According to research carried out by the US Department of Energy's Brookhaven National Laboratory [Checco et al., Nano Lett. (2010) doi: 10.1021/nl9042246], the discovery could lead to a new class of nanostructured non-stick materials.
Conventionally, non-stick surfaces are made by applying a coating of polytetrafluorethylene, or a related material, to a smooth surface. Recently, however, scientists have looked to the water-repellant surfaces of lotus leaves and insect wings to attempt to emulate their surface structure in order to make superhydrophobic materials.
Non-stick surfaces are important to technologists and engineers for reducing the accumulation of materials on a surface such as ice or bacteria and in reducing drag and friction between the moving parts of machinery.
To catch a glimpse of nanobubbles on a superhydrophobic surface and how they affect wetting the BNL team created a regular array of pits, nano-cavities, on an otherwise flat surface. The pits are made in silicon using a mask obtained by self-assembled polymers, and then coated it with a thin film of wax-like surfactant, octadecyltrichlorosilane. The textured surface could trap nanobubbles which reduce the area available for liquid to contact with the surface. This forces the water to form tiny droplets that only interact weakly with the surface and so quickly roll off, with just the slightest inclination of the surface. “It's the combination of pits and hydrophobic coating that makes the surface superhydrophobic,” explains Checco, “The pitted silicon surface alone is hydrophilic.
The BNL team used X-ray measurements at the National Synchrotron Light Source to image small surface features and to show that the surface pits were filled mostly with air. The study showed that water could penetrate a mere 5 to 10 nanometres into these cavities, a depth of just 15 to 30 layers of water molecules, irrespective of the depth of the cavities.
This is the first direct evidence showing the morphology of such small bubbles that give rise to superhydrophobicity. Their measurements suggest that the bubbles are about 10 nanometres in diameter and have almost flat tops, unlike micrometre-sized bubbles, which are rounded on top.
“This flattened configuration is appealing for a range of applications because it is expected to increase hydrodynamic slippage past the nanotextured surface,” Checco explains. “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.”