Sub-Newtonian coalescence in polymeric fluids.

We present a theoretical framework for capturing the coalescence of a pendant drop with a sessile drop in polymeric fluids. The framework is based on the unification of various constitutive laws under a high Weissenberg creeping flow limit. Our results suggest that the phenomenon comes under a new regime, namely, the sub-Newtonian regime followed by the limiting case of arrested coalescence with the arrest angle θarrest ∝ Ec-1/2-1, where Ec-1 is the inverse of Elasto-capillary number. Furthermore, we propose a new time scale T* integrating the continuum variable Ec-1 and the macromolecular parameter Ne, the entanglement density to describe the liquid neck evolution. Finally, we validate the framework with high-speed imaging experiments performed across different molecular weights of poly(ethylene oxide) (PEO).

Universality in coalescence of polymeric fluids.

A pendant drop merging with a sessile drop and subsequently forming a single daughter drop is known to exhibit complex topologies. But their dynamics are yet to be probed for fluids exhibiting characteristic relaxation time scales while undergoing the deformation process. Here, we unveil a universal temporal evolution of the neck radius of the daughter drop during the coalescence of two polymeric drops. Such a generalization does not rely on the existence of previously explored viscous and inertial dominated regimes for simpler fluids but is fundamentally premised on a unique topographical evolution with essential features of interest exclusively smaller than the dominant scales of the flow. Our findings are substantiated by a theoretical model that considers the drops under coalescence to be partially viscous and partially elastic in nature. These results are substantiated with high-speed imaging experiments on drops of polyacrylamide (PAM), polyvinyl alcohol (PVA), polyethylene oxide (PEO), and polyethylene glycol (PEG). The observations herein are expected to hold importance for a plethora of diverse processes ranging from biophysics and microfluidics to the processing of materials in a wide variety of industrial applications.