Evaporation of ethanol-water droplet at different substrate temperatures and compositions

TL;DR

This study experimentally explores how substrate temperature and mixture composition influence the evaporation dynamics of ethanol-water droplets, revealing complex behaviors including multiple evaporation stages, non-monotonic lifetime trends, and interfacial instabilities.

Contribution

It provides new insights into evaporation stages and dynamics of binary droplets at various temperatures, supported by experimental data and comparison with theoretical models.

Findings

Binary droplets exhibit two evaporation stages at room temperature.

Increasing substrate temperature reduces droplet lifetime logarithmically.

Interfacial instability and droplet breakup occur at higher temperatures for some compositions.

Abstract

We experimentally investigate the evaporation dynamics of sessile droplets of a fixed volume consisting of different compositions of ethanol-water binary mixture at different substrate temperatures (T_s). At T_s=25oC, we observe pinned-stage linear evaporation for pure droplets, but a binary droplet undergoes two distinct evaporation stages: an early pinned stage and a later receding stage. In the binary droplet, the more volatile ethanol, evaporates faster leading to a nonlinear trend in the evaporation process at the early stage. The phenomenon observed in the present study at T_s=25oC is similar to that presented by previous researchers at room temperature. More interesting dynamics is observed in the evaporation process of a binary droplet at an elevated substrate temperature. We found that the lifetime of the droplet exhibits a non-monotonic trend with the increase in ethanol concentration in the binary mixture, which {can be attributed to} the non-ideal behaviour of water-ethanol binary mixtures. Increasing T_s decreases the lifetime of the (50\% ethanol + 50 \% water) binary droplet in a logarithmic scale. For this composition, at T_s=60oC, we observed an early spreading stage, an intermediate pinned stage and a late receding stage of evaporation. Unlike T_s=25oC, at the early times of the evaporation process, the contact angle of the droplet of pure water at T_s=60oC is greater than 90. Late stage interfacial instability and even droplet break-up are observed for some (though not all) binary mixture compositions. The evaporation dynamics for different compositions at T_s=60oC exhibit a self-similar trend. Finally, the evaporation rates of pure and binary droplets at different substrate temperatures are compared against a theoretical model developed for pure and binary mixture droplets.