A numerical model on secondary flow and mixing in rotating microfluidics

The fluid mechanics of mixing different species in a closed rotating microfluidics chamber was investigated. Complicated secondary flow, in form of vortices, in a rotating chamber was generated for mixing different species by continuous changing the rate of rotation over time until uniform mixing of the different species in the chamber has been attained. Three vortices are observed when reference to the rotating chamber - one main vortex in the R-θ planes, generated from the d'Alembert force, was responsible for momentum and mass transfer while two pairs of toroidal vortices, generated from the Coriolis force, is responsible for momentum and mass transfer in the direction parallel to the rotational axis. These secondary flow and vortices help to reduce the mixing length between species of a given concentration. With a much smaller mixing length, diffusion can further effect the remaining mixing in a more reasonable time for the mixture to attain a more uniform or homogenous condition of the species in the microfluidic chamber. Numerical simulation using ANSYS-CFX was used to solve the transient Navier-Stokes equation in the rotating frame simulating the Newtonian laminar flow pattern for linear acceleration and deceleration (in rotation) schemes. Mass transfer was also modeled using the convective-diffusion equation governing movement of the different species in the chamber after the NS and continuity equations have been solved for the velocity field. Using the numerical model, parametric study of the rotating truncated pie-shaped chamber (radius and angular span) have been carried out to investigate the effect of momentum and mass transfer. An index, the mixing quality based on concentration distribution for the whole domain, was used respectively to quantitatively evaluate the mixing performance.