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 contact 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 jitter in the onset of reconfigurations of the contact line as it pulls free from surface heterogeneities and moves rapidly across the surface. This time jitter causes the contact line configurations to become irreproducible during the short times they are falling. As the contact line 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. The presence of metastable states with temporally evolving energy barriers lead to the observed cyclical reproducibility/irreproducibility of the microscopic contact line configurations. In view of the reproducibility of the microscopic contact line configurations, we suggest methods of making reproducible macroscopic contact angle measurements on ambient surfaces.