We
report the surface-energy-dependent wetting transition characteristics
of an evaporating water droplet on surface-energy-controlled microcavity
structures with functional nanocoatings. The droplet wetting scenarios
were categorized into four types depending on the synergistic effect
of surface energy and pattern size. The silicon (Si) microcavity surfaces
(γ
Si = 69.8 mJ/m2) and
the polytetrafluoroethylene (PTFE)-coated microcavity surfaces (γ
PTFE = 15.0 mJ/m2) displayed
stable Wenzel and Cassie wetting states, respectively, irrespective
of time. In contrast, diamond-like carbon (DLC)-coated (γ
DLC = 55.5 mJ/m2) and fluorinated diamond-like
carbon (FDLC)-coated (γ
FDLC = 36.2
mJ/m2) surfaces demonstrated a time-dependent transition
of wetting states. In particular, the DLC-coated surface showed random
filling of microcavities at the earlier time point, while the FDLC-coated
surface displayed directional filling of microcavities at the late
stage of drop evaporation. Such dynamic wetting scenarios based on
surface energy, in particular, the random and directional wetting
transitions related to surface energy of nanocoatings have not been
explored previously. Furthermore, the microscopic role of nanocoating
in the wetting scenarios was analyzed by monitoring the time-dependent
deformation and movement of the air–water interface (AWI) at
individual cavities using the fluorescence interference-contrast (FLIC)
technique. A coating-dependent depinning mechanism of the AWI was
responsible for variable filling of cavities leading to time-dependent
wetting scenarios. A capillary wetting model was used to relate this
depinning event to the evaporation-induced internal flow within the
droplet. Interestingly, FLIC analysis revealed that a hydrophilic
nanocoating can induce microscopic hydrophobicity near the cavity
edges leading to delayed and variable cavity filling. The surface
energy-dependent classification of the wetting scenarios may help
the design of novel evaporation-assisted thermodynamic and mass-transfer
processes.
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