Heat flux distribution beneath evaporating hydrophilic and superhydrophobic droplets

•Local heat transfer to a hydrophilic and superhydrophobic evaporating droplets.•Infrared thermography and optical imaging employed in characterising heat flux.•Hydrophilic droplet has a greater total power dissipation.•Superhydrophobic droplet has a greater average heat flux.•Average droplet heat flux proportional to the contact line length density. An experimental investigation of the heat and mass transfer to an evaporating hydrophilic water droplet using thin-foil thermography and droplet shape analysis is reported. These results have been compared with that of a superhydrophobic evaporating droplet. The hydrophilic droplet initially evaporated with a pinned contact line before unpinning and evaporating with a receding contact line. The largest heat flux is observed at the contact line region for both droplets. The hydrophilic droplet evaporated 34% faster than its superhydrophobic counterpart due to its greater contact line length, liquid-gas interface temperature and solid-liquid surface area for the majority of its evaporation. In general, the hydrophilic droplet dissipated a greater total power due to its larger contact line length and solid-liquid surface area, while the superhydrophobic droplet had a greater average heat flux due to its larger contact line length density for the majority of its evaporation time. The average heat flux to the evaporating droplets was demonstrated to vary as a linear function of the contact line length density.