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Scientific Unit
In underwater adhesion of a topographically patterned surface with a very soft material such as human skin, the elastic deformation can be large enough to achieve solid-on-solid contact not only on top of the hills but also in the valleys of the substrate topography. In this context, we have studied the dynamics of dewetting of a thin liquid film confined between a rigid, periodic micropillar array and a soft, elastic sphere. In our experiments, we observed two very distinct dewetting morphologies. For large ratios of array period to micropillar height and width, the dewetted areas tend to have a diamond-like shape and expand with a rate similar to a flat, unpatterned substrate. When the array period is reduced, the morphology of the dry spot becomes irregular and its expansion rate is significantly reduced. We developed a fully coupled numerical model of the dewetting process that reproduces the key features observed in experiments. Moreover, we performed contact mechanics simulations to characterize the deformation of the elastomer and the shape of the dewetted area in a unit cell of the micropillar array.
Micropatterned dry adhesives rely mainly on van der Waals interactions. In this paper, we explore the adhesion strength increase that can be achieved by superimposing an electrostatic field through interdigitated subsurface electrodes. Micropatterns were produced by replica molding in silicone. The adhesion forces were characterized systematically by means of experiments and numerical modeling. The force increased with the square of the applied voltage for electric fields up to 800 V. For larger fields, a less-than-quadratic scaling was observed, which is likely due to the small, field-dependent electrical conductivity of the materials involved. The additional adhesion force was found to be up to twice of the field-free adhesion. The results suggest an alternative method for the controlled handling of fragile or miniaturized objects.
