Gurrala, Pradeep and Katre, Pallavi and Banerjee, Sayak and Balusamy, Saravanan and et al, .
(2019)
Evaporation of ethanol-water droplet at different substrate temperatures and compositions.
arXiv.org.
Abstract
We experimentally investigate the evaporation dynamics of sessile droplets of a fixed volume (5µl) consisting of different compositions of ethanol-water binary mixture at different substrate temperatures (Ts). The experiments are conducted on a cellulose-acetate substrate placed on a customised goniometer. The surface roughness studied by an atomic-force-microscopy (AFM) and the micro-scale images taken using a scanning-electron-microscope (SEM) show that the substrate considered in the present study is stable even at high temperatures. It is well known that in a binary mixture, the differential rates of evaporation of the individual components result in a complex evaporation process. We found that the complexity is even more pronounced at elevated temperatures. In order to compare the dynamics for different compositions and at different substrate temperatures, it is necessary to perform systematic experiments at a fixed condition. Such an attempt is made in the present study. At Ts = 25◦C, we observe pinned-stage linear evaporation for pure droplets, but a binary (50% ethanol + 50 % water) 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 Ts = 25◦C 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 (Ts = 60◦C). 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 Ts decreases the lifetime of the (50% ethanol + 50 % water) binary droplet in a logarithmic scale. For this composition, at Ts = 60◦C, we observed an early spreading stage, an intermediate pinned stage and a late receding stage of evaporation. Unlike Ts = 25◦C, at the early times of the evaporation process, the contact angle of the droplet of pure water at Ts = 60◦C is greater than 90◦ (hydrophobic). 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 Ts = 60◦C exhibit a self-similar trend. It is also found that at Ts = 60◦C the normalised volumetric evaporation rate is early constant for the entire evaporation process, indicating that the evaporation dynamics of a binary droplet of a given composition at Ts = 60◦C is equivalent to that of another pure fluid with a higher volatility at room temperature. 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. The model predictions were found to be quite satisfactory for the steady evaporation phase of the droplet lifetimes.
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