Evaporation induced demixing in binary sessile drops

Abstract The interplay between evaporation and liquid–liquid phase separation (demixing) in binary sessile drops of partially miscible liquids is investigated. To determine the onset of the demixing phenomenon, a simple model is developed, which predicts a considerable temperature reduction (∼20 °C) in the mixture due to evaporative cooling. Temperature reduction alongside with the change of composition lead to demixing in the mixtures. The model explains why a mixture at room temperature is able to demix, whilst the demixing upper critical temperature is at 6.3 °C. Five stages of the process are identified and explained. For the cases studied here, once the demixing begins through nucleation, a growing fingering pattern is formed at the contact line. The length of the fingers and the final area of deposition increase with the initial concentration. Experimental tests were performed using a double telecentric setup.