Velocity fluctuations of a heavy sphere falling through a sedimenting suspension

Abstract

The motion of a test sphere falling through an otherwise monodisperse suspension of different settling velocity has been examined experimentally. Marked test spheres of different sizes and densities were tracked in a monodisperse suspension of unmarked glass spheres, made optically transparent by matching the index of refraction of the suspending fluid to that of the glass spheres. The ratio, Rs, between the Stokes’ velocity of the test particle and that of an isolated sphere of the suspension was varied from 1 to 13 while the particle volume fraction of the suspension was kept constant at 20%. Statistical analyses of the velocities of the test sphere yield the mean settling velocities, the velocity fluctuations and the velocity autocorrelation functions. The long time motion of the test sphere is shown to be diffusive in nature. Correlation times and self-diffusivities are measured as a function of Rs. A change in the behavior of the motion of the test particle is observed for Rs≥5.