Stick-slip: Wet versus dry

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 approximate to 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 approximate to 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, Uc. For hard lenses we observe smooth sliding and high friction below Uc, and low friction above Uc. 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.