Experimental study on Rayleigh-Bénard-Marangoni convection characteristics in a droplet during mass transfer

Abstract

Rayleigh-Bénard-Marangoni (RBM) convection in an aqueous droplet during the mass transfer process of acetic acid (solute) from the droplet to the butyl acetate in a vertical slit are investigated in the parallel visualization experiments via adopting the DPIV method and the Schlieren method. The real-time RBM convection structures are observed and quantitatively analyzed in details. The influence of the initial solute concentration (i.e., initial Marangoni number (Ma0) and initial Rayleigh-Bénard number (Ra0)) in the droplet on the RBM convection characteristics, mass transfer performance and droplet morphology are studied. The results indicate that the RBM convection in the droplet is determined by the competition between the buoyancy/gravity and interface gradient during the mass transfer process, which includes three main modes, i.e., Marangoni effect dominant mode (MEDM) characterized by some counter-rotating small vortices, transition mode (TM) characterized by several small co-rotating vortices, and Rayleigh-Bénard effect dominant mode (REDM) characterized by two large counter-rotating vortices. When Ma0 ≥ 0.816 × 107, MEDM, TM and REDM appear successively during the mass transfer process and MEDM tends to disappear when Ma0 ≤ 0.719 × 107. Especially, only REDM occurs at Ma0 ≤ 0.627 × 107. The REDM convection tends to stabilize fluid convection in the droplet while the MEDM convection makes the fluid flow turbulent. Owing to the downward fluid flow caused by gravity and interfacial tension gradient caused by the local high solute concentration for the REDM and TM convection, the shrinking of the droplet top is faster than the other parts, which induces the faster droplet shrinking in the vertical direction than that in the horizontal direction during the mass transfer process.