(452h) Energetics of the Cassie/Wenzel Transition On Parallel Cylinders: Applications in Characterization of Bird Feathers and in Designing Robust Friction Reducing Meshes

Srinivasan, S., Harvard University
McKinley, G. H., Massachusetts Institute of Technology
Cohen, R. E., Massachusetts Institute of Technology

In this work, we present a unifed model to evaluate the external pressure at which a wetting transition occurs on a liquid-solid composite interface that rests on an array of parallel cylinders. We extend previous work in characterizing three possible modes of Cassie/Wenzel transition (i.e, depinning, meniscus sagging and coalescence) on an array of parallel cylinders, and develop analytical expressions to determine the magnitude of the energy barrier at the onset of each transition in terms of the radius of the cylinder R, a dimensionless parameter D* that characterizes the geometric spacing, and the equilibirum contact angle θe.

We then apply this analytical framework to explain two wetting phenomena: (i) Characterization of the hierarchical topography of bird feathers using goniometric measurements in order to explain the diving and wing spreading behavior of underwater diving birds using the thermodynamic analysis developed above. We perform contact angle measurements using a set of polar and non-poplar probing liquids on dip-coated feathers of 6 bird species, and apply our model to determine the dimensionless spacing ratio (1 < D* < 2) and an effective radius of the quasi-heirarchial structure Reff. (ii) Towards engineering optimal design geometries of superhydrophobic mesh surfaces that can maximize giant liquid slip in laminar flow while simultaneously providing robustness against the Cassie-Wenzel transition.