Mass transfer enhancement at deformable droplets due to Marangoni convection

One of the key design parameters in liquid/liquid extraction is the mass transfer coefficient. A complex list of parameters including fluid dynamics, drop size distribution, chemical properties of the involved species, local interfacial instabilities (Marangoni convection) is required in order to determine the transient evolution of the mass transfer coefficient. The influence of Marangoni convection on single drop mass transfer cannot yet be described in an analytical manner, and empirical correlations available in literature fail to predict the mass transfer process. In the present study, experimental investigations on deformable single droplets in the toluene/acetone/water system are presented which shows strong interfacial instabilities. Parameters varied are the drop diameter, the initial solute concentration and the mass transfer direction. Experimental results are compared with the well-known models by Kronig and Brink and Handlos and Baron. The Kronig and Brink model cannot describe Marangoni dominated systems, but comparisons reveal the influence of deformation on the mass transfer enhancement. In contrary, with a slight modification to the Handlos and Baron model, the mean droplet concentration of the transferred component was successfully modelled as a function of Fourier number.