  This paper reports the dynamic wetting behavior and heat transfer
characteristics for impinging droplets on heated bi-phobic surfaces
(superhydrophobic matrix with hydrophobic spots). A non-patterned
superhydrophobic and a sticky hydrophobic surface acted as control wettability
surfaces. As expected, differences in wetting and heat transfer dynamics were
noticeable for all surfaces, with the most pronounced variation during the
receding phase. During spreading, inertia from the impact dominated the droplet
dynamics and heat transfer was dominated by convection at the contact line and
internal flow. As contact line velocities decreased over time, evaporative
cooling at the contact line gained importance, especially for the bi-phobic
surfaces, where liquid remained trapped on the hydrophobic spots during
receding. These satellite droplets increased the contact area and contact line
length, and assisted heat transfer and substrate cooling after lift-off of the
main droplet. Compared with the hydrophobic surface, the contribution of the
contact line heat transfer increased by 17 to 27% on the bi-phobic surfaces,
depending on the location of impact relative to the hydrophobic spots.
Nonetheless, the bi-phobic surfaces had a lower total thermal energy transfer.
However, compared with the plain superhydrophobic surface, heat transfer was
enhanced by 33% to 46% by patterning the surface. Depending on the application,
a trade-off exists between the different surfaces: the sticky hydrophobic
surface provides the best cooling efficiency, yet is prone to flooding, whereas
the superhydrophobic surface repels the liquid, but has poor cooling
efficiency. The bi-phobic surfaces provide a middle path with reasonable
cooling effectiveness and low flooding probability.
