Bulk and Interfacial Shear Thinning of Immiscible Polymers

Nonequilibrium molecular dynamics simulations are used to study the shear thinning behavior of immiscible symmetric polymer blends. The phase separated polymers are subjected to a simple shear flow imposed by moving a wall parallel to the fluid-fluid interface. The viscosity begins to shear thin at much lower rates in the bulk than at the interface. The entire shear rate dependence of the interfacial viscosity is consistent with a shorter effective chain length $s^*$ that also describes the width of the interface. This $s^*$ is independent of chain length $N$ and is a function only of the degree of immiscibility of the two polymers. Changes in polymer conformation are studied as a function of position and shear rate.Shear thinning correlates more closely with a decrease in the component of the radius of gyration along the velocity gradient than with elongation along the flow. At the interface, this contraction of chains is independent of $N$ and consistent with the bulk behavior for chains of length $s^*$. The distribution of conformational changes along chains is also studied. Central regions begin to stretch at a shear rate that decreases with increasing $N$, while shear induced changes at the ends of chains are independent of $N$.Comment: 8 pages, 8 figure

Non-Newtonian behavior and molecular structure of Cooee bitumen under shear flow:A non-equilibrium molecular dynamics study

The rheology and molecular structure of a model bitumen (Cooee bitumen) under shear are investigated in the non-Newtonian regime using non-equilibrium molecular dynamics simulations. The shear viscosity, normal stress differences, and pressure of the bitumen mixture are computed at different shear rates and different temperatures. The model bitumen is shown to be a shear-thinning fluid at all temperatures. In addition, the Cooee model is able to reproduce experimental results showing the formation of nanoaggregates composed of stacks of flat aromatic molecules in bitumen. These nanoaggregates are immersed in a solvent of saturated hydrocarbon molecules. At a fixed temperature, the shear-shinning behavior is related not only to the inter- and intramolecular alignments of the solvent molecules but also to the decrease of the average size of the nanoaggregates at high shear rates. The variation of the viscosity with temperature at different shear rates is also related to the size and relative composition of the nanoaggregates. The slight anisotropy of the whole sample due to the nanoaggregates is considered and quantified. Finally, the position of bitumen mixtures in the broad literature of complex systems such as colloidal suspensions, polymer solutions, and associating polymer networks is discussed

Adsorption-desorption kinetics in nanoscopically confined oligomer films under shear

The method of molecular dynamics computer simulations is employed to study oligomer melts confined in ultra-thin films and subjected to shear. The focus is on the self-diffusion of oligomers near attractive surfaces and on their desorption, together with the effects of increasing energy of adsorption and shear. It is found that the mobility of the oligomers near an attractive surface is strongly decreased. Moreover, although shearing the system forces the chains to stretch parallel to the surfaces and thus increase the energy of adsorption per chain, flow also promotes desorption. The study of chain desorption kinetics reveals the molecular processes responsible for the enhancement of desorption under shear. They involve sequences of conformations starting with a desorbed tail and proceeding in a very fast, correlated, segment-by-segment manner to the desorption of the oligomers from the surfaces.

Nonlocal shear stress for homogeneous fluids

It has been suggested that for fluids in which the rate of strain varies appreciably over length scales of the order of the intermolecular interaction range, the viscosity must be treated as a nonlocal property of the fluid. The shear stress can then be postulated to be a convolution of this nonlocal viscosity kernel with the strain rate over all space. In this Letter, we confirm that this postulate is correct by a combination of analytical and numerical methods for an atomic fluid out of equilibrium. Furthermore, we show that a gradient expansion of the nonlocal constitutive equation gives a reasonable approximation to the shear stress in the small wave vector limit

Dynamical properties of a confined diatomic fluid undergoing zero mean oscillatory flow: Effect of molecular rotation

In this paper we investigate the spatiotemporal dynamics of a diatomic fluid undergoing zero mean oscillatory flow in a slit pore. The study is based on nonequilibrium molecular dynamics simulations together with two limiting solutions to the Navier-Stokes equations which include the effect of molecular rotation. By examining the viscoelastic properties of the system we can estimate the extent of the Newtonian regime, and a direct comparison between the molecular dynamics data and the solutions to the Navier-Stokes equations is then possible. It is found that the agreement is excellent, and that the vortex viscosity can be estimated by fitting the data obtained in the molecular dynamics simulations to the solutions to the Navier-Stokes equations. The quantitative effect of the coupling between the linear momentum and the spin angular momentum on flow is also investigated. We find that the maximum flow can be reduced up to 3-4 % due to the coupling

Molecular spin in nano-confined fluidic flows

IIn this paper, we study the effect of molecular spin on the fluid dynamics of molecular nano-confined fluids using the extended Navier–Stokes equations. We show that the effect of spin is non-negligible for non-steady flows and we then discuss two examples, namely, a zero mean oscillatory flow and an oscillatory lid driven cavity flow. In the discussion of the former, we propose a dimensionless quantity that qualitatively predicts the effect of the spin. From this it is shown that only for sufficiently small system sizes and extremely high frequencies will molecular spin be relevant, depending on the molecular fluid’s rotational inertia and the rotational viscosity. In the lid driven cavity flow we observe that the thermodynamic energy dissipation due to molecular spin undergoes period doubling when increasing the Reynolds number and, under some circumstances, it may be negative