(367a) Collective Dynamics of Particle Clusters Flowing in a Quasi-Two-Dimensional Microchannel

Control of flowing suspensions of particles is central to many emerging microfluidic applications, including cell sorting, information processing, and assembly of complex structures. Likewise, spatially ordered equilibrium states – crystals – and their excitations – phonons – are the mainstay of condensed matter physics. Flowing, non-equilibrium crystalline states of microparticles and droplets are desirable for microfluidic logic, assembly, and control, and have been achieved in recent work via exploitation of viscous hydrodynamic interactions in geometric confinement. For the most part, these studies considered large ensembles of particles and, accordingly, large scale collective modes arising from small displacements of individual particles. Via theoretical modeling and computational simulations, here we show that for small clusters of flowing particles tightly confined in a shallow, “quasi-two-dimensional” microchannel, new types of ordered behavior emerge, with complex and individualistic particle motion. We elucidate principles and techniques for the a priori construction or rapid numerical discovery of these states, which could be exploited for the orchestration of particle motion in lab-on-a-chip devices and other applications. Our theoretical studies are complimented by experiments in which we can “print” custom-shaped particles in predefined positions in a microchannel.   The experiments allow for the unique ability to precisely set initial conditions for a cluster and then watch it evolve in flow.