Numerical simulations of bursting bubbles: effects of contamination on droplet ejection and micro- and nanoplastics transport

As air is entrained (e.g. surface wave breaking, waterfalls) or injected (e.g. wastewater aeration) into water, bubbles are formed and either dissolve or rise back to the surface, collapse, and eject droplets. These bubbles that burst at the water surface represent a key contribution to aerosol formation and facilitate the exchange of mass, momentum and energy between water bodies and the atmosphere with significant implications for weather and climate. In addition, environmental and industrial water bodies contain a large number of suspended materials, such as micro- and nanoplastics, pollutants and diverse microorganisms, which can be entrained in the ejected droplet and thereby pose major environmental and health risks.
While recent numerical studies have focused primarily on clean interfaces, the contaminants present in natural and industrial settings affect both the bubble size distribution and droplet ejection mechanisms through Marangoni stresses. The present work aims to characterise drop ejection dynamics in the presence of contaminants, with a focus on Marangoni stresses that lower surface tension and rigidify the interface.  
Numerical simulations of bubbles bursting at a free surface are performed with the open source finite volume solver Basilisk. The mass and momentum conservation equations are solved on a Cartesian grid, using a Volume of Fluid method to capture the air-liquid interface while Adaptive Mesh Refinement allows to capture the jet dynamics. The effects of surface active agents or contaminants, often overlooked until recently in numerical simulations of bursting bubbles, are implemented and validated against experiments. 
Droplet ejection mechanism and the number of drops produced are analysed through regime maps spanning a wide range of Bond, Ohnesorge and Marangoni numbers, which characterise the bubble size, the fluid properties and the contamination effects. In the jetting regime, drops dynamics are characterised in terms of size, velocity, and maximum height. Initial results highlight the crucial damping effects of contaminants on capillary waves and the resulting jet during cavity collapse. The entrainment of micro- and nanoplastics is discussed as a function of particle sizes and concentration, providing insight into the coupling between interfacial physics and aerosol generation.