Mixing remains a challenging task in microfluidic channels because of their inherently small length scale. In this work, we propose an efficient microfluidic mixer based on the chaotic vortex dynamics of a viscoelastic flow in a straight channel with side wells. When the inertia and elasticity of a dilute polymer solution are balanced (i.e., the Reynolds number Re and Weissenberg number Wi are both on the order of 10 1 ), chaotic vortices appear in the side wells (inertio-elastic flow

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... y), enhancing the mixing of adjacent fluid streams. However, there is no chaotic vortex motion in Newtonian flows for any flow rate. Efficient mixing by such an inertio-elastic instability is found to be relevant for a wide range of Re values. V C 2016 AIP Publishing LLC. [http://dx.