(518a) Computational Analysis of Pinch-Off Dynamics and Printability of Simple and Complex Fluids

Liquid transfer and drop formation/deposition processes underlying printing involve complex free-surface flows, including the formation of columnar necks that undergo spontaneous capillary-driven instability, thinning and pinch-off. For simple (Newtonian and inelastic) fluids, a complex interplay of capillary, inertial and viscous stresses determines the nonlinear dynamics underlying finite-time singularity, satellite drop formation as well as self-similar capillary thinning and pinch-off dynamics. In rheologically-complex fluids, extra elastic stresses as well as non-Newtonian shear and extensional viscosity dramatically alter the nonlinear dynamics. Stream-wise velocity gradients that arise within the thinning columnar neck create an extensional flow field, and complex fluids exhibit a much larger resistance to elongational flow than simple fluids with the same zero shear viscosity. Using FLOW-3D, we simulate flows within columnar necks or stretched liquid bridges formed by dripping, by applying step strain to fluid between two parallel plates, and by dripping-onto substrate. Using these three prototypical cases, we simulate free-surface flows realized in printing as well as in extensional rheometry devices used for studying pinch-off dynamics and the influence of microstructure and viscoelasticity. In contrast with often-used 1D or 2D models, FLOW-3D allows a robust evaluation of the magnitude of the underlying stresses and extensional flow field (both uniformity and magnitude). We find that the simulated radius evolution profiles match the scaling laws and pinch-off dynamics that are experimentally-observed and theoretically-predicted for Newtonian fluids. Finally, we describe our experiments and FLOW-3D simulations to elucidate how viscoelasticity as well as non-Newtonian shear and extensional viscosity influence interfacial and nonlinear flows underlying pinch-off dynamics, extensional rheometry and printability of complex fluids.