Fluid mixing is common in large scale chemical processing. However, many processes, especially applications in bioengineering, are now carried out in microfluidic systems. At the micro-scale, mixing of solutes is predominantly a diffusion process due to the laminar nature of fluid. Many different mixing strategies have been employed to effectively decrease the characteristic length for diffusion. For example, the Staggered Herringbone Array  uses carefully designed grooves on the surface of the channel to laterally fold and stretch the fluid, and an even mixing of solutes/solvents can be achieved relatively quickly. However, particle (or cell) mixing behavior in suspension is still not well understood.
To assess the critical factors behind neutrally buoyant particle mixing or focusing, we implemented a series of experiments with varying flow conditions, fluid and particle properties, and geometries. We established an experimental method providing information about particle distribution in three dimensions without using confocal microscopy. Studying the particle distribution at varying Reynolds numbers allows us to discern the relative effects of inertial and viscous forces. Similar experiments have been performed previously by other groups in square channels describing the phenomena at relatively high Reynolds numbers or in channels with relatively large grooves only. Thus, in these situations only one major parameter controls the particle distribution. In contrast, in our experiments where more than one factor is important and no simplifications can be made, our analysis integrates many parameters through dimensional analysis and scaling.
To further understand the physical forces on each particle, Computational Fluid Dynamics models were also used to predict the particle flow characteristics and to calculate relevant forces. The modeling results are able to explain the equilibrium distribution behavior observed in the experiments as well as the paths the particles follow in the process.
By studying particle mixing of various particles under different flow conditions, we hope to provide a more comprehensive view of the particle-fluid behavior in laminar flow. With this knowledge, efficient unit operations in multiphase systems, including mixing and separation, can be designed, particularly in microfluidic technologies for many biological and medical applications that handle cells and beads.
. A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezic, H. A. Stone and G. M. Whitesides, Science, 2002, 295, 647–651
See more of this Group/Topical: Engineering Sciences and Fundamentals