Wormlike micelles under shear flow: A microscopic model studied by nonequilibrium-molecular-dynamics computer simulations.

Aqueous surfactant solutions are known to form elongated micelles under certain thermodynamic conditions characterized by surfactant concentration, salinity or temperature [1,2]. In the semidilute regime these linear and flexible particles, with persistence lengths varying from 15 to 150 nm [3], form an entangled viscoelastic network. Recently observed phenomena such as shear banding structure, shear induced structure, phase transitions, thixotropy [4-6] are not completely understood, for example, the data are interpreted without taking into account the possible effect of flow on the size of the micelles or on their polydispersity (distribution of lengths [7,8]). We propose a microscopic model for solutions of wormlike micelles and report results from NonEquilibrium Molecular Dynamics (NEMD) computer simulations under shear flow. Our model (’FENE-C’) introduces the conceptof scission-recombination and is an extension of a model established for polymer melts [9-12]. In this study our interest is focused on the relevance of the microscopic model by comparing with experimental results and mesoscopic theories. The simulated behavior shows an exponential distribution for the micellar length and a quasi exponential dependence of the average length against the scission energy [12]. These results are in accordance with those calculated from Cates mesoscopic theory [13,14] C(L) / exp( L̄ L ); (1)