Drift of elastic hinges in oscillating shear flows

Elastic filaments are prevalent in many natural and industrial systems, including bacterial flagella, polymer chains, and natural and synthetic fibers advected in flows. Due to their elastic nature, they deform under the influence of hydrodynamic stresses as they are advected by flows. This deformation leads to more complex dynamics than that of a rigid particle. We are particularly interested in cases of drift, where passive particles by virtue of their shape and deformation self-propel in directions perpendicular to the mean flow direction. We use an elastic hinge, consisting of two rigid rods joined by an elastic torsional spring, as an analog for an elastic fiber. Similar rigid hinge shapes have been shown to drift in steady shear flows when they are very asymmetric. However, no such rigid particles drift in flows with a shear rate that is a sinusoidal function of time. Here, we demonstrate that symmetric hinges can drift in oscillating shear flows and that the magnitude and direction of drift can be selected by choosing the hinge material properties, geometry, and frequency of oscillation. This opens the possibility of tuning both particles and flows to control particle motion, leading to enhanced particle separation, mixing, or self-assembly of more complex structures.