Stick-Slip: Wet Versus Dry

Abstract A rubber lens (polydimethylsiloxane) is pressed against silanated or bare glass plates (Johnson-Kendall-Roberts (JKR) contact). As the plate slides with a velocity U, we measure the friction on the lens using a "macro Atomic Force Microscope (AFM)", where the cantilever is a thin rectangular glass rod and the tip is the rubber lens. We observe the contact area via optical interferometry. In air for "hard" lenses (Young's modulus E ≈ 1 MPa), we find smooth sliding on a model substrate, and a transition to stick-slip on a hysteretic substrate above a threshold velocity, V M . For soft lenses (E ≈ 0.1 MPa), we observe Schallamach waves and stick-slip depending on normal force and the plate's velocity, U. When immersed in a liquid (silicone oils, water-glycerol mixtures), the contact remains dry at low velocities, but is invaded by a liquid film above a critical velocity, U c . For hard lenses we observe smooth sliding and high friction below U c , and low friction above U c . For soft lenses, we find wet Schallamach waves for U < V M and stick-slip instabilities at large velocities. In the stick-slip regime, the contact is wet in the slip phase, and dewets in the stick phase. We measure the period of the stick-slip cycle as a function of the liquid viscosity. We interpret the stick-slip process by the formation and rupture of adhesive bonds (between the rubber polymer chains and active sites on the glass). Using a recent model, we can explain most of the data for the stick-slip period and slip threshold velocity. Keywords: LubricationRubber/solid frictionSliding instabilitiesStick-slip One of a Collection of papers honoring Liliane Léger, the recipient in February 2007 of The Adhesion Society Award for Excellence in Adhesion Science, Sponsored by 3m. Notes 1If N is the number of units between cross links in bulk rubber, we can estimate ζ ≈ ηR/D where nR ≈ n 0 N and D ≈ N 1/2 a is the mesh size of the rubber. †Deceased.

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