Universal behavior of mirror-symmetric particles in confined Stokes flow

Deeper understanding of particle motion in microfluidic devices would unlock novel technologies for continuous, high throughput separation of distinctly-shaped particles such as cells and crystal polymorphs. Particles interact hydrodynamically with each other or with confining walls, thus altering their trajectory. These hydrodynamic interactions strongly depend on particle shape and can be tuned to guide a particle along a specific path. We focus on particles with a single mirror plane, produced via stop flow lithography in a shallow, quasi-2D microfluidic channel. A particle lags the surrounding flow due to strong confinement from the horizontal channel walls. Regardless of their exact shape, all studied particles have identical modes of motion: exponentially-decaying in-plane rotation and cross-streamline migration along a bell-shaped path. Both modes are characterized by timescales, unique to a particle’s geometry. Scaling rotation and translation of all considered shapes by their respective timescales, yields a single trajectory, universal to mirror-symmetric particles. We propose a minimalistic qualitative model capable of predicting the trajectory of various particles with one mirror plane in a shallow microfluidic flow.