Collaborative Research: Towards a Molecular-Scale Understanding of Flow-Induced Gelation in Thread-Like Micelle Solutions

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0853662LarsonDespite substantial progress achieved in understanding the microstructural basis of transport and rheology in many classes of complex fluids, including polymers, colloids, glasses, liquidcrystals and others, the flow behavior of one of the most important classes of complex fluids, surfactant solutions, remains mysterious. In particular, it has been known for at least two decades that translucent solutions of thread/rod like surfactant micelles undergo a phase transition to form viscoelastic gels under sufficiently strong shear or extensional flows. Lacking an understanding even of what these structures are, they are simply given the name Flow-Induced Structures, or FIS. While progress has been made towards simulating thread/rod like(nanoscale) micelles at the molecular level, and towards simulating the macroscopic consequences of the presence of FIS, such as banded structures in the shear flow, there is an almost complete lack of theoretical connection between molecular structures and the possibility and conditions under which such nano structures might manifest.Intellectual Merit. This is a collaborative investigation between two universities to help close gaps in understanding through both experiments and a multi scale set of simulations encompassing length and time scales ranging from atomic (and nano) to continuum. A set of experiments exploring the regimes of transient and novel permanent flow-induced structures, induced by extensional deformation in micro channels, will be carried out. In parallel, the project proposes four different simulation methods. 1) Atomistic Molecular Dynamic Simulations. These can capture the structure and interactions of one or two thread like micelle fragments in a periodic box roughly 10 nm on a side, at the atomic level on timescales of 10 nanoseconds. This is long enough and big enough to determine ionic effects on micellar structure and intermicellar interactions. 2) Coarse Grained (CG) Molecular Dynamics Simulations. Using the Marrink MARTINI model that lumps around four heavy atoms into each bead, a 1000 fold speed up relative to atomistic simulations is attained, reaching nearly to the millisecond time scale, while preserving molecular scale properties through suitably chosen CG potentials. The CG model will allow for the determination of micelle persistence lengths and the stability of thread like micelles as a function of salt concentration. 3) Brownian Dynamics Simulations using pearl necklace micelle model. This model, pioneered by Ryckaert and coworkers, treats the wormlike micelle as a string of beads that can break and fuse end-to-end, and is fast enough to allow for the equilibration of micelle length distributions, with and without flow. We will incorporate into this model the potential for micelle junctions or cross links, and bundling, thereby allowing for the first time a molecular scale simulation of flow induced gel formation. 4) Kinetic Model and Constitutive Equation. We will attempt to draw from the simulations the ingredients necessary to build a kinetic model and, if possible, a full nonlinear constitutive equation for flow of thread like micelles. Through this set of interlocking simulations, each aimed at different length and time scales, complemented by experiments, the investigators have developed a roadmap to bridge between molecular properties and macroscopic flow effects such as flow induced gelation and shear banding.Broader impacts include a collaboration with scientists at Proctor and Gamble, whose nterest is in understanding, modeling, and controlling the properties of thread like micellar solutions. They plan annual meetings between P&G scientists and our team of graduate and undergraduate students and faculty as well as month long student interships at P&G. This will lead to fruitful exchange of ideas, bringing practical commercial concerns to the attention of students, and carrying novel fundamental ideas and new modeling methods into the corporate world. They plan to also recruit UG (REU) as well as school students including minority students (through STARS program at Washington University) and involve them in developing modules driven by fast GROMACS and MARTINI engines with coarse grained potentials to help learn self assembly in surfactant solutions.