The Yield Transition in Gels: Accounting for Structural Breakdown

Gels are materials comprised of a majority fluid phase, but which exhibit solid-like mechanical properties arising from a percolating network structure. When subjected to external stresses or strains, these materials undergo a yield transition in which the material no longer elastically deforms but viscously flows. This transition is often accompanied by a commensurate breakdown in the material structure to a state that depends on the shear history. For many classes of gels, the structural recovery from this yielded state back into a percolating network occurs over long time scales, resulting in time-dependent mechanical properties that obscure the yield transition and deleteriously affects the performance of gels in applications ranging from additive manufacturing to tissue engineering and drug delivery. Here, we develop a novel rheological protocol to account for structural breakdown and to precisely quantify the yield transition in a variety of materials, including colloidal gels, physical gels, and a physicochemical gel comprised of polymer-linked emulsion droplets. The gelation of these materials is characterized through standard linear oscillatory rheology, but the yield transition is measured through a series of stress-controlled creep measurements in which the time it takes to yield the sample depends on the quiescent recovery time and applied stress. From these measurements, we quantify the structural evolution of the gels through a bifurcation in the creep response and unambiguously define the yield transition according to the divergence of yield times. Our findings elucidate the unique mechanisms of structural recovery that depend on gel physicochemistry, provide insight into the origins of the yield transition, and quantifies the thixotropic recovery of mechanical properties.