8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P8.31The transient response of supercooled colloidal fluids toexternal shearKevin Mutch, 1 Marco Laurati, 1 Georgios Petekidis, 2 Nikos Koumakis, 2 and StefanEgelhaaf 11 Heinrich-Heine-Universität, Universitätsstr. 1, 40225, Düsseldorf, Germany2 Foundation for Research & Technology - Hellas, Crete, GreeceThe transient response of concentrated colloidal suspensions to an external shear field has beenstudied experimentally by confocal microscopy and rheology experiments. Interesting features inthe dynamics of the system after the application or cessation of a step-rate deformation can belinked to a macroscopic stress response; these can be understood within a mode coupling theory(MCT) framework. Stepped flow rheological experiments display a stress overshoot before thesteady sheared state is reached, at accumulated strains of less than 10%. At comparable strainsthe microscopic dynamics exhibit super-diffusion, which can be related to the breaking of nearestneighbour cages, leading to a stress relaxation. When the shear field is switched off the systemrelaxes on a timescale inversely related to the shear rate, ˙γ. Rheology data suggest that, after aninitial relaxation, some frozen in stresses remain for considerable times and these increase uponapproaching and entering the glassy regime. The particle dynamics below the glass transitionshow a complete relaxation on a timescale related to 1/˙γ and are sub-diffusive on an intermediatetimescale. This can be related to localisation, which occurs on a length scale larger than atequilibrium, seeming to relate to the onset of caging in the relaxing system and the presence ofresidual stresses. The time dependence of the rheology and microscopic dynamics suggests thesehard sphere systems exhibit a ‘memory’ of the previously sheared state.31