Capturing Anisotropic Surface Stresses and Particle Desorption with the Fipi Method-Simulating Complex Particle-Laden Fluid Interfaces on a Laptop

In Pickering emulsions, Bijels and particle-stabilised foams the stabilisation mechanism rests on the formation of a semi-solid skin, composed by one or more layers of particles, on the fluid-fluid interface. Understanding the link between the rheological properties of this skin and the surface particle microstructure is a key challenge of modern multiphase fluid materials science. In this talk we present FIPI, a new method for the fast simulation of particle-interface interaction problems involving bubbles, droplets and complex interfacial structures (e.g. bicontinous phases) interacting with particulate materials. The method enables to simulate a large (O(106)) number of particles in a reasonable time on a common PC, and reach the length scale separation and time scales of realistic experimental systems. The idea of the method is to fully resolve interfacial phenomena and hydrodynamics on scales larger than the particle, and model particle-level phenomena using physically-sound analytical or semi-empirical expressions. After describing the working principle of the method, I will illustrate the application of FIPI to two problems. One problem is that of a pendant drop covered by a monolayer of purely repulsive spherical particles. In this case, the simulations reveal the distribution of surface stresses in a fluid interface presenting complex, non-uniform
curvature, providing insights into where surface stress anisotropy - and thus surface shear elasticity - is important, and how the surface stress evolves in time when the drop pinches off. A second problem is that of the shrinkage of a drop or a bubble covered with a particle monolayer. In this case we use FIPI to investigate the transition between surface buckling and particle desorption when the drop or bubble is shrunk in a quasi-static manner, and the surface pressure is increased beyond a critical buckling threshold. The numerical results reveal a complex dependence on 3 nondimensional parameters, characterising the surface overpressure, the particle-interface adhesion strength, and the particle-to-drop size ratio. The simulations, which enables to track the dynamics of each single particle in the monolayer and capture microscopic phenomena, suggest that desorption and buckling may not be mutually exclusive, in contrast to what is often assumed based on macroscopic observations.