The characteristics of Marangoni eruptions of immobilised single drops

The concentration-induced Marangoni effect on single drops is investigated experimentally. The standard chemical system toluene/water/acetone, known for its strong Marangoni instabilities, was chosen for this study. The toluene drops with acetone as solute are immobilised in water by acoustic levitation. The mass transfer from the dispersed to the stagnant continuous phase is observed optically: qualitatively by rainbow schlieren deflectometry and quantitatively by monochrome imaging. The contactless immobilisation allows precise transient mass transfer measurements of weak Marangoni eruptions, which would be suppressed by standard research approaches with sessile drops. Although the influence of standing waves can slightly alter the experimental results, these measurements provide a unique experimental insight. Observations of Marangoni eruptions of & Oslash; 3/5 mm drops occurring at acetone concentrations up to 35.8/60.6 g/L have shown an average periodicity of 11.5/14.4 s, an average duration of 3.7/4.4 s, and a maximum mass transfer enhancement by a single eruption of 5.0/10.6 g/L. Solute loss during the injection process prior to optical volume acquisition ranged from 22.7 to 67.5%. The results are of significant value invalidating numerical investigations of the partially unquantified Marangoni effect.