Department Colloquium | The secret life of gels

Many materials we eat, spread, squeeze, or 3D print are gels, soft amorphous solids whose solid component comprises self-assembled networks of particles, fibers, or agglomerates of proteins, polymers, and colloids. The space between and within human cells is permeated by self-assembled gel networks, the extra-cellular matrix and the cytoskeleton, whose self- organization and heterogeneity is central to biological functions. Self-assembled gels have adaptive, tunable, and nonlinear rheology determined by a complex interplay between the molecular cohesion and surface interactions, the aggregation kinetics that drive formation of various types of structures, and the effect of external forces that can promote breaking or reforming of the load-bearing backbone. Solidification processes are typically sources of frozen-in stresses and help build a memory of the processing history in these amorphous solids. Disorder and self-organization determine stress localization under load and the feedback between stress heterogeneities, structural disorder, and nonequilibrium conditions is therefore key to the mechanical response of these fascinating and ubiquitous materials.