Dynamic wetting and heat transfer during droplet impact on heated bi-phobic wettability-patterned surfaces

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 notable 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. Evaporative cooling was especially important on 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. By patterning the surface, the single-droplet heat transfer was enhanced by 33% to 46% with respect to the plain superhydrophobic surface, depending on the location of impact relative to the hydrophobic spot. Compared to the sticky hydrophobic surface, the bi-phobic surfaces had a lower thermal energy transfer for single droplets. However, the cooling capacity, that is, change in temperature at the moment of impact, decreased very rapidly on the hydrophobic surface due to flooding. Based on the formation of smaller satellite droplets and the removal of the main droplet, the wettability-patterned surfaces showed more promise for the control and improvement of heat transfer rates during successive droplet impact events.