(141e) Structure and Rheology of Branched Micellar Fluids from Molecular Dynamics Simulations

We extend our previous coarse-grained molecular dynamics (CGMD) simulations of cationic surfactant micelles of cetyltrimethyl ammonium chloride (CTAC) and organic salt sodium salicylate (NaSal) in presence of explicit solvent (water) to understand the shear and extensional rheology of branched micellar fluids. Our simulations preserve the underlying physico-chemical, electrostatic and hydrodynamic interactions of the system; simulation methods are discussed in detail elsewhere [1-3]. CGMD simulations were performed for various surfactant (c_D) and salt (c_s) concentrations over micro second time scales to study equilibrium structure, and over deformation rates spanning several decades to understand the underlying structure-dynamics-rheology relationships.

It has been shown in previous studies [4-6] that the addition of highly binding organic salt, such as NaSal, results in a double peak in the variation of the zero-shear viscosity (η₀) of the solution with increasing salt to surfactant concentration ratio R. From the analysis of MD simulation trajectories, we offer a mechanism underlying this experimentally observed non-monotonic variation in the zero-shear viscosity. Specifically, branch formation at relatively large salt concentration (R»1) results in a significant reduction in the solution viscosity. The second maximum in the viscosity vs. R curve is attributed to the formation of a fully connected micellar network [3]. In this presentation, we will discuss how the hyperbolic interfaces at branch points are energetically stabilized by a relatively larger density of condensed Sal^- counter ions as well as various mechanisms of branch formation. Moreover, the simulations revealed pronounced dynamic heterogeneity associated with the motion of the surfactant molecules. Specifically, the ensemble averaged mean square displacement of all the surfactant molecules in the system suggests sub-diffusive dynamics. However, time averaged mean square displacements of individual surfactants exhibit a broad range of dynamical behavior including super-diffusion as proposed in Ref. [5], which has been attributed to the presence of Lévy flight-like trajectories [5]. This is the first time such super-diffusive motion has been observed directly in molecular simulations. This motion stems from the sliding of micelle branches along the main contour which has been proposed in the literature as an alternative mechanism for stress relaxation in branched micellar fluids. At higher surfactant concentrations, micelle entanglements inhibit the motion of surfactant molecules, resulting in sub-diffusive behavior.

In the second part of the talk, we will discuss the deformation of a micellar network in a steady uniform shear and uniaxial extensional flow. First, we will briefly discuss the dynamics of (flexible) wormlike and cylindrical micelles in an extensional flow [7]. Under uniaxial extension, transition from a folded to a stretched state is observed beyond a Weissenberg number-independent critical strain, after which micelle scission occurs through a mid-plane thinning mechanism. The effect of the binding counter ions on micelle stiffness will also be discussed. Extending our previous single micelle simulations [7], we have performed a large number of simulations to probe the dynamics of micelle networks under elongational flow. The effects of the rate of deformation and salt concentration on the extensional rheology and breakage of micelle networks will be discussed.

Acknowledgements: The authors gratefully acknowledge NSF grants CBET-1049454 and CBET-1049489 as well as the computational resources provided by XSEDE.

References:

[1] A. V. Sangwai and R. Sureshkumar, Langmuir, 27, 6628 (2011); 28, 1127 (2012)

[2] A. Sambasivam, A. V. Sangwai, and R. Sureshkumar, Phys. Rev. Lett., 114, 158302 (2015); Langmuir, 32, 1214 (2016)

[3] S. Dhakal and R. Sureshkumar, J. Chem. Phys., 143, 024905 (2015)

[4] D. Sachsenheimer, C. Oelschlaeger, S. Müller, J. Küstner, S. Bindgen and N. Willenbacher, J. Rheol., 58 (6), 2017 (2014)

[5] J. P. Bouchaud, A. Ott, D. Langevin and W. Urbach, Journal de Physique II, 1(12), 1465 (1991)

[6] S. A. Rogers, M.A. Calabrese and N.J. Wagner, Curr. Opin. Colloid Interface Sci., 19, 530 (2014)

[7] S. Dhakal and R. Sureshkumar, ACS Macro. Lett. , 5, 108 (2016)