  This paper presents an experimental study on thermal transport to single
water droplets evaporating on heated bi-phobic surfaces consisting of a
superhydrophobic matrix with a circular hydrophobic pattern with strong contact
line pinning. A single water droplet of 8 ul volume is placed on a preheated
surface and allowed to evaporate in an open laboratory environment. We
investigate the influence of substrate orientation (horizontal and vertical) on
evaporation dynamics. Using optical and infrared imaging, we report droplet
fluid dynamics and heat transfer characteristics of the evaporating droplet.
Overall, evaporation is more efficient on the vertical surface, exhibiting
higher total heat transfer rates and up to 10% shorter evaporation times.
Counterintuitively, on the vertical surface, the substrate-droplet interfacial
heat flux was higher near the lower contact line than in the upper region,
despite a high contact angle and an expected wedge effect at the bottom. At the
same time, the temperature is colder in the lower part of the droplet. We
attribute this apparent anomaly to the competition between sensible heating and
evaporation, and a modified convective flow signature (both within the droplet
and the gas phase) compared to a horizontal surface. We also show that the
thermal signature becomes uniform once the contact angles at the upper and
lower contact lines become equal towards the end of the evaporation process.
Insights from this work can guide the design of spray cooling devices, or be
used to alter particle deposition patterns during evaporation-based fabrication
techniques and ink-jet printing.
