Capillary rise in porous media. Part I. a problem

Capillary rise experiments have been performed with homogeneous packings of coarse rotund particles, yielding a capillary rise at equilibrium of the order of 10 cm. Water, toluene, and other organic liquids have been used as the wetting liquid, and glass beads, polystyrene beads, and sand to make the packings. The phenomena that are observed can be classified using two parameters: the saturation gradient and the time to reach equilibrium:(i) When, after capillary rise, equilibrium is reached between gravity and capillarity, sometimes a wide saturation gradient is observed, while in other cases there is no saturation gradient, i,e. sometimes there is a sharp separation between the dry part of the porous medium and the completely saturated part, and sometimes there is a very gradual change from the saturated to the dry part.(ii) In some cases equilibrium is reached after a few minutes; in other cases it may take several weeks.All four possible combinations of behaviour occur. Hence, there seem to be four kinds of capillary rise. This constitutes a problem, the more so because one system may fall into different classes.