This coated, nanoscale textured surface was much more water repellant than the flat surface alone, suggesting the existence of nanobubbles. However, because the nanoscale is not accessible using ordinary microscopes, little is known about these nanobubbles.
To unambiguously prove that these ultra-small bubbles were present, the Brookhaven team carried out x-ray measurements at the [http://www.nsls.bnl.gov] National Synchrotron Light Source . "By watching how the x-rays diffracted, or bounced off the surface, we are able to image extremely small features and show that the cavities were mostly filled with air," said Brookhaven physicist Elaine DiMasi.
Checco added, "We were surprised that water penetrates only about 5 to 10 nanometers into the cavities an amount corresponding to only 15 to 30 layers of water molecules independent of the depth of the cavities. This provides the first direct evidence of the morphology of such small bubbles."
According to the scientists' observations, the bubbles are only about 10 nanometers in size about ten thousand times smaller than the width of a single human hair. And the team's results conclusively show that these tiny bubbles have nearly flat tops. This is in contrast to larger, micrometer-sized bubbles, which have a more rounded top.
"This flattened configuration is appealing for a range of applications because it is expected to increase hydrodynamic slippage past the nanotextured surface," Checco said. "Moreover, the fact that water hardly penetrates into the nano-textures, even if an external pressure is applied to the liquid, implies that these nanobubbles are very stable."
Therefore, in contrast to materials with larger, micrometer-sized textures, the surfaces fabricated by the Brookhaven team may exhibit more stable superhydrophobic properties.
|Contact: Karen McNulty Walsh|
DOE/Brookhaven National Laboratory