Droplet Nucleation on a Well-Defined Hydrophilic–Hydrophobic
Surface of 10 nm Order Resolution
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Abstract
Water condensation on a hybrid hydrophilic–hydrophobic
surface
was investigated to reveal nucleation mechanisms at the microscale.
Focused ion beam (FIB) irradiation was used to change the wettability
of the hydrophobic surface with 10 nm order spatial resolution. Condensation
experiments were conducted using environmental scanning electron microscopy;
droplets, with a minimum diameter of 800 nm, lined up on the FIB-irradiated
hydrophilic lines. The heterogeneous nucleation theory was extended
to consider the water molecules attracted to the hydrophilic area,
thereby enabling explanation of the nucleation mechanism under unsaturated
conditions. Our results showed that the effective surface coverage
of the water molecules on the hydrophilic region was 0.1–1.1
at 0.0 °C and 560 Pa and was dependent on the width of the FIB-irradiated
hydrophilic lines and hydrophobic area. The droplet nucleation mechanism
unveiled in this work would enable the design of new surfaces with
enhanced dropwise condensation heat transfer