiphobic surface and the particle reveal strong attractive interactions, where the range and magnitude decrease with liquid surface tension. Comparison of free energy calculations based on the capillary menisci shapes and the force measurements suggest that under our dynamic measurements the gas pressure in the capillary is slightly below ambient.

When is a hydrophobic gate not a hydrophobic gate?

The flux of ions through a channel is most commonly regulated by changes that result in steric occlusion of its pore. However, ion permeation can also be prevented by formation of a desolvation barrier created by hydrophobic residues that line the pore. As a result of relatively minor structural changes, confined hydrophobic regions in channels may undergo transitions between wet and dry states to gate the pore closed without physical constriction of the permeation pathway. This concept is referred to as hydrophobic gating, and many examples of this process have been demonstrated. However, the term is also now being used in a much broader context that often deviates from its original meaning. In this Viewpoint, we explore the formal definition of a hydrophobic gate, discuss examples of this process compared with other gating mechanisms that simply exploit hydrophobic residues and/or lipids in steric closure of the pore, and describe the best practice for identification of a hydrophobic gate.

Hydrophobic dewetting in gating and regulation of transmembrane protein ion channels

Water is at the heart of almost all biological phenomena, without which no life that we know of would have been possible. It is a misleadingly complex liquid that exists in near coexistence with the vapor phase under ambient conditions. Confinement within a hydrophobic cavity can tip this balance enough to drive a cooperative dewetting transition. For a nanometer-scale pore, the dewetting transition leads to a stable dry state that is physically open but impermeable to ions. This phenomenon is often referred to as hydrophobic gating. Numerous transmembrane protein ion channels have now been observed to utilize hydrophobic gating in their activation and regulation. Here, we review recent theoretical, simulation, and experimental studies that together have started to establish the principles of hydrophobic gating and discuss how channels of various sizes, topologies, and biological functions can utilize these principles to control the thermodynamic properties of water within their interior pores for gating and regulation. Exciting opportunities remain in multiple areas, particularly on direct experimental detection of hydrophobic dewetting in biological channels and on understanding how the cell may control the hydrophobic gating in regulation of ion channels.

Efficient Gas Transportation Using Bioinspired Superhydrophobic Yarn as the Gas-Siphon Underwater

Inspired by the gas-trapped mechanism underwater of Argyroneta aquatica, we prepared a superhydrophobic yarn with a fiber network structure via a facile and environmentally friendly method. Attributed to the low surface energy, the superhydrophobic fiber network structure on the yarn is able to trap and transport bubbles directionally underwater. The functional yarn has good superhydrophobic and superaerophilic properties underwater to realize the directional transport of bubbles underwater without being pumped. We designed demonstration experiments on the antibuoyancy directional bubble transportation, which indicated the feasibility in the applications of gas-related fields. Significantly, on further testing, where the superhydrophobic yarn is put into a U-shaped pipe, we obtain a gas-siphon underwater with a high flux. The superhydrophobic fiber structure yarn can trap the gas underwater to enable the self-starting behavior while no manual intervention is used. The gas-siphon can convey gas over the edge of a vessel and deliver it at a higher level without energy input, which is driven by the differential pressure. The relationship between the differential pressure and the volume flux of transport bubbles is investigated. The experimental results show that the prepared superhydrophobic yarn has the advantages of good stability, easy preparation, and low cost in bubble continuous transportation underwater, which provides a novel strategy for the development and application of new technologies such as directional transportation, separation, exhaustion, and collection of gases in water.

Wetting Transition on Liquid-Repellent Surfaces Probed by Surface Force Measurements and Confocal Imaging

Superhydrophobic surfaces in the Cassie-Baxter wetting state retain an air layer at the surface which prevents liquid water from reaching into the porous surface structure. In this work we explore how addition of ethanol, which reduces the surface tension, influences the wetting properties of superhydrophobic and smooth hydrophobic surfaces. Wetting properties are measured by dynamic contact angles, and the air layer at the superhydrophobic surface is visualized by laser scanning confocal microscopy. Colloidal probe atomic force microscopy measurements between a hydrophobic microsphere and the macroscopic surfaces showed that the presence of ethanol strongly affects the interaction forces. When the macroscopic surface is superhydrophobic, attractive forces extending up to a few micrometers are observed on retraction in water and in 20 vol % ethanol, signifying the presence of a large and growing gas capillary. Submicrometer attractive forces are observed between the probe particle and a smooth hydrophobic surface, and in this case a smaller gas capillary is formed. Addition of ethanol results in markedly different effects between superhydrophobic and hydrophobic surfaces. In particular, we show that the receding contact angle on the superhydrophobic surface is of paramount importance for describing the interaction forces.

Fabrication of Wettability Mesh with Quasi-Rectangular-Restraining Capacity to Water

A water droplet placed on a surface is usually round owing to surface tension. Restraining a droplet to a rectangle shape has been rarely reported. Herein, we fabricated three meshes with diverse wettability including ordinary mesh, superhydropilic mesh, and quasi-rectangular-restraining mesh. The profiles of water droplets on these three meshes were entirely different from the top view, especially for the quasi-rectangular-restraining mesh, which enables the water droplet on it to achieve the rectangular shape. The surface morphologies and chemical compositions of the meshes were characterized by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Moreover, the influences of processing parameters of the quasi-rectangular-restraining mesh on the quasi-rectangular quality of the water droplet on it were investigated to obtain the relatively optimum processing parameters. The dynamic properties of water droplets on the three meshes were compared, and forces acting on the water droplets during the spreading and shrinking processes on the three meshes were qualitatively analyzed. Additionally, we studied the influences of falling height and water volume on the quasi-rectangular quality of the water droplet on the quasi-rectangular-restraining mesh. Water droplets on the quasi-rectangular-restraining mesh demonstrated good stability under vibration and the droplet could maintain the quasi-rectangular quality on the quasi-rectangular-restraining mesh for about 7 days, revealing a good durability. Further, the large-scaled fabrication of the quasi-rectangular-restraining mesh was realized.