Topologic mixing on a microfluidic chip

Abstract

Mixing two liquids on a microfluidic chip is notoriously hard because the small dimensions and velocities on the chip effectively prevent turbulence. We present a topological mixing scheme that exploits the laminarity of the flow to repeatedly fold the flow and exponentially increase the concentration gradients to obtain fast and efficient mixing by diffusion. It is based on helical flow channels with opposite chiralities that split, rotate, and recombine the fluid stream in a topology reminiscent of a series of Möbius bands. This geometry is realized in a simple six-stage, two-layer elastomer structure with a footprint of 400 μm×300 μm per stage that mixes two solutions efficiently at Reynolds numbers between 0.1 and 2. This represents more than an order of magnitude reduction in the size of microfluidic mixers that can be manufactured in standard multilayer soft lithography techniques.