posted on 2017-11-27, 00:00authored byHyeongyun Cha, Alex Wu, Moon-Kyung Kim, Kosuke Saigusa, Aihua Liu, Nenad Miljkovic
Water
vapor condensation on hydrophobic surfaces has received much
attention due to its ability to rapidly shed water droplets and enhance
heat transfer, anti-icing, water harvesting, energy harvesting, and
self-cleaning performance. However, the mechanism of heterogeneous
nucleation on hydrophobic surfaces remains poorly understood and is
attributed to defects in the hydrophobic coating exposing the high
surface energy substrate. Here, we observe the formation of high surface
energy nanoscale agglomerates on hydrophobic coatings after condensation/evaporation
cycles in ambient conditions. To investigate the deposition dynamics,
we studied the nanoscale agglomerates as a function of condensation/evaporation
cycles via optical and field emission scanning electron microscopy
(FESEM), microgoniometric contact angle measurements, nucleation statistics,
and energy dispersive X-ray spectroscopy (EDS). The FESEM and EDS
results indicated that the nanoscale agglomerates stem from absorption
of sulfuric acid based aerosol particles inside the droplet and adsorption
of volatile organic compounds such as methanethiol (CH3SH), dimethyl disulfide (CH3SSCH), and dimethyl trisulfide
(CH3SSSCH3) on the liquid–vapor interface
during water vapor condensation, which act as preferential sites for
heterogeneous nucleation after evaporation. The insights gained from
this study elucidate fundamental aspects governing the behavior of
both short- and long-term heterogeneous nucleation on hydrophobic
surfaces, suggest previously unexplored microfabrication and air purification
techniques, and present insights into the challenges facing the development
of durable dropwise condensing surfaces.