Boundary Condition Induced Passive Chaotic Mixing in Straight Microchannels

Abstract: Mixing in low Reynolds number flow is difficult because in this laminar regime it occurs mostly via slow molecular diffusion. This letter reports a simple way to significantly enhance low Reynolds number passive microfluidic flow mixing in a straight microchannel by introducing asymmetric wetting boundary conditions on the floor of the channel. We show experimentally and numerically that by creating carefully chosen hydrophobic slip patterns on the floor of the channels we can introduce stretching, folding and/or recirculation in the flowing fluid volume, the essential elements to achieve mixing in absence of turbulence. We also show that there are two distinctive pathways to produce homogeneous mixing in microchannels induced by the inhomogeneity of the boundary conditions. It can be achieved either by: 1) introducing stretching, folding and twisting of fluid volumes, i.e., via a horse-shoe type transformation map, or 2) by creating chaotic advection, through manipulation of the hydrophobic boundary patterns on the floor of the channels. We have also shown that by superposing stretching and folding with chaotic advection, mixing can be optimized by significantly reducing mixing length, thereby opening up new design opportunities for simple yet efficient passive microfluidic reactors.