PEELING STRUCTURED SURFACES IN VISCOUS ENVIRONMENTS: THE ROLE OF DEFORMATION AND DRAINAGE CHANNELS
Dhong, Charles B
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Tree frogs exhibit remarkable adhesion in completely flooded environments without the aid of chemical adhesives, interlocking supports, or capillary forces. This ability has been attributed to the specialized structures on their toe pads, which consist of a hexagonal array of hierarchical structures. The arrangement of these structures form drainage channels that aid in removing the fluid between the toe pad and surface to make rapid and reversible contact. While it has been shown that many animals, such as geckos, take advantage of patterned toe pads to enhance adhesion in dry and wet environments, less is known about how the drainage channels on the tree frog’s toe pads are able to modulate its adhesion in completely flooded environments. These insights could be relevant to many natural and industrial processes including hydrofracture, micro-contact printing, self-assembly and soft robotics. In addition to these structured surfaces, 1) their toe pads are highly deformable, 2) they approach a surface in the normal direction, but detach in a peeling mode, and 3) their toe pads are subject to both viscous forces and van der Waals interactions once in contact. To investigate this coupled phenomenon, we created a custom-made peeling apparatus to mimic the tree frog toe pad by using micro-patterned flexible plates (e.g. glass coverslips). We systematically deconstruct these coupled mechanisms by altering our samples or the fluid bath. By building the complexity methodically, we maintain a link between each new investigation and the results of the preceding study. First, we chose materials that minimize van der Waals forces to isolate the coupling between the mechanical properties of unpatterned plates (the Young’s modulus and the rigidity) and the viscous forces. We developed scaling arguments that explain how the peak force and work needed to separate the plate scale with the viscosity and rigidity. We also explain why deformable samples will further reduce the viscous forces compared to samples of the same rigidity. We then micro-fabricated rigid structures on these plates that do not alter the bulk mechanical properties of the plates, but create drainage channels that alter their approach and detachment from a bottom surface. We have found that structured surfaces reduced forces compared to flat samples, but only in regimes where the fluid can enter channels. We are able to correlate these regimes to the structure geometry. Finally, we fabricated the same structures out of a more deformable material which reduced the viscous forces further than their rigid counterparts. We attribute this reduction to the fluid flow around the pillars, as opposed to a mechanical response from the plate. Then, by coating these deformable structures in a silicate layer and changing the fluid, we amplify the van der Waals forces. We examine the competing effects of the structured surfaces, which reduce viscous forces, but enhance adhesion from van der Waals forces. We summarize how the mechanical properties of the plate, drainage channels and deformable structures alter the viscous forces and van der Waals forces associated with detaching in flooded environments.