(142bz) Rapid Mixing in Microchannel Due to Flow Instability Induced by Flexible Wall

An important limitation in microfluidic applications is the slow mixing due to the laminar nature of the flow. Mixing takes place by molecular diffusion, and the time for mixing across a distance L scales as (L²/D), which is approximately 10 to 1000 seconds when we use a length  in the range (100 micron-1 mm), and with typical diffusion coefficient of 10^-9 m²s^-1. Here, we report the development of a fast and homogenized microfluidic mixer, where cross-stream mixing is induced by a flow instability due to the flexibility of one of the wall of a channel of rectangular cross-section. The flexible wall of microfluidic channel is prepared with varying range of shear modulus (17–550 kPa) of PDMS gel and the pressure difference between the inlet and outlet is used to drive fluid flow. It is found that the laminar flow becomes unstable, and there is a transition to a chaotic flow with large cross-stream fluctuations. This results in decrease of mixing times by five orders of magnitude than those in a laminar flow. The mixing is characterized by two methods: a macroscopic dye-stream image analysis technique based on images of the channel, and a microscopic ionic conductivity of the fluid at the outlets. The results of both procedures show very good mixing of the fluid streams within milliseconds after the transition. The Reynolds number for transition decreases as the shear modulus of the wall is decreased, and it is about 200 for the softest gels used here. Wall oscillations are also detected at the onset of mixing using a laser scattering technique where a laser beam is scattered off microbeads embedded in the flexible wall. Even though there is near complete mixing, the increase in dissipation is modest, and the pressure drop is only about 50% higher than that for a laminar flow in the same channel. These experiments show that use of soft walls is practically feasible and a low-cost method to induce instability and generate fast and effective mixing in microfluidic applications. This is also advantageous over conventional passive methods which typically involve large increase in pressure gradients and complicated fabrication, and also over active methods since there are no moving parts.