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W. Briels: „Simulations of transient forces in soft matter and the rheology of tri-block co-polymer telechelics”
University Twente, The Netherlands

In order to perform simulations of flowing soft matter in complex geometries, a minimum number of degrees of freedom must be used. A possible strategy in this situation is to simulate only the movements of the centers of mass of all particles, and to envisage the remaining degrees of freedom as constituting a thermodynamic bath. Such a coarse graining procedure inevitably leads to very soft interactions and usually to equations of motion that include memory of the recent history of the moving particles. In order to circumvent dealing with memory effects, we introduce an additional set of variables describing the thermodynamic state of the bath. Deviations of these variables from their equilibrium values give rise to transient forces in addition to the, usually very soft, potentials of mean force. The model will be applied to study the rheology of solutions of tri-block copolymers in a solvent that does not accept the end blocks of the polymer, but does accept the middle block. In such a case the end blocks gather together in micellar spheres, with the middle blocks dissolved in the solvent. At low concentrations both end blocks of each individual polymer are connected to the same micelle, with the middle blocks distributed like leaves around the central sphere. Under these circumstances the mesoscopic particles are called flowers. At high concentrations, micelles are connected by polymers with both end blocks taking part in different micelles, in which case the system forms a telechelic network. Such systems exhibit shear banding and sometimes even fluid fracture. It turns out that model parameters may be chosen such that all these phenomena are indeed observed in the simulations as well. From the simulations some characteristics of the banding to fracture transition will be obtained.