(107c) MIXING ANALYSIS of Neutrally Buoyant PARTICLES of Finite SIZE IN COMPLEX FLOW

Microfluidic devices is widely used for chemical and biological applications. Chaotic mixing using the staggered herringbone (SHB) mixer, for example, is a common strategy for effective mixing of solutes. In mixing particles for neutrally buoyant cells of finite size, however, the distribution is dissimilar to that of the solutes and not uniform after long mixing time. To better understand the physics and evaluate particle mixing strategies in dilute suspensions, we developed both experimental and modeling techniques. We use a simple epifluorescent technique - Single-field Three-dimensional Epifluorescent Particle (STEP) imaging - which can find the particle depth (z) from the fluorescence intensity patterns of the moving particles and generate 3D particle distributions. The exact location of individual particles can be recognized and particle tracking experiments can also be performed easily and less expensively than confocal microscopy. We found that particle and solute distributions after a number of cycles in a slant groove mixer (SGM) under the same flow conditions are dissimilar. We found that many factors affect the particle distribution: flow Re, particle size and particle Re, and groove geometry. Mixing patterns are particularly sensitive to the particle size and groove geometry within the Re number range examined. We hypothesize that the finite particles have a defined set of accessible streamlines; streamlines that come within a distance equal to the particle's radius to the wall cannot contain particles. Preliminary computational fluid dynamics simulations support this hypothesis. The experimentation and modeling tools allow us to further understand the detailed mechanisms behind this phenomenon and will allow us to efficiently design microfluidic unit operations (mixing, separation and sample concentration) on particle or cell suspension of finite size.