Dynamics of Contact Line Pinning in Capillary Rise and Fall

The dynamics of water columns rising and falling in vertical glass capillaries are studied near the pinning threshold. In tubes with rough interior walls, the column height $h$ as a function of time $t$ exhibits stick-slip behavior. In tubes with smooth walls, falling experiments show no pinning effects, but rising experiments exhibit critical behavior with nonunique exponents. We argue that this is due to the fact that fluid has to be transported in a microscopic wetting film which does not have a stationary structure. Similar restrictions can affect avalanche dynamics in other systems.