From experiments to simulations of shear-thinning droplet dynamics in microfluidic platforms

Unlike Newtonian fluids, which typically favour the squeezing mode for droplet formation, in this work, we unveil that this mode is entirely absent for shear-thinning fluids. Following experimental observations as well as $3D$ numerical investigations, we establish the reason for this absence by analyzing the evolution of viscous shear stress $(\tau ^{*})$ at the droplet interface. We identify a threshold flow ratio of $Q\sim0.45$ (where $Q=Q_{d}/Q_{c}; Q_{d}$ and Qc represent the flow rates of the dispersed and continuous phases), below which any subtle variations in $\tau ^{*}$ favour the dripping mode. However, beyond this threshold, $\tau ^{*}$ increases sharply, shifting the flow mode directly to jetting and bypassing squeezing mode. Our study further reveals how varying shear-thinning degree influences jet development and flow transitions, as illustrated through a flow regime map. We believe these insights will offer valuable guidance for developing lab-on-a-chip platforms for shear-thinning droplet formation, largely used in applications, including biochemistry, material synthesis, and emulsification.