REPRODUCIBILITY OF CONTACT LINE MOTION ON SURFACES EXHIBITING CONTACT-ANGLE HYSTERESIS

We examine the quasi-static microscopic motion of the contact line on an ambient surface which exhibits contact angle hysteresis. When the c ontact line configuration is static or moving slowly, the microscopic contact line configurations are highly reproducible between two cycles of immersion of the surface. Mechanical vibrations produce a time jit ter in the onset of reconfigurations of the contact line as it pulls f ree from surface heterogeneities and moves rapidly across the surface. This time jitter causes the contact line configurations to become irr eproducible during the short times they are falling. As the contact li ne repins on new defects, the contact line configurations again become reproducible from one macroscopic cycle to the next. The observations are explained by energy models used to describe wetting hysteresis. T he presence of metastable states with temporally evolving energy barri ers lead to the observed cyclical reproducibility/irreproducibility of the microscopic contact line configurations. In view of the reproduci bility of the microscopic contact line configurations, we suggest meth ods of making reproducible macroscopic contact angle measurements on a mbient surfaces.