  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.
