Flows inside polymer microfluidic droplets: Role of elasticity

Abstract

The role of elasticity on the flow topology inside viscoelastic (Boger) droplets moving in a rectangular microchannel is examined experimentally by means of micro Particle Image Velocimetry (μPIV). Polyacrylamide (PAAM) water -glycerol solutions of different concentrations are employed to vary droplet elasticity. Varying the Wi number alters the flow topology inside the Boger microdroplets, progressively reducing the number of vortical structures observed until their complete disappearance. The flow structure resembles that of Newtonian inelastic droplets for low Wi and elasticities. However, when the Wi and elastic number increase above one-i.e. elastic effects become more important- a new flow structure is observed at the front of the droplets characterized by two recirculating regions either side of the droplet centreline. Spatiotemporal maps show that the flow in this new regime fluctuates periodically around the centreline indicating the onset of an elastic instability. This flow transition is attributed to the well documented coupled effects of polymer stretching and curved streamlines and resultant hoop stresses. The findings highlight the importance elasticity can have on the nature of microdroplet flows of complex fluids and the potential of tuning elasticity to engineer flow structures for given microfluidic applications.