(380a) Flow, Arrest and Yielding in Dense Colloidal Suspensions – Glasses Vs. Gels

Rheology modification in complex fluids is important for a wide range of applications including formulation of engineering fluids and consumer products. The ways in which such fluids evolve after cessation of flow and during yielding under flow have important implications for the properties and performance of the fluid in its intended application. Here we consider flow properties, dynamical arrest, and yielding in systems with repulsive and attractive interactions, i.e. systems which form soft colloidal glasses and colloidal gels, respectively. Rheological studies chart the evolution from metastable fluid-like states into arrested solid-like states as a function of time following cessation of flow, or a thermal quench. We observe distinct signatures that are associated with dynamical arrest in both cases. For colloidal glasses, arrest occurs due to a sudden onset of elasticity in which the viscosity of the system goes through a maximum as the elastic contribution becomes dominant at large times. Notably, the temporal width of the peak in the viscous modulus of the system is a strong function of stress, which signals that the stress modifies the distribution of arrest timescales in the system. In colloidal gels, arrest proceeds via the formation of a critical gel. The scaling exponents describing the approach to the critical state can be captured through a systematic set of time-resolved measurement, revealing strong similarities to the critical behavior of many commonly studied polymer gels. We examine the yielding response in a model suspension as the system is deliberately tuned from a soft colloidal glass to an attractive colloidal gel by adjusting ionic strength. The colloidal glass is marked by rapid recovery of rheological properties following yielding, whereas the gel is subject to much longer transients and exhibits path-dependence in its rheological properties due to the sensitivity of the aggregation state to shear flow. The implications of dynamical arrest and yielding are discussed in the context of formulating complex fluids and rheology modification for various applications. A novel approach to rheology modification is discussed, based on the deliberate design of systems featuring hierarchical dynamics.