Berlin 2008 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 1: DRG-DPG SYMPOSIUM Rheology I

CPP 1.4: Talk

Monday, February 25, 2008, 10:30–10:45, C 130

Macro- and micro- rheology of a soft colloidal suspension with tunable glassines — •Dirk van den Ende, Eko Purnomo, Siva Vanapalli, and Frieder Mugele — Physics of Complex Fluids, University of Twente, The Netherlands

We studied both the macro- and micro- rheology of soft thermosensitive microgel suspensions that can be tuned continuously and reversibly between the glassy state at low and the liquid state at high temperature. In the glassy state, the rheological properties (G^′ , G^{′ ′} , and J) of the suspensions depend strongly on their age [1]. They can be described quantitatively by the soft glassy rheology (SGR) model. The underlying mechanism for the aging is the increase of the structural relaxation time τ _s as the system ages. However, τ _s is experimentally inaccessible by small amplitude frequency sweep experiments. Therefore we tested the recently introduced strain rate frequency superposition (SRFS) technique. The evolution of τ _s(t) as determined with SRFS is consistent with the predictions of the SGR model based on the linear measurements. Hence, no macroscopic inhomogeneities occur in the sample. The relaxation time τ _s is also determined from the mean square displacement (MSD) of probe particles, embedded in the system, using a Confocal Scanning Laser Microscope. This technique provides not only the MSD values but also the displacement distributions, which are indicative for heterogeneity of the suspension.

[1] E.H. Purnomo, D. van den Ende, J. Mellema, and F. Mugele, Europhys. Lett. 76, 74 (2006).