In-Situ Measurement of Contact Area and Deformation of Soft Materials: Probing Adhesion Hysteresis

In this paper, we zoom in on the points of contact between the two materials in order to truly understand the surface-to-surface interface. We choose aluminum oxide (Ruby/Sapphire) spheres and polydimethylsiloxane (PDMS) discs to investigate the variability of real adhesive contact. An in-situ optical tribometer is built and implemented for use in obtaining forces of adhesion between PDMS and Ruby and comparing against the Johnson-Kendall-Roberts (JKR) contact model. Adhesion hysteresis is explored and compared to results found in the literature. High resolution in-situ imagery is coupled with custom data acquisition software to examine the relationship of contact area to applied force, loading rate, dwell time, and unloading rate. Contact area and pull-off force are shown to be dependent on applied force, dwell time, and unloading rate, while loading rate shows no major effect. Newton Rings in captured in-situ contact images are used to create three-dimensional models and height maps of the tensile region. This method provides accurate representations of surface and bulk behavior in a variety of contact conditions. A tensile zone is found at the edge of contact for all testing scenarios.