Influence of shear flow on the formation of rings in wormlike micelles: a nonequilibrium molecular dynamics study.

We study the influence of shear flow on the formation of rings in a generic reversible polymer (FENE-C) model, representative for wormlike micelles. Under equilibrium conditions, rings are dominating in dilute solutions, while linear chains are dominating in strongly overlapping and concentrated solutions. We find that shear flow induces a net shift of micellar mass from linear chains to rings. At the same time, the average aggregation size of linear chains is decreasing, while the average aggregation size of rings is increasing. We hypothesize that the increased abundance and size of rings are caused by a decreased entropy gain associated with ring opening under shear flow. Linear chains and rings are elongated in the flow direction and contracted in the gradient direction. This leaves an essentially two-dimensional free volume, which two newly created chain ends can explore after being disconnected. We study the ratio of ring and linear chain distribution functions to substantiate this hypothesis. Finally, we study the rheology and discuss how the observed increase of ring abundance can provide a positive feedback between strain and ring connectivity. Such a positive feedback can contribute to shear thickening behavior, observed in micellar solutions near the overlap concentration.