Universal motion of mirror-symmetric microparticles in confined Stokes flow

Significance

Particles of all shapes and sizes flowing through tight spaces are ever present in applications across length scales ranging from blood flow through tissue capillaries to industrial-scale processes. To date, separating these particles relies on methods employing external force fields. Currently underexplored, omnipresent fluid–structure interactions hold the key to shape-based separation independent of external intervention. By leveraging experiments, theory, and simulations, we show how the symmetry of a particle determines its overall trajectory: In particular, mirror-symmetric particles, both strongly and weakly confined, follow a universal path. We propose minimalistic scaling relations to describe how particle shape affects the parameterization of the universal path. These findings could be used to “program” particle trajectories in lab-on-a-chip devices and industrial separation processes.

Abstract

Comprehensive understanding of particle motion in microfluidic devices is essential to unlock additional technologies for shape-based separation and sorting of microparticles like microplastics, cells, and crystal polymorphs. Such particles interact hydrodynamically with confining surfaces, thus altering their trajectories. These hydrodynamic interactions are shape dependent and can be tuned to guide a particle along a specific path. We produce strongly confined particles with various shapes in a shallow microfluidic channel via stop flow lithography. Regardless of their exact shape, particles with a single mirror plane have identical modes of motion: in-plane rotation and cross-stream translation along a bell-shaped path. Each mode has a characteristic time, determined by particle geometry. Furthermore, each particle trajectory can be scaled by its respective characteristic times onto two master curves. We propose minimalistic relations linking these timescales to particle shape. Together these master curves yield a trajectory universal to particles with a single mirror plane.