Inchworms and Tree Frogs: Modulation of Interfacial Properties Using External Fields and Surface Topography ( More about this event )
Speaker: Joelle Frechette, Johns Hopkins University
Department: Chemical & Biological Engineering
Location: Engineering Quadrangle A224
Date/Time: Wednesday, December 5, 2012, 4:00 p.m. - 5:00 p.m.
This presentation will discuss two of the avenues we employ to modulate and understand interfacial properties: the application of an external stimulus and the design of surface structure or heterogeneities. First, we will discuss our efforts to direct the motion of droplets by mimicking inchworm motion using surfaces with tunable contact angle hysteresis. The shape and motion of drops on surfaces is governed by the balance between the driving and pinning forces. We demonstrate control over the motion of droplets, when subjected to a uniform force field (gravity or pressure drop), by exerting control over the contact angle hysteresis. The external modulation of contact angle hysteresis is achieved through a voltage-induced local molecular reorganization within the surface film at the solid-liquid interface. We show that tuning contact angle hysteresis alone is sufficient to direct and deform drops in the absence of a pre-defined surface energy gradient or pattern. We also show that the observed stretching and contraction of the drops mimic the motion of an inchworm. Such reversible manipulation of the pinning forces could be an attractive means to direct drops, especially with the dominance of surface forces at micro/nanoscale.
Second, we will discuss our efforts to understand the mechanisms behind the adhesion of the tree frog under wet conditions. The locomotion mechanisms employed by tree frogs under flooded conditions offer the ultimate solution for the need of strong, reversible, reusable, tunable, and water tolerant adhesives. The presence of structured toe-pads (hexagonal epithelial cells separated by channels) has been proposed to facilitate drainage of fluid and reduce hydrodynamic repulsion during approach. These channels may remain closed during retraction for hydrodynamic adhesion and open for pull-off, both of which require active/passive control of toe-pad deformation. We present results from our investigation of the role of draining channels on normal hydrodynamic interactions in the absence of elastohydrodynamics (rigid surfaces). The surface force apparatus is employed to measure hydrodynamic forces between a smooth silver film and a structured array driven towards or away from each other at a constant drive velocity. The array consists of cylindrical posts. The hydrodynamic forces are analyzed within the framework of Reynolds continuum approach in the lubrication limit for smooth surfaces in cross-cylinder geometry. We observe a reduction in repulsion upon approach with respect to that predicted by Reynolds theory. Our results are analyzed using a scaling argument based on the geometric parameters associated with the structures (diameter of posts, channel width and depth, and surface coverage). Viscosity-dominated drainage of fluid in the presence of a network of channels intrinsically results in competition between drainage from the contact region and through the network of channels. The scaling argument can then be used to predict regimes of preferential drainage of fluid through the network of channels versus those with drainage of fluid from the contact region for a single post or the cross-cylinders as a whole as the surface separation changes.