Morphological Peculiarities of Active and Partially Active Microgels

Active polymer gels provide useful models for biological and synthetic nonequilibrium soft matter, yet most theoretical and simulation studies of such systems have focused on effectively infinite active networks and have therefore neglected free-surface effects. Here, we use coarse-grained computer simulations within the active Brownian particle framework to investigate single microgels in which activity is assigned directly to individual monomer units. We examine the effects of subchain length, active-monomer fraction, localization of activity on subchains or cross-links, and radial distributions of active units. Activity induces pronounced microgel compression, which becomes stronger with increasing subchain length and, among the systems studied, reaches its greatest magnitude at a low active fraction of 5%. The collapsed structures can exhibit a dense, heterogeneous core surrounded by a shell of strongly extended subchains. Activity localized at cross-links produces substantially weaker compression than activity distributed along subchains, whereas surface-localized activity enhances collapse and core-localized activity suppresses it. By mapping the Péclet number onto an effective Flory–Huggins parameter, we show that the swelling behavior of partially active microgels near the θ point can be related to Flory–Rehner theory, although important structural differences from passive collapse remain.