Impact of Hydrodynamic Interactions on the Collapse-Expansion Kinetics of Nanogels

Nanogels, nanoscale interconnected polymer networks, exhibit shape changes in response to external triggers, offering diverse applications. In fluid environments, hydrodynamic interaction (HI) plays a crucial role in nonequilibrium kinetics, yet its influence on the collapse and expansion kinetics of nanogels remains elusive. Here, we investigate these kinetics using fluid particle dynamics simulations. During the initial stage of collapse, HI accelerates chain contraction, leading to rapid cluster formation around the periphery. However, in the later stages, short-range HIs, known as squeezing flow effects, slow further collapse. Conversely, during expansion, HI initially impedes monomer separation due to inverse squeezing flow effects but subsequently facilitates diffusion, thereby speeding up swelling. Interestingly, including electrostatic interactions reduces the effects of short-range HIs, including both squeezing and inverse squeezing flow effects, significantly accelerating the overall kinetics. Our findings contribute to the design of responsive nanogels and facilitate electrohydrodynamic modeling of soft materials in aqueous solutions.