Elastic flow instability, curved streamlines, and mixing in microfluidic flows

Flow instabilities are well known to occur in macroscopic flows when elastic fluids flow along curved streamlines. In this work we use flow visualization to study the mechanism underlying a purely elastic flow instability for Poiseuille flow in a micro (μ)channel having a zigzag path (curved streamlines) and quantitatively investigate its implications for fluid mixing (studied by fluorescence microscopy) in the μchannel. We find that the instability enhances mixing over the range of applied flow rates. For Newtonian streams, mixing occurs by molecular diffusion, and, as expected, mixing worsens with increasing flow rate because of decreasing residence time. However, for elastic fluid streams, we find substantial enhancement of mixing at sufficiently high throughputs, which indicates a strategy to counter the loss of diffusive mixing at high throughputs by exciting an elastic flow instability. Flow visualization is done using neutrally buoyant non-Brownian tracer particles added to the elastic fluids and a...