Suspension plug in oscillatory pipe flow

Non-colloidal particles under shear are known to exhibit irreversible, cross-stream migrations at even low Reynolds numbers. We experimentally investigate this phenomenon in a suspension plug of finite lengths inside a cylindrical tube, subjected to an oscillating pressure-driven flow. The plug consists of neutrally buoyant, non-colloidal particles in the viscous fluid. By applying the refractive index matching techniques, we visualize the particle dynamics on a center plane of the tube and track the trajectories of individual particles under applied strain. Our preliminary results demonstrate the extension of the suspension plug in both streamwise and spanwise directions after multiple oscillations, which are directly tied to the irreversible migration of particles across streamlines. In this talk, we discuss our new experimental findings and discuss the physical mechanism based on a simplified model.