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.

Influence of evaporation on the morphology of a thin film of a partially miscible binary mixture

“…The fluids properties used in the model are those corresponding to our experiments and already described previously in [43] (see section 3 in [43] for more details). They give ≈ 20 and ≈ 0.4 and these two values are used throughout this work.…”

“…The evaporation of thin films has attracted many interests because it commonly happens in natural phenomena and in many industrial processes such as drying of paint films [29][30][31] ink-jet printing [32][33][34], packaging [35,36], heat exchangers [37,38], and fabrication of solar cells [39,40]. When a thin film consists of a binary mixture of partially miscible liquids, either a temperature gradient [41], or a concentration gradient [42], or both as in the case of evaporation [43], can induce demixing. The situation of evaporation, which is analyzed in the present paper, provides an example of phase separation induced by phase change.…”

“…There is limited information in the literature on non-isothermal demixing induced by evaporation, in particular on the path to the unstable zone in the phase diagram. In our previous work [43], we analyzed experimentally and theoretically the evaporation induced demixing in sessile drops of binary mixtures of partially miscible liquids. We proposed a simple model with the aim to analyze the influence of the initial conditions on the possible onset of demixing (but we did not model the demixing phase separation itself).…”

“…The liquids used in this work are n-hexane and 2-(2-Ethoxyethoxy)ethanol, also known as Diethylene Glycol Monoethyl Ether (DGME), used as purchased from Sigma-Aldrich company. The components are partially miscible and there is a considerable difference between their volatility [43], which allows considering that only the n-hexane evaporates. The binodal curve defining the miscibility gap in the phase diagram of the two partially miscible liquids is available in the literature [87].…”

“…Then, the binodal curve, and later the spinodal one, are crossed, which makes the system unstable and allows the beginning of phase separation. Note that some initial conditions (for instance high temperature, see [43] for more details) could prevent the system from crossing these curves. In such situations, phase separation does not occur.…”