Mechanical and Civil Engineering Seminar

Understanding kinetically arrested phase transition in complex media, and its influence on structure-property relationships, has been identified as one of the grand challenges for the future of soft matter science. Fundamental discovery in this area will advance the next frontier in reconfigurable 'smart' materials developed for their hierarchical structure, amenability to biological functionalization, and extraordinarily flexible delivery. Colloidal gels and glasses are an important class of such materials and are the subject of an emergent field of study in which much focus is placed on predicting yield behavior. More fundamentally, colloids serve as a paradigmatic model system for molecular phase transitions, where a vast separation in timescales between colloid and solvent particles provides a powerful means by which to "slow down" fast relaxation processes to study phase behavior. Colloidal equilibrium phase diagrams have thus been constructed following this idea. However, colloidal gels represent "arrested" states of phase separation, where the same interparticle attractions that promote phase separation also inhibit it, freezing in a non-equilibrium microstructure to form a visco-elastic network. In contrast to attempts to place them on equilibrium phase diagrams, we argue that such gels must exit the equilibrium phase diagram. We show that when interparticle bonds are O(kT), thermal fluctuations enable ongoing particle migration and a (logarithmically) slow march toward full phase separation. However, as will be shown, external fields and forces open a pathway from arrested to equilibrium phases. I will propose a non-equilibrium phase diagram as the foundation for "phase mechanics", a new view of states of arrested colloidal matter. My talk will center on our large-scale dynamic simulation studies of flow-induced phase transitions in colloidal gels, both as mechanical solid-to-liquid yield and as force-activated release from kinetic arrest. Our simulations reveal the surprising result that gel yield can occur with loss of fewer than 0.1% of particle bonds, with no network rupture; rather, localized re-entrant liquid regions permit yield and flow. Analysis of the evolving osmotic pressure and potential energy reveals the interplay between bond dynamics and external stress that underlies mechanical yield. Discussion will include our developing non-equilibrium phase transition theory.