Capillary Rise on Legs of a Small Animal and on Artificially Textured Surfaces Mimicking Them

The wharf roach Ligia exotica is a small animal that lives by the sea and absorbs water from the sea through its legs by virtue of a remarkable array of small blades of micron scale. We find that the imbibition dynamics on the legs is rather complex on a microscopic scale, but on a macroscopic scale the imbibition length seems to simply scale linearly with elapsed time. This unusual dynamics of imbibition, which usually slows down with time, is advantageous for long-distance water transport and results from repetition of unit dynamics. Inspired by the remarkable features, we study artificially textured surfaces mimicking the structure on the legs of the animal. Unlike the case of the wharf roach, the linear dynamics were not reproduced on the artificial surfaces, which may result from more subtle features on the real legs that are not faithfully reflected on the artificial surfaces. Instead, the nonlinear dynamics revealed that hybrid structures on the artificial surfaces speed up the water transport compared with non-hybrid ones. In addition, the dynamics on the artificial surfaces turn out to be well described by a composite theory developed here, with the theory giving useful guiding principles for designing hybrid textured surfaces for rapid imbibition and elucidating physical advantages of the microscopic design on the legs.