How to make droplets run spontaneously and fast on surfaces
Abstract：Thousands of plant and animal species have been observed to have superhydrophobic surfaces that lead to various novel behaviors, such as keeping dry and self-cleaning. Superhydrophobicity is an enhanced effect of surface roughness. A mystery is why all the observed naturally occurring superhydrophobic surfaces have micro-submicron roughness. It was recently found that scaling-down roughness into the micro-submicron range is a unique and elegant strategy to not only achieves superhydrophobicity but also to increase its stability against environmental disturbances. For a rough surface with a given topology, there exists a critical size defined by the interface energies and three-phase line tension, at which the surface has the minimum adhesion and thus allows easier rolling than at other scales. How to make small droplets move spontaneously and directionally on solid surfaces is a challenge in lab-on-chip technologies, drug screening, and DNA analysis. The best-known mechanism, a wettability gradient, does not move droplets rapidly enough for most purposes and cannot move droplets smaller than a critical size defined by the contact angle hysteresis. We show a new general mechanism that is particularly effective at accelerating small droplets. Using this mechanism, we observed speeds of two and four orders of magnitude higher than obtained by wettability gradient in sub-millimeter and nano scales, respectively.
About the Speaker
Dr. Quanshui Zheng is a chair professor of the Engineering Mechanics Department at Tsinghua University, had serviced for seven years as the department chair. He is the funding Director of two multidisciplinary research institutes: the Center for Nano and Micro Mechanics at Tsinghua University, and the Institute for Advanced Study at Nanchang University. He is the Editor-in-Chief of Acta Mechanica Sinica. Dr. Zheng had visited UK, France, Germany, USA, and Australia for more than five years as a Royal Society fellow, Alexander von Humboldt fellow, visiting professor, or joint professor. The major interests of his group include: novel solutions for nano- and micro-scale devices, transportations, and flow; mechanics of heterogeneous and/or anisotropic materials; and theory of tensors and tensor functions and rational mechanics.