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Complex fluids display a variety of interesting rheological properties, shear thinning and shear banding being perhaps two of the most prominent of these features. This apparently common behavior, however, may arise from substantially different microscopic mechanisms [1-3] so that a generalization may appear too simplistic. There are, nevertheless, a number of interesting cases, where a common understanding of the phenomenon can be achieved. This particularly applies to the so called soft glassy materials, where details of microscopic interactions seem to play a minor role as compared to the effects arising from the lack of long range order and the collective dynamics at high densities. In these systems, a complex spatio-temporal response to external perturbation may arise from an interplay of simple microscopic mechanisms such as shear-induced diffusion and pressure-induced transport. In this talk, we first briefly address features common to a large class of complex fluids. We then take the case of a hard sphere glass, and provide evidence for a complex spatio-temporal flow behavior in such a simple system. The problem is studied both via event driven molecular dynamics [4] simulations and lattice Boltzmann modelling [5]. Dynamical equations governing the time evolution of the system response are also analyzed in the limit of small perturbations around the steady and homogeneous solution. Predictions are made regarding the stability of the steady homogeneous response. Simulations are in qualitative agreement with these predictions [4].
[1] F. Varnik, L. Bocquet, J.-L. Barrat, L. Berthier, Phys. Rev. Lett. 90, 095702 (2003). [2] R. Besseling, L. Isa, P. Ballesta, G. Petekidis, M.E. Cates, W.C.K. Poon, Phys. Rev. Lett. 105, 268301 (2010). [3] P.C.F. Moeller, S. Rodts, M.A.J. Michels, D. Bonn, Phys. Rev. E 77, 041507 (2008). [4] S. Mandal, M. Gross, D. Raabe, F. Varnik, Phys. Rev. Lett. 108, 098301 (2012). |