We report on an investigation of the structural and dynamical properties of n-nonyltrimethylammonium chloride (C9TAC) and erucyl bis [2-hydroxyethyl] methylammonium chloride (EMAC) micelles in aqueous solution. A fully atomistic description was used, and the time evolution was computed using molecular dynamics. The calculations were performed in collaboration with Silicon Graphics Inc. using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code [1] on a range of massively parallel platforms. Simulations were carried out in the isothermal-isobaric (N,P,T) ensemble, and run for up to 3 nanoseconds. Simulated systems contained approximately 50 surfactant cations and chloride counterions, surrounded by 3000 water molecules. Starting from different initial configurations (spherical micelle, wormlike micelle) in the case of the C9TAC molecule, we observe shape transformations on the timescale of nanoseconds, micelle fragmentations, and surfactant-monomer exchange with the surrounding medium. Starting from a random distribution of surfactant molecules in the solution, we observe the mechanism of micelle formation at the molecular level. The mechanism of self-assembly or fragmentation of a micelle is interpreted in terms of generalised classical nucleation theory. Our results indicate that, when these systems are far from equilibrium and at high surfactant concentration, the basic aggregation-fragmentation mechanism is of Smoluchowski type (cluster-cluster coalescence and break up); closer to equilibrium and at lower surfactant concentration, this mechanism appears to follow a Becker-Döring process (stepwise addition or removal of surfactant monomers). In the case of the EMAC molecule, we have characterised two different structures (spherical and cylindrical) of the micelle, and have found that water penetration is not important. We have also studied the effect of the introduction of co-surfactant (salicylate) molecules to the EMAC system; hydrogen bonds between surfactant head groups and co-surfactant molecules were observed to play an important role in stabilizing wormlike micelles.2
This paper outlines the benefits of computational steering for high performance computing applications. Lattice-Boltzmann mesoscale fluid simulations of binary and ternary amphiphilic fluids in two and three dimensions are used to illustrate the substantial improvements which computational steering offers in terms of resource efficiency and time to discover new physics. We discuss details of our current steering implementations and describe their future outlook with the advent of computational grids.
We investigate spinodal decomposition and structuring effects in binary immiscible and ternary amphiphilic fluid mixtures under shear by means of three-dimensional lattice Boltzmann simulations. We show that the growth of individual fluid domains can be arrested by adding surfactant to the system, thus forming a bicontinuous microemulsion. We demonstrate that the maximum domain size and the time of arrest depend linearly on the concentration of amphiphile molecules. In addition, we find that for a well-defined threshold value of amphiphile concentration, the maximum domain size and time of complete arrest do not change. For systems under constant and oscillatory shear we analyze domain growth rates in directions parallel and perpendicular to the applied shear. We find a structural transition from a sponge to a lamellar phase by applying a constant shear and the occurrence of tubular structures under oscillatory shear. The size of the resulting lamellae and tubes depends strongly on the amphiphile concentration, shear rate, and shear frequency.
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