Berlin 2012 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 16: Poster: Colloids and Complex Liquids

CPP 16.37: Poster

Tuesday, March 27, 2012, 18:15–20:45, Poster A

Effective Confinement as Origin of the Equivalence of Kinetic Temperature and Fluctuation-Dissipation Ratio in a Dense Shear Driven Suspension — •Boris Lander¹, Udo Seifert¹, and Thomas Speck² — ¹II. Institut für Theoretische Physik, Universität Stuttgart, Germany — ²Institut für Theoretische Physik II, Heinrich-Heine-Universität Düsseldorf, Germany

In equilibrium, the fluctuation-dissipation theorem states that the ratio of a correlation function and the corresponding response function is time independent, its value - the fluctuation dissipation ratio (FDR) - is given by the thermal energy. Out of equilibrium, this time independence is not expected to hold anymore. However, studying response and velocity autocorrelation functions for a tagged particle in a shear driven colloidal suspension, we found a broad regime where this time independence still holds approximately to a very good degree. The approximately constant FDR is the kinetic temperature. We can now explain this a priori surprising observation, using the idea of an effective confinement in dense suspensions [1]. Exploiting a time-scale separation, we can derive an approximate form of the FDT involving the kinetic temperature as a constant scaling factor and an additive correction term. We show numerically that the latter is negligible in a broad parameter range. We also compare the system to a simple toy model, consisting of a single colloidal particle, trapped in a harmonic potential, subjected to shear flow. For this simple model, we also find a regime in which the FDR is approximately time-independent.

[1] B. Lander, U. Seifert, and T. Speck arXiv:1111.3589