Sedimentation of bidispersed glass fibre suspension

We study experimentally the settling of rigid glass fibre suspensions for insight into fibre fraction. Experiments were performed using a novel method for measuring movement of tracer glass fibres in a suspension where the majority of the glass particles were made invisible through an index of refraction matching technique. The sedimentation event was visualized simultaneously from two orthogonal images captured by a CCD camera from which we could construct a 3D-velocity field. Tests were conducted with mixtures of 2 mm and 5 mm glass fibres at consistencies up to 0,5 volume percent and with a number of different fluids with known viscosity. It was found that for very dilute suspensions the mean settling velocity increased as the fibre volume fraction increased. In this region, the settling velocity actually exceeded the terminal velocity of an isolated fibre. When the concentration was increased further, the mean sedimentation velocity was hindered. It was also found that for all cases tested, the velocity fluctuations increased with fibre concentration. Finally, fractionation could only be achieved under extremely dilute conditions