Molecular dynamics simulations of nanofluids and their chaotic properties
Creators: name: Bernardi, Stefano
Publisher: name: Swinburne
Year: 2024

description: In this work we analyze several aspects of atomic fluids, in either homogeneous or inhomogeneous conditions, using non-equilibrium molecular dynamics simulation techniques (NEMD). The main aim is to characterize the chaotic properties of nanofluids under several types of constraints and forces. To achieve this, in conjunction with NEMD we use the Lyapunov spectral analysis. Lyapunov exponents allow us to quantify the degree of chaoticity of a dynamical system measuring the sensitivity to small changes in initial conditions. NEMD allows us to study mechanical properties of atomic fluids but also to view them as dynamical systems so that the tools of dynamical systems theory can be used. Many studies have been successfully performed in the past to characterize the chaoticity of homogeneous fluids, less were performed on inhomogeneous fluids in highly confined geometries and we focus our work on this class. We analyze if and how the chaoticity changes with confinement including analysis of the atomistic walls' phase space. In particular we look at two types of flow: Poiseuille and Couette. The former is typically described by a fluid in a channel driven by gravity-like forces or pressure gradients, while the latter describes a fluid trapped between two surfaces sliding in opposite directions. Both are important in many engineering applications. In the framework of confined fluids we also analyze the issue of how to properly thermostat such systems. When performing simulations of systems driven away from equilibrium, it is necessary to extract the heat produced by viscous heating. Several types of thermostats have been derived, and essentially they all act by modifying the velocity or acceleration of the particles to which they are applied, mimicking the collisions with the surroundings that would otherwise absorb the excess kinetic energy. In homogeneous systems the thermostat is usually applied to all particles, but in confined systems the situation is more complex because of spacial inhomogeneities and the difficult evaluation of streaming velocities even at low fields. We analyze these issues showing that thermostatting the confining walls, as representative of nature where the heat is always dissipated through the container’s walls, is a preferable procedure from either a mechanical or dynamical point of view, rather than thermostatting the fluid itself. For homogeneous flows, we present a new algorithm for infinite time simulations of fluids under mixed flow: a linear combination of pure shear flow and pure elongational flow. Both flows are found in many industrial and biological processes, and their models, even if idealizations, have been successfully employed to compute transport coefficients for complex fluids and other meaningful physical quantities. However in real situations they are often combined together, sometimes in addition to rotational and/or elliptical flow. It is therefore important to be able to characterize combinations of arbitrary flows. We derive new periodic boundary conditions (PBCs) able to simulate a bulk region of fluid under mixed flow for an indefinite amount of time (for the first time to our knowledge), without introducing nonphysical effects in the mechanical properties. Given that Lyapunov exponents are properly defined only in the infinite time limit, the development we have made to enable the simulation of mixed flow for an infinite period means, in theory, that for the first time it is possible to characterize its chaotic properties and to understand its phase space structure viewed as a whole and as a sum of pure flows. Lyapunov spectra for both planar shear flow and planar elongational flow have been studied at length before and a comparison with previous results is also reported.