We seek to harness interfacial phenomena to achieve external, reversible, and local control of wetting and adhesion properties. The large surface to volume ratios provided when devices are shrunk to the micro- and nanoscale create particularly exciting opportunities for exerting control via tunable surface interactions.
To achieve this goal, we explore two separate avenues for the control of surface and interfacial properties: control of electrostatic interactions and design of surface structure. The importance of electrostaticsis approached by studying the nanoscale limits of electrowetting on dielectric, the design of responsive films that can be employed to move drops, and the use of surface charge as a means to control the assembly of nanoparticles at the oil-water interface. Our efforts in the control of surface structure have been focused on the understanding of the mechanisms for the adhesion of tree frogs under flooded condition, and on the importance of partial contact line pinning on the morphology of capillary bridges
Measurement and Scaling of Hydrodynamic Interactions in the Presence of Draining Channels
Authors: Rohini Gupta and Joelle Frechette
Langmuir 2012, 28, 14703-14712
Central to the adhesion and locomotion of tree frogs are their structured toe pads, which consist of an array of 10 μm hexagonal epithelial cells separated by interconnected channels that are 1 μm wide and 10 μm deep. It has been proposed that the channels facilitate the drainage of excess ﬂuid trapped between the toe pads and the contacting surface, and thus reduce the hydrodynamic repulsion during approach. We performed direct force measurement of the normal hydrodynamic interactions during the drainage of ﬂuid from the gap between a structured and a smooth surface using surface force apparatus. The structured surface consisted of a hexagonal array of cylindrical posts to represent the network of interconnected channels. The measured hydrodynamic drainage forces agree with the predictions from Reynolds’ theory for smooth surfaces at large separations. Deviations from theory, characterized by a reduction in the hydrodynamic repulsion, are observed below some critical separation (hc), which is independent of drive velocity. We employ a scaling analysis to establish the relationship between structural features (channel depth, width, and post diameter) and the critical separation for the onset of deviations. We ﬁnd agreement between our experiments and the scaling analysis, which allows us to estimate a characteristic length scale that corresponds to the transition from the ﬂuid being radially squeezed out of the nominal contact area to being squeezed out through the network of interconnected channels.