Dynamics of droplet formation and flow regime transition in a T-shaped microfluidic device with a shear-thinning continuous phase

A numerical and experimental study of the process of droplet formation in a T-junction microchannel with a shear-thinning continuous phase is presented. Simulations are performed using a three-dimensional lattice Boltzmann multicomponent model. The shear-thinning behavior is modeled using the Carreau-Yasuda model and validated with our experiments involving both Newtonian and non-Newtonian continuous phases. Using the validated numerical framework, a parametric study is performed to quantify the effect of the rheological parameters, namely, the power-law index n and the Carreau number Cr, on the droplet size for two different capillary numbers. To understand the variation in the droplet size and shape with the shear-thinning parameters, the dynamics of the droplet breakup process is analyzed in terms of the upstream pressure buildup and the shear stresses acting on the interface of the two fluids. Our numerical results show that the dynamics of droplet formation shifts from the shear-dominated dripping regime to the pressure-dominated squeezing regime for continuous phase fluids having lower values of n and higher values of Cr. Hence, an increase in the droplet size and a change in its shape from spherical to plug are observed for fluids showing a higher shear-thinning tendency (higher Cr and lower n). Finally, an analysis of the transition in the droplet flow regimes with a shear-thinning continuous phase at high capillary numbers is presented. For Ca >= 0.1, an increase in the shear thinning of the continuous fluid is shown to shift the droplet flow pattern from parallel flow to droplets in the channel to eventually forming droplets at the T junction.