Werner Loose

Rheology and structural changes of polymer melts via nonequilibrium molecular dynamics

Results of nonequilibrium molecular dynamics computer simulations of a planar Couette flow are presented for the multibead anharmonic‐spring model. The finitely extensible nonlinear elastic force law is used to connect the up to 100 beads of a chain molecule. Rheological data (shear viscosity, normal pressure differences) are discussed and compared with quantities describing the chain conformation (e.g., alignment tensor, static structure factor). This renders possible a test of the theoretical approaches which connect these quantities. In agreement with recent experiments, the static strucure factor exhibits characteristic elliptical distortions of the polymer coil whose magnitude depends on the distance from the gyration center. In our simulations the zero‐shear‐rate viscosity is found to scale linearly with the number of beads N up to chains with N=60. A weak upturn of the viscosity per bead for N=100 is found which may indicate the onset of the reptation regime.

Order in fluids: Shear-induced anisotropy in dense fluids of spherical particles and in gases of rotating molecules

After some general remarks on order in fluids and the anisotropy of distribution functions, results obtained by theoretical considerations and by nonequilibrium molecular dynamics computer simulations are presented for two specific examples. These are the influences of a shear flow, firstly, on the pair correlation function and the static structure factor of a dense fluid of spherical particles and, secondly, on the orientation of the rotational angular momenta of a gas of linear molecules. In the first case, the short and long range partial positional ordering is compared with scattering experimenrs from colloidal solutions. The second example is related to the flow birefringence measurements in gases.

Anisotropy in velocity space induced by transport processes

Nonequilibrium molecular dynamics computer simulations and the kinetic theory of gases are employed to investigate the interrelation between the (macroscopic) transport properties of dilute gases and the underlying characteristic distortions of the (microscopic) velocity distribution function. The anisotropy in velocity space is illustrated by intensity plots and by partial distribution functions which stem from a directional expansion of the nonequilibrium velocity distribution function with respect to irreducible Cartesian tensors. The two fundamental transport processes investigated are the transport of energy (heat flow) and of momentum. In the latter case special emphasis is laid on the ordering phenomena in velocity space which accompany non-Newtonian flow behavior.

Slip flow and slip boundary coefficient of a dense fluid via nonequilibrium molecular dynamics

The velocity slip of a dense fluid undergoing a plane shear flow is investigated by nonequilibrium molecular dynamics. The flow is driven by moving walls. The particles in the walls are pulled by localized force fields. The resulting velocity profiles exhibit a well-defined velocity slip which depends linearly on the velocity gradient in the bulk.

The constant-temperature constraint in nonequilibrium molecular dynamics and the non-newtonian viscosity coefficients of gases

Abstract The non-newtonian viscosity coefficients of a Lennard-Jones gas undergoing isothermal planar shear flow are calculated by kinetic theory and nonequilibrium molecular dynamics. Excellent agreement over a wide range of shear rates is obtained. The Boltzmann equation is used to derive a new analytic formula for the shear rate dependence of the viscosity coefficients. An essential feature of this presentation is the incorporation of the constant-temperature constraint of the computer simulation into the kinetic equation.

Shear-induced ordering revisited

The simulation of a stationary homogeneous shear flow is probably the most thoroughly investigated application of nonequilibrium molecular dynamics (NEMD). Well-probed algorithms are at hand which are discussed in several excellent reviews [1] and conference proceedings [2]. Even for the nonlinear regime their validity was established by means of nonlinear response theory for transient phenomena [3] and by comparing the steady-state response of a sheared gas with predictions of the Boltzmann equation [4,5]. Conversely, for the homogeneous heat flow a kinetic theory analysis was recentiy used to prove that no homogeneous driving force exists which generates the correct nonlinear heat conductivity [6].