An Experimental Approach to Study Drainage in Relation to Geometrical Features of Artificial Adhesives

Abstract

With growing interest in wet adhesion, animals such as tree frogs are often used as paradigm for creating grippers in wet environments. Tree frogs are capable of attaching to surfaces under several conditions (smooth, rough, dry, wet, and flooded) due to the microstructure of their toe pads. This microstructure consists of pillar-shaped hexagonal cells separated by thin and deep channels. Drainage is an important phenomenon to consider, since too much liquid between the toe pad and surface results in a low adhesive force. Therefore, understanding the relationship between pillar-shaped microstructures and drainage can lead to the creation of adhesive pads that are functional in wet conditions and to other drainage related applications.

Adhesives with a pillar-shaped microstructure were fabricated and their drainage speed was experimentally studied against a glass substrate. Gathering experimental data was accomplished by using Particle Image Velocimetry, and by using fluid velocity as a proxy for drainage. A custom-made set-up was built to accurately control the speed at which the adhesive and the substrate approached each other. Adhesives with various pillar diameters, pillar spacings, and pillar alignments were tested with fluids of various viscosities. It was found that the pillar diameter most likely has a significant effect on drainage, whereas the effect of pillar spacing and the effect of fluid type could not be determined. Pillar alignment did not have a significant effect on drainage.

In future work, it is recommended to complement the findings of this thesis with adhesion and friction measurements to understand how drainage associates to adhesive force.