Ordering in sediments

Abstract

A carefully dialysed silica suspension with an initial volume fraction of about 4% in pure ethanol has been studied for its sedimentation behaviour using both an optical diffraction method and an acoustic time-of-flight technique. The results show that when the suspension sediments under gravity, the densely-packed layer of sediment becomes highly ordered, and thus diffracts light well, giving sharp Bragg peaks. The time taken for this order to become measurable is about one week, and the intensity and line width of the diffraction peaks suggest that this ordering improves with time over the next two weeks or so. The acoustic measurements show a rather different state of order in the sediment: Until a time of some 47 days has elapsed, the acoustic signal has great difficulty in passing through the packed layer. Quite suddenly, this acoustic opacity disappears, and a clear signal is transmitted. We have not been able to establish exactly how rapid this transition is, but it appears to occur on a time scale of hours or less. Within the sediment, the density of packing varies with sediment depth, becoming greater as the depth increases, until a height of a few millimetres from the base of the cell. This density increase may be partly due to the pressure developed by the particles in the sediment above the height at which the measurements are made, partly to the polydispersity of the silica sol, and possibly to accumulation of a coexisting and more dense crystalline phase at the base of the cell. A further possible explanation is that solvent extrusion takes place slowly, and at long times, this variation would disappear as the system reached equilibrium, but this has not been experimentally verified.