Analysis of fluid separation in microfluidic T-channels

The behaviour of a fluid, which may contain particle suspensions, flowing in micro-dimensional channels is governed by both viscous and surface tension forces as well as high shear rates and geometric effects such as bifurcations, constriction, and high surface-to-volume ratio. This paper discusses some of the key design factors affecting fluid behaviour in micro-engineered products containing a main channel, constriction and side channel bifurcations. Differences in fluid behaviour at the macro and micro-scales are discussed. The dynamic bulk fluid behaviour is characterised in terms of: (i) fluid properties, (ii) governing physics and (iii) microchannel geometric features. At this stage of the analysis the fluids are assumed to be Newtonian and single phase, where any particle suspension is represented through a bulk density and viscosity. Based on these assumptions Computational Fluid Dynamics (CFD) is used to investigate the effect of both product inlet and outlet boundary conditions on the bulk flow behaviour. Discussions are provided on how these boundary conditions can affect particle separation efficiency. In particular, the so called pull-design whereby the fluid is pulled out of the device at the outlet, is shown to offer better performance compared to the mode of operation where the fluid is pushed into the device at the inlet. It is also observed that increasing the pressure at the outlet of the main channel can achieve a balanced flow rate ratio which leads to a uniform separation performance among all bifurcations.