Self-Organizing Microfluidic Crystals

We consider how to design a microfluidic system in which suspended particles spontaneously order into flowing crystals when driven by external pressure.  Via theory, numerics, and experiments, we find that particle-particle hydrodynamic interactions drive self-organization under suitable conditions of particle morphology and geometric confinement.  Small clusters of asymmetric "tadpole" particles, strongly confined in one direction and weakly confined in another, spontaneously order in a direction perpendicular to the external flow, forming one dimensional lattices.  Large suspensions of tadpoles exhibit strong density heterogeneities and form jammed aggregates.  By rationally tailoring particle shape, we tame this aggregation and achieve formation of large two-dimensional crystals. Key aspects of the theoretical/numerical results are confirmed by Stop Flow Lithography experiments.