  We experimentally investigate the evaporation characteristics of a sessile
ethanol droplet containing Al$_2$O$_3$ and Cu nanoparticles of sizes 25 nm and
75 nm on a heated substrate using shadowgraphy and infrared imaging techniques.
Our results demonstrate that the droplet contact line dynamics resulting from
the presence of various nanoparticles plays a dominant role in the evaporation
process. This is in contrast to the widely-held assumption that the enhanced
evaporation rate observed in sessile nanofluid droplets is due to the higher
thermal conductivity of the added nanoparticles. We observe that even though
the thermal conductivity of Al$_2$O$_3$ is an order of magnitude lower than
that of Cu, droplets containing 25 nm-sized Al$_2$O$_3$ exhibit pinned contact
line dynamics and evaporate much more rapidly than droplets containing Cu
nanoparticles of both sizes and 75 nm Al$_2$O$_3$ nanoparticles that exhibit
stick-slip behaviour. We also found that the droplets with different
nanoparticles display distinct thermal patterns due to the difference in
contact line behaviour, which alters the heat transfer inside the droplets. We
establish this counter-intuitive observation by analysing the temporal
variations of the perimeter, free surface area, and deposition patterns on the
substrate.
