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:

• Lubrication
• Rubber/solid friction
• Sliding instabilities
• Stick-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 ≈ η_R/D where n_R  ≈ n ₀ N and D ≈ N ^1/2 a is the mesh size of the rubber.

^†Deceased.