Image courtesy of Charles Black, Antonio Checco, and Atikur Rahman
(Top left) Arrays of tapered-cone and (bottom left) cylindrical nanostructures that create superhydrophobic or water-repellant surfaces. Air pockets between the structures give rise to the hydrophobic properties. (Right) High-speed photographs of a falling water droplet on a nanostructured surface (top) before, (middle) during, and (bottom) after impact.
Conventional superhydrophobic coatings that repel liquids by trapping air inside microscopic surface pockets tend to lose their properties when liquids are forced into those pockets. In this work, extremely water-repellant or superhydrophobic surfaces were fabricated that can withstand pressures that are 10 times greater than the average pressure a surface would experience resting in a room. The surfaces resist the infiltration of liquid into the nanoscale pockets.
The extent to which nanometer-size textured, superhydrophobic coatings can withstand elevated pressures is largely determined by the geometry of the texturing. This work shows that by careful tuning of the nanoscale geometry, substantial gains in the durability and applicability of these structures for solar panels, highly robust, self-healing coatings, and anti-icing applications could be realized.
Superhydrophobic coatings repel liquids by trapping air inside microscopic surface textures. However, the resulting composite interface is prone to collapse under external pressure. Nanometer-size textures should facilitate more resilient coatings owing to geometry and confinement effects at the nanoscale. This study uses in situ x-ray diffraction to investigate the extent to which the superhydrophobic state in arrays of ~20 nanometer-wide silicon textures with cylindrical, conical, and linear features persists under pressure. The research revealed that the upper bounds of the superhydrophobic state are reached when the liquid pressure is raised above a critical value, which depends on texture shape and size. This infiltration is modeled quantitatively by accounting for the actual geometry of the texture and macroscopic capillary theory. Another important finding is that the infiltration is irreversible for all but the conical surface textures, which exhibit a spontaneous, partial reappearance of the trapped gas phase upon liquid depressurization. This phenomenon appears to be influenced by the kinetics of gas-liquid exchange. These results have profound implications for the understanding and the design of nanosized multiphase (liquid/vapor) systems, including more effective superhydrophobic coatings.
Charles T. Black
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY
This research is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences in the Materials Sciences and Engineering Division and at the Center for Functional Nanomaterials under Contract No. DE AC02 98CH10886. Financial support is acknowledged from the European Union via 7th Framework International Program Research Staff Exchange Scheme Marie-Curie Grant No. PIRSES-GA-2010-269181.
A. Checco, B.M. Ocko, A. Rahman, C.T. Black, M. Tasinkevych, A. Giacomello, and S. Dietrich, “Collapse and Reversibility of the Superhydrophobic State on Nanotextured Surfaces.” Physical Review Letters 112, 216101 (2014).
Brookhaven National Laboratory Article
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