  Condensing atmospheric water vapor on surfaces is a sustainable approach to
potentially address the potable water crisis. However, despite extensive
research, a key question remains: what is the physical mechanism governing the
condensation from humid air and how significantly does it differ from pure
steam condensation? The answer may help define an optimal combination of the
mode and mechanism of condensation as well as the surface wettability for best
possible water harvesting efficacy. Here we show that this lack of clarity is
due to the differences in heat transfer characteristics during condensation
from pure vapor and humid air environments. Specifically, during condensation
from humid air, the thermal resistance across the condensate is non-dominant
and the energy transfer is controlled by vapor diffusion and condensate
drainage. This leads to filmwise condensation on superhydrophilic surfaces,
offering the highest water collection efficiency. To demonstrate this, we
measured condensation rate on different sets of superhydrophilic and
superhydrophobic surfaces in a wide degree of subcooling (10 - 26 C) and
humidity-ratio differences (5 - 45 g/kg of dry air). The resulting condensation
rate is enhanced by 57 - 333 % on the superhydrophilic surfaces as compared to
the superhydrophobic ones. The findings of this study challenges the nearly
century-old scientific ambiguity about the mechanism of vapor condensation from
humid air. Our findings will lead to the design of efficient atmospheric water
harvesting systems.
