Interfacial propulsion by directional adhesion

Authors: Manu Prakash, John W. M. Bush

Link: https://www.sciencedirect.com/science/article/abs/pii/S0020746210001836?via%3Dihub

DOI: https://doi.org/10.1016/j.ijnonlinmec.2010.12.003

Abstract: The rough integument of water-walking arthropods is well-known to be responsible for their water-repellency [1], [2], [3], [4]; however, water-repellent surfaces generally experience reduced traction at an air–water interface [5], [6], [7], [8]. A conundrum then arises as to how such creatures generate significant propulsive forces while retaining their water-repellency. We here demonstrate through a series of experiments that they do so by virtue of the detailed form of their integument; specifically, their tilted, flexible hairs interact with the free surface to generate directionally anisotropic adhesive forces that facilitate locomotion. We thus provide new rationale for the fundamental topological difference in the roughness on plants and water-walking arthropods, and suggest new directions for the design and fabrication of unidirectional superhydrophobic surfaces.

Additional Information:

So you are sitting on a pond, watching tiny rain drops hit the surface and ripple along. It's all peace and quiet on the surface of a pond. But suddenly comes a "water strider" a cheetah of the surface tension world zipping along the water like nobody's business (~0.5 m/sec). But wait, didn't all the scanning electron microscopy images show that the legs of a water strider (and almost all the 1800 other species) are superhydrophobic. So if you don't touch the fluid interface, how do you generate such high traction forces. I built a fluid-interface force spectroscopy setup to measure direct propulsion forces generated by individual superhydrophobic surface - and "aha" to our surprise, water strider legs exhibit unidirectional anisotropy. What that means is the surface has a preferential direction in which a fluid contact line would happily move along one direction on a surface but have a high resistance when moving along another. This work has now inspired a large number of "unidirectional superhydrophobic surfaces" with commercial applications. But wait, water striders thought of this idea millions of years ago.