(483h) Ultra-Fast Microfluidic Mixing By Soft-Wall Turbulence

Experiments in a soft-walled microchannel show that there is a laminar-turbulence transition at a Reynolds number significantly lower than that for a rigid channel due to the fluid-wall interactions. The transition Reynolds number could be as low as 226 for a channel with smallest dimension 35 microns, in contrast to the transition Reynolds number of 1200 for a rigid channel. The flow after transition shows many of the characteristics of a turbulent flow, including the large maximum in the streamwise velocity fluctuations close to the wall, and the non-zero Reynolds stress. There are also significant differences with wall bounded turbulence --- the fluctuating velocities and the Reynolds stress do not decrease to zero at the soft wall. The turbulence production appears to be a maximum at the wall itself indicating that the turbulence generating mechanism involves wall motion; this is very different from the generation of turbulence due to bursting of eddies in the near-wall region in hard wall bounded turbulence. After transition, the mixing time across a channel of width 0.5 mm is smaller, by a factor of 10⁵ , than that for a laminar flow, and complete mixing is achieved within a channel length of 2 cm. The increased mixing rate comes at very little energy cost, because the pressure drop is comparable to that required in current microfluidic devices, and it increases continuously and modestly at transition. This is because the channel length required to achieve complete mixing, 2 cm, is much smaller than that used in microfluidic devices that employ diffusive mixing; in addition, the deformation of the soft wall decreases the resistance to flow. Thus, soft-wall turbulence provides a simple and powerful strategy for ultra-fast mixing in a microfluidic applications.