Cooperative size sorting of deformable particles in porous media

Diverse applications-ranging from enhanced oil recovery, filtration, and lab on a chip sorting-rely on the flow-induced transport of deformable particles in porous media. However, how fluid flow can force such particles to squeeze through pore constrictions of complex geometries is poorly understood. Here, we study the transport of model deformable particles in millifluidic porous media with constrictions of tunable aspect ratio. We find that multiple particles can unexpectedly squeeze through large-aspect ratio constrictions, even when isolated particles cannot. This phenomenon arises from pairwise flow-mediated interactions between the particles: when one particle is trapped at a constriction, the increased fluid flow around it enables a second to squeeze past due to locally increased hydrodynamic stresses. This cooperative mechanism causes the particles to ultimately sort themselves by size through the pore space. By revealing a new mode of deformable particle transport in porous media, our work helps to inform real-world applications and provides a straightforward way to sort particles based on size.