STRUCTURE, DYNAMICS AND RHEOLOGY OF SURFACTANT MICELLES AND MICELLE-NANOPARTICLE SOLUTIONS: A MOLECULAR DYNAMICS STUDY

Surfactant micelles are widely used in a number of industrial, commercial and household products and processes. Understanding flow-microstructure coupling in micellar systems can benefit applications ranging from targeted drug delivery and detergency to enhanced oil recovery and hydrofracking. Amongst micellar fluids, wormlike micelles (WLMs) are extremely interesting due to their structural similarity to polymers and their ability to constantly undergo scission and recombination at equilibrium. More recently, much has been generated in studying the effect of adding colloidal particles to WLMs. Colloidal particles can not only add functionality to the fluid but also act as viscosity modifiers. Such solutions can be used to design active nanomaterials for applications in energy harvesting and sensing. While several theories and continuum-level computational models have been developed to study the dynamics and rheology of WLMs, molecular-level explorations of the flow-structure coupling in such solutions is lacking. Further, in the case of mixtures of colloidal particles and WLMs, there are only a handful of attempts to develop theoretical/computational frameworks capable of describing their thermodynamics, self-assembly and phase behavior. The goals of this thesis are to uncover mechanisms by which WLMs interact with colloidal particles and to determine how these interactions affect the macroscopic properties of mixtures of model WLMs and colloidal nanoparticles (NPs) using molecular dynamics (MD) simulations. Coarse-grained (CG) molecular models and corresponding force-fields are employed to describe the NP, cationic cetyltrimethylammonium chloride (CTAC) surfactant, hydrotropic sodium salicylate (NaSal) salt, solvent and the underlying physico-chemical interactions. Results are first presented for the dynamics of a single self-assembled rodlike micellar aggregate under shear flow. The effect of shear rate on the configurational dynamics, e.g. orientation distribution of the end-to-end vector and tumbling frequency are presented and compared to experimental observations as well as predictions from stochastic simulations and mesoscopic theories. Further, a relationship between micelle length and stretching force is presented and compared with experimental estimates of similar forces in biological systems. Finally, a shear rate-independent energy barrier for micelle scission is identified for relatively large shear rates. We also show that the addition of NPs to surfactant solutions can result in the formation of NP-surfactant complexes (NPSCs). The effect of NP charge and surface chemistry on the nature of the self-assembly is discussed. Further, such NPSCs can further interact with WLMs, in the presence of NaSal salt, to form electrostatically stabilized micelle-NP junctions via an end cap attachment mechanism. The dynamics, energetics and stability of such junction formation is also described in detail. These junctions can give rise to unique rheological modifications of WLMs such as significant buildup in viscosity and viscoelasticity. Large-scale equilibrium and non-equilibrium MD simulations consisting of several NPs and WLMs are performed to study the flow-microstructure coupling in such systems. The relationship between the zero-shear viscosity, NP volume fraction and salt concentration at a fixed surfactant concentration is presented. Shear thinning behavior is observed for all of the systems studied. Shear thinning is accompanied by flow-alignment and shear-induced isotropic-to-nematic transitions in micellar systems. Further, the evolution of the first normal stress difference, N1, is presented as a function of time and shear rate, and compared with experimental observations for similar systems. The results of this work provides insight into the mechanisms of self-assembly in WLMs and colloidal NPs and demonstrate that rheological properties of WLMs can be uniquely controlled by the addition of NPs

Elongational perturbations on nematic liquid crystal polymers under a weak shear

The article of record as published may be located at http://dx.doi.org/10.1063/1.2794002The two-dimensional Smoluchowski equation is employed to study the effect of elongational perturbations on nematic liquid crystal polymers under a weak shear. We use the multiscale asymptotic analysis to show that (1) when the elongational perturbation is small relative to the weak shear, the orientational probability density function (pdf) tumbles periodically only in an intermediate range of polymer concentration; outside this intermediate range (i.e., for very small and very large polymer concentration) the orientational pdf converges to a steady state and there is no tumbling. (2) When the elongational perturbation is about 20% of the shear rate or larger, the intermediate range of tumbling disappears and the orientational pdf always converges to a steady state regardless of the polymer concentration. Our theoretical predictions are consistent with various earlier results based on the Leslie - Ericksen theory [C. V. Chaubal and L. G. Leal, J. Non-Newtonian Fluid Mech. 82, 22 (1999)] or analogous 3D numerical simulations (M. G. Forest, R. Zhou, and Q. Wang, Phys. Rev. Lett. 93, 088301 (2004); M. G. Forest, Q. Wang, R. Zhou, and E. Choate, J. Non-Newtonian Fluid Mech. 118, 17 (2004)]. (C) 2007 American Institute of Physics

Stochastic semi-Lagrangian micro–macro calculations of liquid crystalline solutions in complex flows

A general method for the simulation of complex flows of liquid crystalline polymers (LCPs) using a stochastic semi-Lagrangian micro–macro method is introduced. The macroscopic part uses a spatial-temporal second order accurate semi-Lagrangian algorithm, where ideas from the finite element and natural element methods are mixed in order to compute average quantities. The microscopic part employs a stochastic interpretation of the Doi–Hess LCP model, which is discretized with a second order Richardson extrapolated Euler–Maruyama scheme. The new method is validated and tested using the benchmark problem of flow between rotating eccentric cylinders. In a decoupled analysis, a discussion on the sensibility of the scalar order parameter to the macroscopic flow is offered. For the coupled situation, the proposed method predicts disclinations at certain regions of the geometry, as well as an accentuated abatement of the flow as the strength of the micro–macro interaction increases. Further examples are provided at different Peclet and concentration numbers to gain insight on the behavior of complex flows of LCPs in the eccentric cylinder geometry. The generality and robustness of the method, as well as its accurate prediction of LCP behavior under complex flows are main features of the implementatio

Non-existence of mean-field models for particle orientations in suspensions

We consider a suspension of spherical inertialess particles in a Stokes flow on the torus $\mathbb T^3$. The particles perturb a linear extensional flow due to their rigidity constraint. Due to the singular nature of this perturbation, no mean-field limit for the behavior of the particle orientation can be valid. This contrasts with widely used models in the literature such as the FENE and Doi models and similar models for active suspensions. The proof of this result is based on the study of the mobility problem of a single particle in a non-cubic torus, which we prove to exhibit a nontrivial coupling between the angular velocity and a prescribed strain.Comment: All comments welcom