An investigation of the Marangoni-Rayleigh-Bénard convection characteristics during mass transfer within compound droplets

Marangoni-Rayleigh-Bénard (MRB) convection characteristics during mass transfer within compound droplets were studied by integrating digital particle image velocimetry for flow visualization and schlieren imaging for concentration field analysis. Through comparative analysis of MRB convections in single-phase droplets, the hydrodynamic mechanisms underlying interfacial evolutions and vortex-cellular convection patterns in compound droplets were elucidated. Furthermore, the influences of initial solute concentration ( c 0 ) and core-shell ratios ( r / R ) on MRB convection behaviors, mass transfer properties, and morphological evolution of compound droplets were revealed. Three MRB convection patterns were identified: the Marangoni effect-dominated pattern (MD-pattern) with multiple pairs of counter-rotating vortex cells, the transition pattern (T-pattern) distinguished by a couple of co-rotating vortex cells, and the Rayleigh-Bénard effect-dominated pattern (RBD-pattern) characterized by a set of counter-rotating large vortex cells. At the initial Marangoni number Ma 0  ≥ 0.669 × 10 7 ( c 0  ≥ 20 wt%), the convection transitioned from the MD-pattern to the T-pattern and finally to the RBD-pattern, whereas at Ma 0  ≤ 0.361 × 10 7 ( c 0  ≤ 10 wt%), only the RBD-pattern was observed. For Ma 0  = 0.530 × 10 7 (15 wt%), a limited Marangoni effect competes with the Rayleigh-Bénard effect, resulting in the T-pattern. The convection intensity in droplets was enhanced by increased interfacial tension gradients, thereby significantly promoting mass transfer and resulting in pronounced droplet contraction. Conversely, the core-shell structure mitigated droplet contraction by reducing solute content that shortens the mass transfer duration while imposing spatial confinement that inhibits internal convection development. Notably, the vertical contraction far exceeded horizontal contraction due to the combined effects of lateral fluid migration driven by interfacial tension gradients at the droplet bottom and gravity-induced downward flow of fluid at the upper region.