Modeling necking dynamics of viscoelastic microgels

Microgels can deform and interpenetrate and display colloid/polymer duality. Our previous studies show that the effective interaction of microgels in the collapsed state is governed by the interplay of polymer–solvent interfacial tension and bulk elasticity. A connecting neck is observed to mediate microgel interaction, but its temporal evolution has not been addressed. In this study, we systematically studied the necking dynamics using mesoscale hydrodynamic simulations and theoretical modeling. The results reveal a crossover in the coalescence dynamics reflecting the viscoelastic signature of microgels. In contrast to the common knowledge that viscoelastic materials respond elastically on short time scales, the early expansion of the microgel neck exhibits a linear behavior, similar to the viscous coalescence of liquid droplets. However, the late regime with arrested dynamics resembles the sintering of solid particles. Through an analytical model relating microgel dynamics to neck growth, we show that the long-term behavior is governed by stress relaxation of the polymers in the neck region and predict an exponential decay in the rate of growth, which agrees favorably with the simulation. Different from coalescence, the thread thinning in microgel breakup primarily highlights its polymeric characteristics.