(315j) Long-Range Interparticle Forces in Complex Particle Arrays Assembled By Inertial Microfluidic Flows

Microfluidic inertial focusing provides the passive alignment of small particles without the need for external actuation or sheath fluid by predictably positioning particles to well-defined lateral and longitudinal locations.  The benefits of inertial focusing have quickly enabled the development of miniaturized flow cytometers, size-selective sorting devices and other high-throughput particle screening tools.  Rudimentary design of inertial focusing devices requires only knowledge of the fluid properties and particle to channel size ratio.  Equilibrium behavior of inertially focused particles has been extensively characterized and the constitutive phenomena described by scaling relationships for straight channels of square and rectangular cross-section.  At the limits of inertial focusing, however, interparticle repulsions give rise to complex particle ordering that, while interesting and potentially useful, dramatically diminishes the technique’s effectiveness for high-throughput particle handling applications.  We have identified three scaling regimes with increasing degrees of geometrical ordering between focused particles.  To explore the limits of inertial focusing and identify the origins of these long range interparticle forces, we have explored both equilibrium focusing behavior and particle dynamics during focusing.  Experimental results for highly concentrated particle solutions identify equilibrium thresholds to focusing that scale weakly with concentration and strongly with channel geometry.  These results have in turn motivated experiments to quantify geometry-dependent inertial forces that drive lateral particle migration at the dilute particle limit.  Balancing inertial forces with interparticle repulsive forces now provides a complete picture of pattern formation within inertially focused particles and enhances our understanding of the fundamental limits of inertial focusing for technological applications.