(51h) Fantastic Gels and Where to Find Them: Toward Thermomechanical Processing of Colloidal Gels

Thermodynamic phase instability provides a versatile toolkit for developing non-equilibrium structure in atomic and molecular mixtures, where the tendency for kinetic arrest underlies thermomechanical processing strategies for biphasic materials. Repeated annealing and quenching produce materials that combine the properties of individual phases, such as ductility, strength and memory. While such techniques have existed since antiquity for processing of materials like metals and ceramics, the use of sophisticated thermal processing of colloidal systems is in its infancy yet holds potential for engineering the properties of colloidal gels. To enable more complex thermal processing in these systems, a better understanding of how to precisely establish locations of non-equilibrium states on a colloidal phase diagram need to be developed. Here, we present a new method for doing this that uses modulated quenches in attraction strength to determine the minimal conditions for gelation for a model experimental colloidal system and in particle simulations. In all in vitro and in silico systems we identify three distinct regimes of gelation set by an interplay of percolation, equilibrium phase separation and glassy arrest that encode the ability to form gels. In addition, this new method can be used to determine microscopic properties of the arrested biphasic gel like the final volume fraction of particles in the dense phase. We find that the dense phase volume fractions determined from this method extend the attractive glass line outside the phase boundary into the region of phase instability. Using this information, we dissect the equilibrium phase boundary, gelation threshold, percolation line, and glass line to define a complete colloidal phase diagram with district structure and mechanics in regions where these states intersect. This new understanding of colloidal phase behavior will enable thermal processing in the “hot” region of arrested phase separation to tune the arrested length scales of colloidal gels and thereby tune various mechanical properties such as gel stiffness and toughness, akin to analogous processes in hard materials.