Multiscale modelling using molecular dynamics and interfacecapturing methods for two-phase flow simulation of droplets covered with surfactants or asphaltenes, and applications to electrocoalescence

Abstract

THE WORK PRESENTED in this thesis attempts to improve our fundamental understanding of the interfacial properties of water drops in crude-oil. Model fluids consisting of aliphatic oil with surfactants, as well as real crude components such as asphaltenes, are considered. The case of a single drop being deformed, either by an electric field or by a needle deflating the drop, is studied in detail using simulations. Close comparisons with experiments are made, in order to increase confidence in the simulations as a faithful representation of the physical world. Combining methods at different scales, namely the molecular and the continuum, a simple but effective multi-scale method is developed to predict and explain the behaviour of drops with complex interfaces, e.g. water drops in oil covered with asphaltenes. The first part of this study concerns systems with a deforming drop of water in a model oil with surfactant, where one has a more-or-less complete understanding of the physics and chemistry. These systems are studied in detail using computational fluid dynamics simulations, and direct comparison with experiments is performed. The effect of surfactants on the transient deformation of drops, and in particular on the damping of oscillations, is studied here for the first time. To enable future studies of larger systems with several drops interacting, a 3D parallel version of the code has been developed using a domain decomposition approach. The case of a single falling drop in the presence of surfactants is also considered with a theoretical approach, and exact solutions are obtained for this system. The second part is the multiscale simulations of systems with a water drop in crude oil. The developed multiscale method consists of coarse-grained molecular dynamics simulations, using the SAFT- Mie approach, which are used to provide interfacial properties for the continuum simulations. At the continuum scale, a novel hybrid level-set/ghostfluid /immersed-boundary method has been developed for the simulation of complex fluid-fluid interfaces. Together, these approaches enable a direct link between the chemical composition of the crude oil and the interfacial properties. The molecular structure of crude oil components, and its effect on interfacial properties, is still under debate in the literature. The simulation approach developed here will enable detailed hypothesis testing, which may help settle the debate. A configuration of particular interest in this context is the “crumpling drop”, a phenomenon observed when a water drop in crude oil is drained by means of a pipette. This system is simulated, and good agreement is found with experiments reported in the literature. For the coarse-grained molecular dynamics approach used in the multiscale approach, namely the SAFT- Mie force field, two computational tools have been developed. They are called Bottled SAFT and raaSAFT. Together they provide an unprecedented ease-of-use for obtaining models for molecular simulation and for setting up and running these simulations.