Speaker:

George Petekidis

IESL-FORTH and Department of Material Science and Technology, University of Crete, Greece

Topic:

Colloidal gels as model systems to study shear induced tuning of thixotropic soft materials

Abstract:

Attractive colloids form out-of-equilibrium states such as gels and attractive glasses, which may exhibit thrixotropic effects. Their mechanical properties are affected by pre-shear history and show a time evolution after nonlinear steady or oscillatory shear due to structural changes and subsequent restructuring and relaxation after shear cessation. Along these lines, specific preparation protocols are often used empirically in similar type industrial systems to fine material’s properties. Therefore, pre-shear can be used to tune the structure and mechanical properties, driving the system into metastable states not accessible via a change of thermodynamics parameters. A related question is under what conditions strong shear can erase any preparation history i.e. fully rejuvenate an-of-equilibrium soft matter system in an equivalent way with an temperature jump, often used in computer simulations to switch-off attractions?

To answer the above questions we present a combination of experiments and computer simulations on colloidal gels with different attraction strengths and volume fractions. We discuss the implications of steady and oscillatory pre-shear on the microstructure as well as on the linear and non-linear rheological properties. In comparison with tests where a thermal quench is implemented we show when the two protocols (shear and thermal rejuvenation) are equivalent at the microstructural level and with respect to the macroscopic mechanics, and when shear induced structural anisotropy or residual stresses introduce differences. Finally, as several real systems comprise of non-spherical particles we present experiments with rodlike colloids and compare them with those of their spherical counterparts.

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