AVERAGE SPREADING PARAMETER ON HETEROGENEOUS SURFACES

We study experimentally the behavior of a liquid deposited on heterogeneous nonwetting substrates characterized by large contact-angle hysteresis, delta = cos theta(r) - cos theta(a) (theta(a) and theta(r) being respectively the advancing or receding angle). By vibrating the substrate, we are able to measure an average spreading coefficient. We used a wide range of surfaces and found that the final contact angle thetaBAR (after vibrating the substrate) is roughly related to theta(a) and theta(r) by cos thetaBAR = 1/2(cos theta(a) + cos theta(r)).