Elasto-viscous regime in coalescence of viscoelastic droplets

We report a regime transition in the coalescence of concentrated polymeric droplets in a pendant-pendant configuration. While Newtonian droplet coalescence has been extensively studied with distinct identification of viscous and inertial regimes, the presence of polymers introduces additional regimes governed by elasticity and molecular relaxation effects. The coalescence process is typically characterized by the neck radius, $R$, of the liquid bridge connecting the two droplets, following a power-law relation with time: $R=at^{b}$. Most of the existing studies, including Newtonian and non-Newtonian fluids, report a unique value of $b$ for a given fluid. In contrast, our findings reveal that elasticity induces a temporal transition from one $b$ values to another, marking a shift in the coalescence regime. In particular, our measured $b$ value falls in the sub-Newtonian regime, highlighting the role of elasticity in governing the dynamics. We conducted two-dimensional simulations using a volume-of-fluid framework with the exponential Phan-Thien-Tanner model, which quantitatively reproduced Newtonian benchmarks and accurately captured viscoelasticity induced neck growth in close agreement with experiments. Furthermore, we determined the curvature experimentally, as the assumptions typically employed in the literature to approximate axial curvature are not universally valid.