Computational nanofluidics: nonlocal viscosity kernel
Creators: name: Puscasu, Ruslan M.
Publisher: name: Swinburne
Year: 2024

description: In this thesis, we use equilibrium molecular dynamics (EMD) methods to investigate the nonlocal viscosity of atomic and molecular fluids. Firstly, we present an extended analysis of the wavevector dependent shear viscosity of monatomic and diatomic (liquid chlorine) fluids over a wide range of wavevectors and for a variety of state points. The analysis is based on EMD simulations, which involves the evaluation of transverse momentum density and shear stress autocorrelation functions. The results suggest that the real space viscosity kernels ofmonatomic and diatomic fluids depend sensitively on the density, the potential energy function and the choice of fitting function in reciprocal space. It is shown that the reciprocal space shear viscosity data can be fitted to two different simple functional forms over the entire density, temperature andwavevector range: a function composed of n-Gaussian terms and a Lorentzian type function. Overall, the real space viscosity kernel of these systems has a width of 3 to 6 atomic diameters which means that a generalized hydrodynamic constitutive relation is required for fluids with strain rates that vary nonlinearly over distances of the order of these dimensions. Secondly, the wavevector dependent shear viscosity for more complex molecular fluids such as butane and polymer chains have been determined. The density, temperature and chain length dependencies of the reciprocal and real space viscosity kernels are presented. We find that the density has a major effect on the shape of the kernel. The temperature range and chain lengths considered here have by contrast less impact on the overall normalized shape. Functional forms that fit the wavevector dependent kernel data over a large density and wavevector range have also been tested. Finally, a structural normalization of the kernels in physical space is considered. Overall, the real space viscosity kernel of these systems has a width of roughly 3-6 atomic diameters, similar to the diatomic case. Lastly, we study the nonlocal viscosity kernels of polymer melts upon cooling towards the glass transition. Previous results available in the literature for the temperature dependence of the self-diffusion coefficient and the value of the glass transition temperature are confirmed. We find that it is essential to include the attractive part of the interatomic potential in order to observe a strong glass transition. The width of the reciprocal space kernel decreases dramatically near the glass transition, being described by a delta like function near and below the glass transition, leading to a very broad kernel in physical space. It is found that a dynamic scaling factor obtained from the reciprocal space viscosity kernels reduces the real space kernels to a unique form unlike the static scaling factors based on pair distribution functions. Thus, spatial nonlocality turns out to play an important role in polymeric fluids at temperatures near the glass transition temperature. Finally, we make some concluding remarks and suggestions for future work.