RHEOLOGY OF SEVERAL HUNDRED RIGID BODIES

A novel nonequilibrium molecular dynamics, originating in mesoscopic t heory of suspensions, is introduced to investigate the behavior of mod el polymeric fluids consisting of several hundred ellipsoids of revolu tion (spheroids) that interact via the Gay-Berne potential. This dynam ics is used to generate new microstructural, thermodynamic and rheolog ical data. The microcanonical equtions of motion for the translational and angular momenta as well as for mass-centers and orientational uni t vectors are derived from a Hamiltonian. These expressions are then a ugmented by SLLOD-like and Gaussian thermostat terms added consistentl y to equations for both the rotational and translational degrees of fr eedom; the role of Gaussian thermostat is to maintain constant kinetic temperature of the assembly of spheroids. The thermodynamic results a re calculated along one isotherm (nondimensional temperature T maintai ned at unity). Rheology is investigated for two state points (namely f or particle number density rho equal to 0.25, 0.4 and T set to 1), tha t lie well inside the isotropic phase if no external flow is applied. A state point is defined by the fluid's temperature T, and the concent ration of particles per unit volume rho. As indicated by snapshots of molecular configurations, at the intermediate shear rates (nondimensio nal shear rate approximately 1-2), ellipsoids become aligned to the di rection of flow and the stress tensor begins to be nonsymmetric. At ev en higher shear rates, this configuration breaks down leading to the f ormation of a transitory isotropic-type fluid, and then to the build-u p of a highly ordered structure exhibiting global orientation of parti cles in the direction of the vorticity axis. For rho = 0.4, the first (N1) and the second (N2) normal stress differences are positive and ne gative respectively, but at low densities (rho = 0.25), N1 becomes sli ghtly negative. In addition to the stress tensor, we compute the confo rmation tensor, the order parameter and the components of the pair rad ial distribution function. At high shear rates the radial distribution functions become significantly anisotropic. Furthermore, we investiga te the phenomenon of the stress overshoot at the inception of the simp le shear flow from a molecular perspective, and study the evolution of the distribution of translational velocities as a function of the she ar rate.