Contact line dynamics near the pinning threshold: A capillary rise and fall experiment

We used video microscopy to study the pinning dynamics of air/water contact lines in vertical glass capillaries. Stick-slip behavior and avalanches ar e observed in tubes with rough interior walls and strong pinning forces. In tubes with smooth interior walls, we find that receding contact lines in f alling water columns show no evidence of pinning, but advancing contact lin es in rising water columns exhibit algebraic slow down. The measured value of the critical exponent beta varies from run to run, but it is always larg er than unity. Furthermore, we find that the rise dynamics varies with the waiting time preceding the experiments. These observations led us to conclu de that the wetting film on the surface and other microscopic changes in th e slipping region near the contact line affect the macroscopic dynamics. We discuss the differences between the real system and the existing theories that might explain the results. We also present a brief review of other stu dies of contact line dynamics and a numerical study of a one-dimensional mo del.