Self-organization of spherical particles submerged in an oscillating flow

When a group of spherical particles submerged in a viscous fluid is subjected to oscillations, the particles align themselves in chains perpendicular to the oscillation direction. The driving mechanism of the chain-forming phenomenon is a non-zero residual flow, or steady streaming flow, that remains after averaging over a full oscillation period. We performed experiments using an oscillating box filled with water and stainless steel ball bearings. Our experiments show that the equilibrium particle configurations range from one-particle-thick chains to layered bands, depending on the flow conditions and particle number density. The formation, evolution, and stability of the patterns are characterized by techniques commonly used for colloidal assemblies.

We further supplement our experiments with direct numerical simulations, in which the particles are modeled using an immersed boundary method. The simulations reveal the three-dimensional flow structures and give insight into the underlying physical mechanisms governing the patterns.