Thousands of plant and animal species have been observed to have superhydrophobic surfaces that lead to various novel behaviors. These observations have inspired attempts to create artificial superhydrophobic surfaces, given that such surfaces have multitudinous applications. Superhydrophobicity is an enhanced effect of surface roughness and there are known relationships that correlate surface roughness and superhydrophobicity, based on the underlying physics. However, while these examples demonstrate the level of roughness they tell us little about the independence of this effect in terms of its scale. Thus, they are not capable of explaining why such naturally occurring surfaces commonly have micron-submicron sizes. Here we report on the discovery of a new relation, its physical basis and its experimental verification. The results reveal that scaling-down roughness into the micro-submicron range is a unique and elegant strategy to not only achieve superhydrophobicity but also to increase its stability against environmental disturbances. This new relation takes into account the previously overlooked but key fact that the accumulated line energy arising from the numerous solid-water-air intersections that can be distributed over the apparent contact area, when air packets are trapped at small scales on the surface, can dramatically increase as the roughness scale shrinks. This term can in fact become the dominant contributor to the surface energy and so becomes crucial for accomplishing superhydrophobicity. These findings guide fabrication of stable super water-repellant surfaces.
Recent experiments and molecule dynamics simulations have shown that adhesion droplets on conical surfaces may move spontaneously and directionally.Besides, this spontaneous and directional motion is independent of the hydrophilicity and hydrophobicity of the conical surfaces.Aimed at this important phenomenon,a general theoretical explanation is provided from the viewpoint of the geometrization of micro/nano mechanics on curved surfaces.In the extrinsic mechanics on micro/nano soft curved surfaces,we disclose that the curvatures and their extrinsic gradients form the driving forces on the curved spaces.This paper focuses on the intrinsic mechanics on micro/nano hard curved surfaces and the experiment on the spontaneous and directional motion.Based on the pair potentials of particles,the interactions between an isolated particle and a micro/nano hard curved surface are studied,and the geometric foundation for the interactions between the particle and the hard curved surface is analyzed.The following results are derived:(a)Whatever the exponents in the pair potentials may be, the potential of the particle/hard curved surface is always of the unified curvature form, i.e.,the potential is always a unified function of the mean curvature and the Gaussian curvature of the curved surface,(b)On the basis of the curvature-based potential,the geometrization of the micro/nano mechanics on hard curved surfaces may be realized, (c)Similar to the extrinsic mechanics on micro/nano soft curved surfaces,in the intrinsic mechanics on micro/nano hard curved surfaces,the curvatures and their intrinsic gradients form the driving forces on the curved spaces.In other words,either on soft curved surfaces or hard curved surfaces and either in the extrinsic mechanics or the intrinsic mechanics, the curvatures and their gradients are all essential factors for the driving forces on the curved spaces,(d)The direction of the driving force induced by the hard curved surface is independent of the hydrophilicity and hydrophobicity of the curved
Three-phase line tensions may become crucial in the adhesion of micro-nano or small droplets on solid planes.In this paper we study for the first time the nonlinear effects in adhesion spanning the full range of physically possible parameters of surface tension, line tension,and droplet size.It is shown that the nonlinear adhesion solution spaces can be characterized into four regions.Within each region the adhesion behaves essentially the same.Especially,inside the characteristic regions with violent nonlinearities,the co-existence of multiple adhesion states for given materials is disclosed.Besides,two common fixed points in the solution space are revealed.These new results are consistent with numerical analysis and experimental observations reported in the literatures.
