Drop Retention and Departure in Shear Flow on Structured Superhydrophobic Surfaces

Drops are retained on surfaces due to adhesion forces between the drop and the surface. The adhesion force depends on the surface tension of the liquid, drop geometry, and the contact angle between the drop and the surface. When gravitational or fluid dynamic forces exceed the adhesion force the drop begins to move. Various models have been proposed in previous works to relate when a droplet will depart from a flat plate due to an imposed air velocity. However, no validated model exists that can predict the onset of droplet motion over a large range of static contact angles ranging both hydrophobic and superhydrophobic regimes. In this study we explore the drop departure phenomena for hydrophobic and superhydrophobic (SH) surfaces that exhibit static contact angles from 115° to 170°. A steady air flow (ranging in speed from 1-5 m/s) is directed parallel to a flat plate and particle image velocimetry (PIV) has been utilized to characterize the near wall velocity distribution. The SH surfaces explored are fabricated with circular post microstructures of varying pitch (structure to structure spacing) and cavity fractions. Image analysis is utilized to quantify the instantaneous centroid position and contact angles of the drops as they oscillate and then eventually depart from the surface. Prior models have been evaluated and it has been shown that predictions deviate significantly from experimental results when the models are used outside of the regime (hydrophobic, superhydrophobic) they were developed for.