Dynamics and rheology of wormlike micelles emerging from particulate computer simulations

Abstract

We perform coarse-grained computer simulations of solutions of semidilute wormlike micelles and study their dynamic and rheological properties, both in equilibrium and under shear flow. The simulation model is tailored to the study of relatively large time and length scales (micrometers and several milliseconds), while it still retains the specific mechanical properties of the individual wormlike micelles. The majority of the mechanical properties (persistence length, diameter, and elastic modulus of a single worm) is determined from more detailed atomistic molecular dynamics simulations, providing the link with the chemistry of the surfactants. The method is applied to the case of a solution containing 8% (by weight) erucyl bis(hydroxymethyl)methylammonium chloride (EHAC). Different scission energies ranging from 15.5k(b)T to 19.1k(B)T are studied, leading to both unentangled and entangled wormlike micelles. We find a decrease in the average contour length and an increase in the average breaking rate with increasing shear rate. In equilibrium, the decay of the shear relaxation modulus of the unentangled samples agrees with predictions based on a theory of breakable Rouse chains. Under shear flow, transient over- and undershoots are measured in the stress tensor components. At high shear rates we observe a steady-state shear stress proportional to gamma(1/3), where gamma is the shear rate. This is confirmed by our high shear rate experiments of real EHAC in a parallel-plate geometry.

Transient dynamics in dense colloidal suspensions under shear: shear rate dependence

“…Only some time after the commencement of shear is the steady state of flow established. A transient regime occurs during which the stress-strain relation displays an overshoot [26][27][28][29], which has also been observed in other soft materials [29][30][31][32][33][34][35][36]. At comparable strains, a pronounced structural anisotropy of the cage develops [26,37, 3S] that does not fully relax even in the steady state of shear.…”

Non-Newtonian behavior and molecular structure of Cooee bitumen under shear flow: A non-equilibrium molecular dynamics study

“…This is usually not the case for colloids and for polymer solutions. Theoretical [22,[55][56][57] and numerical [26,[58][59][60][61][62] studies have been carried out on the linear and non-linear rheological properties of solutions of associative polymers, where the size of the assemblies also depends on temperature and shear rate. There is for example a growing literature on the rheology of telechelic polymer networks [22, 55-57, and 60] and of wormlike micelles [58, 59, 61, and 62].…”

Computer simulation of the rheology of concentrated star polymer suspensions

“…The y-dependence of the (implicit) solvent flowfield in the x-direction is dynamically updated by coupling it to the star polymer motion with an averaging time of 10 −3 s, as described in van den Noort and Briels (2008), Padding et al (2008). This algorithm allows any y-dependent flowfield to form.…”