Evaporation of sessile drops under combined diffusion and natural convection

Experiments were conducted to investigate the range of applicability of a commonly used assumption for evaporation models of sessile drops, that the transport mechanism that controls the evaporation is vapor diffusion. The evaporation rates of sessile drops of 3-methylpentane, hexane, cyclohexane, and heptane were measured. The radius of the drop contact line was constant during the measurements and drops of radius from 1 mm to 22 mm were studied. It was found that a diffusion-controlled evaporation model underpredicts the evaporation rate from 36% to 80% depending on the drop size. The increase in the evaporation rate was attributed to a second transport mechanism, natural convection of the vapors, and an empirical model was developed for conditions of combined diffusive and convective transport. Over the broad range of volatilities and drop sizes studied, the evaporation rates computed using the combined transport model agree with the measured values with less than 6% root mean square error.