How superhydrophobicity breaks down

A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes wi...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2013-02, Vol.110 (9), p.3254-3258
Main Authors: Papadopoulos, Periklis, Mammen, Lena, Deng, Xu, Vollmer, Doris, Butt, Hans-Jürgen
Format: Article
Language:English
Subjects:
Air pressure
Coatings
Curvature
Evaporation
Fluorescence
Hydrophobic surfaces
Hysteresis
Liquids
Microscopy
Optical reflection
Physical Sciences
Pressure
Substrates
Visualization
Wetting
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Summary:A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar’s circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water–air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1218673110