Internal circulation in a single droplet evaporating in a closed chamber

Evaporation of a single droplet of a pure liquid in a confined chamber under atmospheric ambient condition is expected to be purely controlled by the rate of diffusion of the vapor into the surrounding. But, it is seen from the experimental results presented in this paper that for several liquids the process is faster than a theoretical estimate of the diffusion-driven process. It is seen from the visualization inside the droplet that these liquids exhibit intense internal circulation during evaporation. From a scaling analysis the temperature variations within the droplet due to surface traction and buoyancy-driven convection during evaporation is estimated. Marangoni and Rayleigh numbers are also obtained from these estimates. The values of these numbers indicate that Marangoni convection aided by buoyancy is probably the reason for the internal circulation induced within the droplet. The average velocity of the internal circulation is measured and is found to compare well with the velocity scale for Marangoni convection.