Structure factor and rheology of chain molecules from molecular dynamics.

Equilibrium and non-equilibrium molecular dynamics were performed to determine the relationship between the static structure factor, the molecular conformation, and the rheological properties of chain molecules. A spring-monomer model with Finitely Extensible Nonlinear Elastic and Lennard-Jones force field potentials was used to describe chain molecules. The equations of motion were solved for shear flow with SLLOD equations of motion integrated with Verlet's algorithm. A multiple time scale algorithm extended to non-equilibrium situations was used as the integration method. Concentric circular patterns in the structure factor were obtained, indicating an isotropic Newtonian behavior. Under simple shear flow, some peaks in the structure factor were emerged corresponding to an anisotropic pattern as chains aligned along the flow direction. Pure chain molecules and chain molecules in solution displayed shear-thinning regions. Power-law and Carreau-Yasuda models were used to adjust the generated data. Results are in qualitative agreement with rheological and light scattering experiments.

Critical phenomenon analysis of shear-banding flow in polymer-like micellar solutions. 1. Theoretical approach.

The shear-banding flow in polymer-like micellar solutions is examined here with the generalized Bautista-Manero-Puig model. The coupling between flow and diffusion naturally arises in this model, which is derived from the extended irreversible thermodynamic formalism. The limit of an abrupt interface is treated here. The model predicts a dynamic master steady-flow diagram, in which all data collapse at low shear rates. Moreover, the model predicts that a nonequilibrium critical line is reached upon decreasing the shear-banding intensity parameter of the model, which corresponds to increasing temperature, increasing surfactant concentration, or varying salt-to-surfactant concentration ratio. By employing nonequilibrium critical theory and the concept of dissipated energy (or extended Gibbs free energy), a set of symmetrical reduced stress versus reduced shear-rate curves are obtained similar to gas-liquid transitions around the critical point. In addition, the nonequilibrium critical exponents are derived, which follow the extended Widom's rule and the extended Rushbroke relationship, but they are nonclassical.

Nonequilibrium molecular dynamics of the rheological and structural properties of linear and branched molecules. Simple shear and poiseuille flows; instabilities and slip.

Nonequilibrium molecular-dynamics simulations are performed for linear and branched chain molecules to study their rheological and structural properties under simple shear and Poiseuille flows. Molecules are described by a spring-monomer model with a given intermolecular potential. The equations of motion are solved for shear and Poiseuille flows with Lees and Edward's [A. W. Lees and S. F. Edwards, J. Phys. C 5, 1921 (1972)] periodic boundary conditions. A multiple time-scale algorithm extended to nonequilibrium situations is used as the integration method, and the simulations are performed at constant temperature using Nose-Hoover [S. Nose, J. Chem. Phys. 81, 511 (1984)] dynamics. In simple shear, molecules with flow-induced ellipsoidal shape, having significant segment concentrations along the gradient and neutral directions, exhibit substantial flow resistance. Linear molecules have larger zero-shear-rate viscosity than that of branched molecules, however, this behavior reverses as the shear rate is increased. The relaxation time of the molecules is associated with segment concentrations directed along the gradient and neutral directions, and hence it depends on structure and molecular weight. The results of this study are in qualitative agreement with other simulation studies and with experimental data. The pressure (Poiseuille) flow is induced by an external force F(e) simulated by confining the molecules in the region between surfaces which have attractive forces. Conditions at the boundary strongly influence the type of the slip flow predicted. A parabolic velocity profile with apparent slip on the wall is predicted under weakly attractive wall conditions, independent of molecular structure. In the case of strongly attractive walls, a layer of adhered molecules to the wall produces an abrupt distortion of the velocity profile which leads to slip between fluid layers with magnitude that depends on the molecular structure. Finally, the molecular deformation under flow depends on the attractive force of the wall, in such a way that molecules are highly deformed in the case of strong attracting walls.