Multi-scale and multi-fidelity numerical study of microfluidics and complex interfacial phenomena

Multiphase flows are crucial to various industrial applications and are an ubiquitous part of the natural world. Particularly significant are multiphase flows characterised by complex interfacial phenomena, such as surface-active contamination and interface breakup, as well as those involving disperse phases exhibiting variability in their properties, known as multiphase polydisperse systems. Accurately predicting the behaviour of these systems poses various challenges due to their tremendous complexity, involving multiscale phenomena, interactions between inertia, viscosity, gravity, and surface tension, nonlinear dynamics, and multiphysics mechanisms. This thesis presents an in-depth computational investigation of three multiphase systems that exemplify these complexities. The first study examines reactive microscale flows through the chemical synthesis of silver nanoparticles. The second centres around the poorly-documented phenomenon of surfactant-laden drop encapsulation within a propagating bubble in a capillary channel. Lastly, this study examines the mechanisms leading to aerosol release from bursting bubbles at a liquid-gas interface, with a focus on the action of surfactants. Regarding nanoparticle synthesis, a modelling framework integrating computational fluid dynamics with population balance (CFD-PBM) is proposed, extending traditional models developed for well-mixed systems to open-flow microchannels, where mixing dynamics critically influence the particle size distribution. The model has been rigorously validated against experimental and numerical data from multiple sources and under various operating conditions, offering insights into the relationships between kinetic and hydrodynamic phenomena affecting the system's outcomes. Drop encapsulation events in propagating bubbles and bursting bubbles are investigated using high-fidelity CFD simulations employing a hybrid front-tracking/level-set approach. Through a comprehensive analysis of various surfactant properties, this work introduces a regime map categorising the observed encapsulation behaviours and demonstrates how surface tension reductions and Marangoni stresses delay encapsulation. For bursting bubbles, a quantitative assessment of surfactant-induced wave retardation and its impact on aerosol suppression is presented, along with a phenomenological mechanism explaining wave delays.