Linear and Nonlinear Rheology of Thread-like Micelles: Multi-scale Simulations, Theory, and Experiments

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Aqueous surfactant (i.e., soap) solutions are very widely used in consumer-care products including shampoos, body washes, and gels, cleansing products, as well as in industrial products, including emulsions and adhesives. Surfactants in solution self-assemble into spherical or cylindrical aggregates called "micelles", whose shape and size control the properties of the surfactant solutions. The micelle sizes and shapes are in turn controlled by the molecular structure of the surfactant and by the concentrations added salts and other ingredients. An ability to predict and control these dependencies is the key to the rapid design of surfactant products. The project will therefore develop molecular simulation tools at multiple levels of scale that, together, will produce a deep understanding of the energies and forces that control surfactant self-assembly allow for predictive design of surfactant products. Software tools developed under this project will be made public and we expect it to be used by companies such as Procter and Gamble, as has been the case with software developed under the previous NSF funded grant. The new project will train Ph.D. students, aided by interactions with P&G, including possible internships there. We will used molecular dynamics (MD) simulations and the Weighted Histogram Analysis Method (WHAM) to extract the scission free energy required to break a thread-like micelle, and the free energy for removal of a micellar branch. This information will be combined with simulations at a coarser scale, using the ?Pointer Algorithm,? which tracks rates of micellar breakage and fusion and branch formation along with rates of sliding of micelles along entanglement pathways. These methods will be used to predict the rheology of ensembles of linear and branched micelles. These predictive tools will be tested by measuring rheological data for well-defined micellar solutions of CTAC (cetyl trimethylammonium chloride) mixed with either NaCl or with sodium salicilate (NaSal), and other surfactant solutions. We will also develop a simulation method and constitutive theory for the nonlinear rheological properties of thread-like micelles that accounts for breakage/fusion, nonlinear deformation, and entanglements. This simulation method will be novel hybrid of Brownian dynamics simulations of breakage/fusion of bead-spring chains, with the slip-spring model of Likhtman to account for entanglements, providing the first simulation method for the nonlinear properties of thread-like micelles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.