Dresden 2009 – wissenschaftliches Programm
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 21: Polymer Physics II
CPP 21.10: Vortrag
Mittwoch, 25. März 2009, 12:00–12:15, ZEU 114
Build-up and relaxation of stresses in a glass-forming soft-sphere mixture — •Jochen Zausch1 and Jürgen Horbach2 — 1Inst. f. Physik, Universität Mainz, Staudinger Weg 7, 55099 Mainz — 2Inst. f. Materialphysik im Weltraum, DLR, Linder Höhe, 51147 Köln
Glass-forming liquids under shear often exhibit the effect of shear thinning, i.e. the decrease of viscosity η upon increasing shear rate γ if γ 1/τ (with τ the relaxation time). The viscosity is related to the shear stress σ=γη that is built up due to the external shear field. Employing extensive non-equilibrium Molecular Dynamics computer simulations of a glass-forming binary Yukawa mixture we aim at understanding the transition from a quiescent state to steady shear. To this end, we monitor the stress as a function of the strain γt after a suddenly commencing or terminating shear flow.
After switching on the shear field, the stress σ shows an overshoot at a strain γt≈ 0.1 that marks the transition from elastic to plastic deformation. The stress increase is closely related to the development of structural anisotropies, which we quantify by a projection of the pair correlation function onto a spherical harmonic of degree 2. After switching off the external drive in the elastic regime, stresses decay to zero on the time scale τ, whereas a switch-off in the plastic regime leads to the much faster decay on time scale 1/γ. We attribute this behaviour to the different distributions of local shear stresses in these regimes. Our results are corroborated by confocal microscopy experiments of colloidal systems and by a theoretical approach in the framework of mode-coupling theory (J. Phys.: Condens. Matter 20, 404210 (2008)).