The retention and suspension of particles in a fluid using oscillatory flow

Experimental observations are reported in which some suspended neutrally buoyant particles are retained within a straight channel through which the suspension is flowing by adding an oscillatory component to the flow. The degree of retention of particles is correlated with the frequency and amplitude of the applied oscillation. Under certain conditions, single particles may be trapped indefinitely at fixed positions within the channel. A similar trapping effect is reported for naturally sedimenting particles. The flow within the channel is laminar, but it is suggested that the suspension effect is related to vortex formation. Trapping within vortices is shown directly in experiments in steady flow through an expansion.