Dewetting through Forests of Micropillars

When a low-viscosity nonwetting fluid displaces a higher-viscosity wetting liquid in a rough surface consisting of a forest of micropillars, a film of the viscous defending fluid is left behind the advancing fluid-fluid front. Here, we investigate experimentally the dewetting of this residual film, and we contrast it with the dewetting that occurs on a smooth surface. We find that there are three regimes: ``no film'' at low capillary number; ``thick film'' at sufficiently high capillary number, and a novel ``thin film'' regime at intermediate capillary number. In this latter regime, the dewetting occurs as the residual viscous fluid retracts as a film of uniform thickness through (and not over) the micropillars. We propose necessary geometric conditions for the emergence of this new dewetting dynamics, and delineate the different dewetting regimes on a phase diagram, which quantitatively matches lab experiments conducted on a microstructured Hele-Shaw cells. The long-term state of the system induced by dewetting is a pattern of residual film clusters confined in the micropillar forest---a fluid arrangement that impacts macroscopic hydrodynamics and heat transfer efficiency.