Fluid dynamics alters liquid-liquid phase separation in confined aqueous two-phase systems

Liquid-liquid phase separation is key to understanding aqueous two-phase systems (ATPS) arising throughout cell biology, medical science, and the pharmaceutical industry. Controlling the detailed morphology of phase-separating compound droplets leads to new technologies for efficient single-cell analysis, targeted drug delivery, and effective cell scaffolds for wound healing. We present a computational model of liquid-liquid phase separation relevant to recent laboratory experiments with gelatin-polyethylene glycol mixtures. We include buoyancy and surface-tension-driven finite viscosity fluid dynamics with thermally induced phase separation. We show that the fluid dynamics greatly alters the evolution and equilibria of the phase separation problem. Notably, buoyancy plays a critical role in driving the ATPS to energy-minimizing crescent-shaped morphologies and shear flows can generate a tenfold speedup in particle formation. Neglecting fluid dynamics produces incorrect minimum-energy droplet shapes. The model allows for optimization of current manufacturing procedures for structured microparticles and improves understanding of ATPS evolution in confined and flowing settings important in biology and biotechnology.Comment: 9 pages, 8 figures, 3 supplementary movies, to appear in Proceedings of the National Academy of Sciences, accompanying code and parameters to generate data available at https://github.com/ericwhester/multiphase-fluids-cod

Phase field modelling of surfactants in multi-phase flow

A diffuse interface model for surfactants in multi-phase flow with three or more fluids is derived. A system of Cahn-Hilliard equations is coupled with a Navier-Stokes system and an advection-diffusion equation for the surfactant ensuring thermodynamic consistency. By an asymptotic analysis the model can be related to a moving boundary problem in the sharp interface limit, which is derived from first principles. Results from numerical simulations support the theoretical findings. The main novelties are centred around the conditions in the triple junctions where three fluids meet. Specifically the case of local chemical equilibrium with respect to the surfactant is considered, which allows for interfacial surfactant flow through the triple junctions

Physically consistent modelling of surface tension forces in the Volume-of-Fluid method for three or more phases

\ua9 2024 The Authors. Multiphase numerical simulations have become a widely sought methodology for modelling capillary flows due to their scientific relevance and multiple industrial applications. Much progress has been achieved using different approaches, and the volume of fluid is one of the most popular methods widely used for modelling two or more phases due to its simplicity, accuracy and robustness. However, when prescribing the forces emerging from three or more fluid-fluid interfaces, the force balance is not guaranteed and can lead to spurious self-propulsion. Here, a new approach to account for the surface tension forces for multiphase flows with a correct force balance is proposed. The newly proposed method is successfully validated for a wide range of tests, including contact angles for the fluid-fluid and fluid-solid triple line. Additionally, complete spreading phenomena of fluid on fluid and fluid on solid have been found to emerge naturally from the newly proposed surface tension force model. Finally, simulation results are compared against experiments of lubricant-impregnated surfaces to demonstrate the practical applicability of the newly proposed method