Dynamic strain measurements in a sliding microstructured contact

A novel experiment is described which measures the tangential strain development across the contact between a PDMS (polydimethylsiloxane) block and a glass surface during the initial stages of sliding. The surface of the PDMS block has been microfabricated to take the form of a regular array of pyramidal tips at 20 µm separation. Tangential strain is measured by means of light scattering from the interface between the block and surface. Three phases are observed in all experiments: initial shear deformation of the whole PDMS block, a pre-sliding tangential compression of the tip array with stepwise increase of the compressive strain, and sliding in stick–slip movements as revealed by periodic variation of the strain. The stick–slip sliding between the regular tip array and the randomly rough counter surface always takes on the periodicity of the tip array. The fast slip can cause either a sudden increase or a sudden decrease in compressive strain.