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

Publisher Website
Google Scholar
Abstract
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 elasticflow 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 μ channel. 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 elasticflow instability.Flow visualization is done using neutrally buoyant non-Brownian tracer particles added to the elastic fluids and also to the Newtonian fluids. In the Newtonian fluids, the tracer particles follow the streamlines. In the elastic fluids, the particles are radially displaced while flowing around bends in the zigzag μ channel, revealing the presence of secondary flow. This radial secondary flow, which promotes mixing between adjacent fluid streams, motivates us to draw an analogy between the instability observed here for the elastic fluids in the μ channel and the elasticinstability that occurs in systems with curved streamlines, e.g., in the viscoelastic (non-inertial) Taylor–Couette, Dean, and Taylor–Dean instabilities.