Evaluations of the preferential evaporation of binary droplet by rainbow technique and simulation

The evaporation behavior of the ethanol and heptane binary droplets was investigated using both, experimental and simulation methods. Droplets with three compositions of ethanol/heptane and three initial diameters were tested at ambient temperatures of 373 and 473 K, respectively. The preferential evaporation behavior of the individual components inside the binary droplet was obtained and confirmed by both, the numerical and experimental results. The heavier component undergoes prior evaporation when the lighter component dominates the liquid mixture. A systematic comparison between the experimental measurements and simulation results was made with respect to the normalized diameter and refractive index. The spatial and temporal dependences of the refractive index on the temperature and composition were obtained. The composition of the liquid phase has a decisive effect on the variation of refractive index near the droplet surface. Temperature controls the refractive index during the initial stage. Furthermore, the experimental results validate the accuracy of the non-ideal equilibrium model for predicting the diameter, temperature, and concentration of binary droplets for the first time. A linear fitting between the refractive index gradient in the liquid phase and the deviations of the equivalent refractive index from the refractive index at the droplet center has been proposed.