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Measuring the evolution of the real area of contact over short timescales provides insights into the onset of sliding.
We previously reported in this column on an elegant method for measuring the real area of a static contact that was developed by professor Jay Fineberg and his group at the Hebrew University of Jerusalem in Israel. The interface between two contacting Plexiglas" blocks was illuminated with a laser beam. The light was reflected from regions that were not in contact but was transmitted at the asperity-asperity contacts, thereby enabling the real area of contact to be measured from the amount of light transmitted.
The Fineberg group has more recently refined the experiment to address the much more complicated issue of what happens to the interface when a shear force is applied. Now the real area of contact is imaged in real-time using a fast video camera. An additional refinement is that displacements of the block are simultaneously measured by focusing another laser beam onto grids attached to the side of...