The undrainable water in quartz sand and glass beads

The extent to which the magnitude of the undrainable water depends on its surface tension in sand and glass beads is studied by a free drainage column and a pressure porous ceramic plate. Undrainable water consists of two main elements or ‘types’: (a) film water, the thickness of which could be as high as 1 μm, depending on the unevenness of the surface, and (b) ring water retained by capillary forces at the points of contact of the particles. Film water predominates in the fine particles, whereas ring water predominates in the coarse particles. When there is some dispersion, both film and ring water should be present in the same proportion in undrainable water. From a simple consideration (h ≈ 1 μm; A_o/A = 1.5 – 2.0), it follows that this dispersion lies within particle diameters of 4 – 6 × 10^?4 m. Both methods show that a decrease in water content is observed concomitantly with a decrease in surface tension only when fine (<0.4 mm) particles are involved, while there is no such decrease with particles >0.4 mm. Apparently, with a decrease in the size of particles due to the overlaying and overlapping of ring and film water, fine capillaries are formed from which drainage is more dependent on surface tension. In any case, the magnitude of the undrainable water increases when the size of the particles decreases.