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Apratim Chatterji: „Dynamics of Ultra-Soft Colloids (Star Polymers) under Shear”
IFF Jülich

We study the dynamics, structure formation, and jamming transitions of ultrasoft colloidal particles under shear. We consider a star polymer as a model system of a colloid with variable softness. The center of a star polymer with a large number of polymer arms f, has a very dense core and, hence, behaves like a rigid colloidal object. The softness can be tuned by varying f as well as the number of monomers N in the individual polymer arms. The transition from polymer-like properties to rigid-colloid like behaviour as a function of f of stars has been investigated previously {1,2}. We now study the effect of packing many star polymers for fixed f and N. Under equilibrium conditions we find that increased packing reduces the effective size of the stars. Under steady state shear, however, the higher packing of stars enhances shear induced deformation of stars as well as increased allignment of the deformed stars with the flow direction. We have observed shear thinning behaviour at high shear rates. We are investigating the possibility of jamming phenomenon for soft colloids/star polymers at higher arm number f.

We use a bead-spring potential with explicit excluded volume interactions to model the monomers of the star polymer. Hydrodynamic interactions are incorporated by coupling the star polymers to a MPC (multi-particle collision) fluid which is a coarse grained description of the solvent.


  1. M. Ripoll, R. G. Winkler, G. Gompper, Phys. Rev. Lett. 96, 188302, (2006).
  2. M. Ripoll, R. G. Winkler, G. Gompper, Eur. Phys.J.E, 23, 349 (2007).