Amann, C.P.; Siebenbürger, M.; Krüger, M.; Weysser, F.; Ballauff, M.; Fuchs, M.: Overshoots in stress-strain curves: Colloid experiments and schematic mode coupling theory. Journal of Rheology : transactions of the Society of Rheology 57 (2013), p. 149-175
10.1122/1.4764000
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Abstract:
The stress vs strain curves in dense colloidal dispersions under start-up shear flow are investigated combining experiments on model core-shell microgels, computer simulations of hard disk mixtures, and mode coupling theory. In dense fluid and glassy states, the transient stresses exhibit first a linear increase with the accumulated strain, then a maximum (stress overshoot) for strain values around 5%, before finally approaching the stationary value, which makes up the flow curve. These phenomena arise in well-equilibrated systems and for homogeneous flows, indicating that they are generic phenomena of the shear-driven transient structural relaxation. Microscopic mode coupling theory (generalized to flowing states by integration through the transients) derives them from the transient stress correlations, which first exhibit a plateau (corresponding to the solid-like elastic shear modulus) at intermediate times, and then negative stress correlations during the final decay. We introduce and validate a schematic model within mode coupling theory which captures all of these phenomena and handily can be used to jointly analyze linear and large-amplitude moduli, flow curves, and stress-strain curves. This is done by introducing a new strain- and timedependent vertex into the relation between the generalized shear modulus and the transient density correlator.