On the influence of gravity on particle accumulation structures in high aspect-ratio liquid bridges

This analysis focuses on the properties of the so-called particle accumulation structures (PAS) in non-cylindrical liquid bridges in normal gravity conditions. In this regard, it follows and integrates the line of inquiry started in Lappa (J. Fluid Mech., vol. 726, 2013, 160) where the main focus was on the effect of high-frequency vibrations. After the discovery (passed almost unnoticed) of a structure (Sasaki et al. Trans. Japan Soc. Mech. Engrs B, vol. 70, 2004) that seems to escape a simple classification on the basis of existing categorizations or paradigms for this type of phenomena, dedicated numerical simulations are carried out in the framework of an integrated Eulerian-Lagrangian approach. Conditions are considered corresponding to isodense and non-isodense particles in a liquid bridge with high aspect ratio, high Prandtl number fluid and hydrostatically deformed interface. Special attention is paid to the very elusive azimuthal wavenumber, for which PAS seem to be possible only in a very restricted range of Marangoni numbers. Two distinct families of particle attractors are identified, which coexist in the space of parameters as multiple solutions. We show that the key ingredient needed to unravel the related basins of attraction is the effect of gravity on particles, which may therefore be regarded as an additional mechanism responsible for the formation of these structures in terrestrial liquid bridges.