LAUSR

Abstract The circulation inside various evaporating methanol – water drops in an inert and quiescent environment, is studied. The circulation is visualized using laser. An oscillatory internal circulation resulting from the opposing effects of thermal and solutal Marangoni convection is observed. The speeding up and slowing down of the convection roll (as visualized from the path-lines of the seeding particles) in the drop interior and around the centre, is termed oscillation. The circulation demonstrates novel feature for various methanol – water drops. The frequency of oscillation increases up to a volume fraction of 0.5 of methanol, and reduces afterwards. From the variation of Marangoni and Rayleigh numbers with volume fraction, it is demonstrated that the variation in frequency results from the complex interaction between the thermal Marangoni, solutal Marangoni, and solutal Rayleigh convection. The opposing effect of the thermal Marangoni and solutal Marangoni convection tries to enhance the oscillatory internal circulation, whereas, the solutal Rayleigh convection makes the circulation steadier. Hence, the frequency increases with the volume fraction initially (up to 0.5), but reduces afterwards. The characteristics of the circulation predominantly depend on the solutal Marangoni convection.