Non-stick surfaces are important to many areas of technology, from drag reduction to anti-icing agents. These surfaces are usually created by applying coatings, such as Teflon, to smooth surfaces. But recently, scientists have realized that a bit of texture can help. By incorporating topographical features on surfaces, they've created extremely water-repellant materials.
This effect, called “superhydrophobicity,” occurs when air bubbles remain trapped in the textured surfaces, drastically reducing the area of liquid in contact with the solid. As a result, water is forced to ball up into pearl-shaped drops, which are weakly connected to the surface and can readily roll off.
To get the first look at nanobubbles on a superhydrophobic surface, the scientists created a regular array of more than a trillion nano-cavities on an otherwise flat surface, and then coated it with a wax-like surfactant at Brookhaven's Center for Functional Nanomaterials.
This coated, nanoscale-textured surface was much more water repellant than the flat surface alone, suggesting the existence of nanobubbles. To prove that these ultra-small bubbles were present, the researchers took transmission small-angle x-ray scattering measurements at NSLS beamline X6B.
They found that the cavities were mostly filled with air, with water penetrating only about 5 to 10 nanometers into the cavities, independent of their depth. This provides the first direct evidence of the morphology of such small bubbles.
The researchers’ results also show that the bubbles are only about 10 nanometers in size — about 10,000 times smaller than the width of a single human hair — and have nearly flat tops.
In contrast to materials with larger, micrometer-sized bubbles (which have rounded tops), the surfaces fabricated by the Brookhaven-led team may exhibit more stable superhydrophobic properties.
The research could lead to a new class of non-stick materials for a range of applications, including improved-efficiency power plants, speedier boats, and surfaces that are resistant to contamination by germs.
A. Checco, T. Hofmann, E. DiMasi, C.T. Black, B.M. Ocko, "Morphology of Air Nanobubbles Trapped at Hydrophobic Nanopatterned Surfaces," Nano Lett., 10(4), 1354 (2010).
Top: In this picture, the central image is the optical profile of a
water drop placed on "nanopitted" silicon; the right image is a scanning electron micrograph
of the nanocavities; and the left image is a cartoon illustrating the nanobubbles' shape
as inferred from x-ray measurements.
Bottom: Brookhaven physicist Antonio Checco