High density ratio Lattice Boltzmann simulations for ternary systems

Bala, Neeru (2021) High density ratio Lattice Boltzmann simulations for ternary systems. Doctoral thesis, Northumbria University.

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Abstract

Ternary fluid systems (and specifically the ones involving two liquids and a gas phase) are of special interest for a variety of practical applications, such as combustion engines, ink-jet printing, and oil recovery. The physics and dynamics of these systems involves a complex interplay of capillary, viscous and inertial forces. For such flows, some fundamental information such as the velocity field, strains, and stresses, usually cannot be observed in experiments. On the other hand, analytic descriptions struggle to account for complex boundary conditions at multiple interfaces. In this context, numerical approaches are able of revealing hidden details, and provide the missing link between experiments and theory. Additionally, numerical approaches allow to easily tune a variety of physical parameters over a broad range of values, usually difficult to access experimentally, allowing a broader and deeper understanding. In this thesis, I employ and further develop a ternary free energy lattice Boltzmann method (LBM) to investigate two fundamental problems involving ternary fluid systems.

The first problem focuses on the fluid flow and contact line dynamics of ternary fluids in presence of solid boundaries. To this aim, I propose three alternative schemes for solid boundaries for ternary fluids within the lattice Boltzmann framework. After benchmarking both static and dynamic properties, I focus on a system consisting of a train of two immiscible drops (bislug) confined within a long channel. By imposing a capillary force imbalance through the wetting boundaries for different phases, the bislug is self-propelled, and the steady velocity can be readily tuned by setting the bislug length. This will allow to extract simultaneous information on the contact line dynamics for all interfaces, evaluating the role of surface tension, density and viscosity contrast between phases.

The second and main problem focuses on the collision between immiscible drops surrounded by a low-density phase. Several experimental, and a few numerical studies have shown that, depending on the balance between viscous, inertial, and surface tension forces, different outcomes of collision are possible, such as Adhesion, Bouncing or Encapsulation. However, a detailed and systematic investigation is currently lacking. To cover this gap, I have performed systematic numerical simulations varying over a wide range of relevant dimensionless numbers tuning the relative surface tension of the fluid phases, the impact speed, the liquid viscosity, and the relative size of the drops. This allows to detect the transition boundaries between collision outcomes within multi-dimensional parameter spaces. Furthermore, I focus on the details of collision dynamics, highlighting the link between physical parameters, collision mechanisms and final outcomes. While most previous studies consider a phase space consisting of Weber number (rescaled velocity) and impact parameter (drop alignment), to the best of my knowledge this is the first work including a systematic variation of the Ohnesorge number (rescaled viscosity) for such ternary system. Among the main results, I have identified a novel type of collision outcome named Delayed Adhesion, not reported previously. This novel outcome is specifically related to the presence of capillary forces acting between immiscible drops.

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