In this work, electrically controlled fully reversible wetting-dewetting transitions on superhydrophobic nanostructured surfaces have been demonstrated. Droplet behavior can be reversibly switched between the superhydrophobic Cassie-Baxter state and the hydrophilic Wenzel state by the application of electrical voltage and current. The nature of the reversibility mechanism was studied both experimentally and theoretically. The reported results can provide a new method of dynamically controlling liquid-solid interactions.

We analytically and numerically consider the hydrodynamic and thermal transport behavior of fully developed laminar flow through a superhydrophobic (SH) parallel-plate channel. Hydrodynamic slip length, thermal slip length and heat flux are prescribed at each surface. We first develop a general expression for the Nusselt number valid for asymmetric velocity profiles. Next, we demonstrate that, in the limit of Stokes flow near the surface and an adiabatic and shear-free liquid–gas interface, both thermal and hydrodynamic slip lengths can be found by redefining existing solutions for conduction spreading resistances. Expressions for the thermal slip length for pillar and ridge surface topographies are determined. Comparison of fundamental half-space solutions for the Laplace and Stokes equations facilitate the development of expressions for hydrodynamic slip length over pillar-structured surfaces based on existing solutions for the conduction spreading resistance from an isothermal source. Numerical validation is performed and an analysis of the idealized thermal transport behavior suggests conditions under which superhydrophobic microchannels may enhance heat transfer.

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We investigate forced convection in a parallel plate-geometry microchannel with superhydrophobic walls consisting of a periodic array of ridges aligned parallel to the direction of a Poiseuille flow. In the de-wetted (Cassie) state, the liquid contacts the channel walls only at the tips of the ridges, where we apply a constant heat flux boundary condition. The subsequent hydrodynamic and thermal problems within the liquid are then analysed accounting for curvature of the liquid-gas interface (meniscus) using boundary perturbation, assuming a small deflection from flat. The effects of this surface deformation on both the effective hydrodynamic slip length and the Nusselt number are computed analytically in the form of eigenfunction expansions, reducing the problem to a set of dual series equations for the expansion coefficients which must, in general, be solved numerically. The Nusselt number quantifies the convective heat transfer, the results for which are completely captured in a single figure, presented as a function of channel geometry at each order in the perturbation. Asymptotic solutions for channel heights large compared to ridge period are compared to numerical solutions of the dual series. The asymptotic slip length expressions are shown to consist of only two terms, with all other terms exponentially small. As a result these expressions are accurate even for heights as low as half the ridge period, and hence useful for engineering applications.

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