The effect of elasticity on drop creation in T-shaped microchannels

Abstract

Polymeric microdrops of low viscosity, elastic fluids have been generated in T-shaped microfluidic devices using a cross-flow shear-induced drop generation process. Dilute ( c/ c * ∼ 0.5) aqueous solutions of polyethylene oxide (PEO) of various molecular weights (3 × 10 5–2 × 10 6 g/mol) were used as the drop phase fluids whilst silicone oils (5 mPa s ≤ η ≤ 50 mPa s) were used as the continuous phase fluids. The effects of viscosity ratio and fluid elasticity on the mechanism of drop detachment and break-up, final drop size and frequency of drop formation were studied in this non-Newtonian–Newtonian multiphase system. The generation of thinning filaments between subsequent drops of these low viscosity fluids signifies the presence of elastic stresses in this low Reynold's number flow. Two distinct regions of filament thinning dynamics, a ‘pre-stretch’ region and an exponential self-thinning region, were observed for the highest molecular weight of PEO studied. The ‘experimental’ relaxation times extracted from the exponential self-thinning region were of the same order of magnitude as the calculated Zimm relaxation time but were shown to increase as the cross-flow shear was increased. This increase is associated with substantial pre-stretching of the polymer molecules within the forming neck prior to the onset of thread self-thinning. The presence of elasticity within these low viscosity fluids resulted in the production of secondary drops of varying sizes upon final breakup. The substantial threads between the primary drop and the nozzle display traditional bead-on-a-string morphologies, with the final drop size and polydispersity being a distinct function of cross-flow rate, dispersed flow rate and polymer molecular weight.