Driven Dynamics of Physical Polymer and Colloidal Gels

Gels assembled by the reversible association of polymers or colloids are a common and fascinating class of soft materials. Here, we explore the diffusive modes of the constituent particles of the gel, drawing analogies to activated glassy dynamics. While our theory and experiments suggest that single-particle (activated) dynamics control long-time relaxation in quiescence, relaxation in driven systems can be qualitatively distinct. For example, using computer simulation, we find that shearing a polymer gel results in a fast (non-activated) diffusive mode in the form of freely-diffusing mulitchain aggregates. The rapid emergence of this mode with applied shear is found to destabilize homogeneous flow for gels sufficiently close to the two-phase boundary. The driving force need not be externally applied. The induced collective motion in colloidal gels subject to internal driving forces (such as the presence of a small fraction of self-propelling colloids) can drive the system from a state of arrested metastablity to a state of lower free energy. Our findings suggest that by carefully tuning the magnitude of internal or external driving forces, new diffusive modes can emerge that can facilitate traversing kinetic barriers that are otherwise insurmountable in quiescence.