Breaking Symmetries

Abstract

The recent advancements in lab-on-a-chip technologies has led to numerous studies on the collective dynamics of micro-particles suspended in a microfluidic channel. The role of hydrodynamics in “self-steering” of the particles in a low Reynolds flows is explicitly dependent only on the particle’s exterior geometry. Thus richer responses can be achieved by controlling the transport and orientation of asymmetric particles under external flow. In this work, we study the universal behaviour of particles with any arbitrary shape, strongly confined along direction involving height. The particle both translates and rotates in-plane, until a stable orientation is reached, after which it drifts cross-stream. By combining analytical, numerical and experimental techniques, we establish the generalised equations of motion for a particle sufficiently far away from the channel walls along width. Thus, the total motion of a particle is characterized by estimating only a few timescales, intrinsic to its exterior geometry. We discuss the shape properties defining the dynamics, and use them to derive an analytical framework. The results are then validated against the numerical solution utilizing an improved 2-Dimnesional Brinkman formulation for highly confined particles. Experiments are performed using stop-flow lithography, where the dynamics of the photo-polymerized particles are observed by driving it out of equilibrium using an externally imposed pressure difference. We confirm the universal behaviour of the particles, except for a few cases, only when close to the stable orientation. Our experimentally observed rotational timescales consistently match the numerically computed values when multiplied by a factor of 1.8.