Coalescence of 3D Polymeric Drops in the Presence of in Situ Photopolymerization

We develop a combined numerical (using direct numericalsimulation)and scaling model to study the dynamics of two identical photocurable,shear-thinning polymeric drops undergoing spreading and coalescence(on a substrate) in the presence of in situ curing.We only consider the shear-thinning nature of the drops but do notaccount for any changes in the drop elasticity during the in situ photocuring process. Two separate cases are studied:(1) & tau;(s) MUCH LESS-THAN & tau;(p) (case 1) and(2) & tau;(s) & SIM; & tau;(p) (case 2) (& tau;(s) and & tau;(p) represent the spreading and photocuringtimescales, respectively). Photopolymerization-driven increase inthe drop viscosity significantly delays the spreading and the coalescenceevents for case 2. Furthermore, for case 2, at any given time duringthe coalescence process, both the height and the width of the "bridge"that forms due to coalescence are always smaller than those for case1. Our scaling analysis establishes three distinct regimes characterizingthe bridge growth for both cases 1 and 2: the initial regime (viscousand capillary forces balance each other, and the bridge growth rateis weak), the intermediate regime (inertia and capillary forces balanceeach other, and the bridge growth rate is enhanced), and the lateregime (viscous and capillary forces balance each other, and the bridgegrowth rate is weak). Also, in regimes where viscous forces play arole (initial and late regimes), the bridge growth is weaker for case2, while in the intermediate regime, the rate of bridge growth issimilar between cases 1 and 2. We also provide the temperature variationand the evolution of the curing front inside the coalescing dropsfor cases 1 and 2: the significantly rapid rate of polymerizationfor case 2 manifests in a noticeable temperature drop, fast propagationof the curing front, and the fluid flow (associated with coalescence)affecting the migration of the curing front inside the coalescingdrops. We anticipate that our study will be crucial in designing polymericdroplet-based additive manufacturing systems and understanding thebehavior of polymeric blends and polymeric emulsions.