thermal capacity and very high surface tension .Up ... Conventionally, non-stick surfaces are made by ... repellant surfaces of lotus leaves and insect wings.
RESEARCH NEWS
Bubbling up water repellence SURFACE
Nanoscopic air bubbles prevent water from wetting a nanopatterned superhydrophobic surface, 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 waterrepellant 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
Nanobubbles.
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.” David Bradley
community as their theory goes against the prevailing theory of water. They posit that water has a dual structure where as most physicists believe water placed under certain extreme conditions may separate into two different structures but that it resumes a single structure under normal conditions. Anders Nilsson says there are several aspects of their research that makes it pioneering. For instance, they have developed different forms of X-ray emission spectroscopy and X-ray experiments to get new and unique insight into the complexities of water. “We show that the established understanding of water as a continuous dispersion around primarily ice-like tetrahedral structures bound to four other molecules in a certain disarray doesn’t fit with our experiment,” says Nilsson. The researchers claim, instead, to have identified two peaks in the spectrum of emitted X-rays which suggests that there could be two separate structures in water. Despite attracting considerable attention from the scientific community, Pettersson and Nilsson’s
theory is still open to considerable debate but may
Why is water so weird? CHARACTERIZATION The Strangest Liquid: Why water is so weird. Stockholm University’s Lars Pettersson, and Stanford University’s professor Anders Nilsson, a guest researcher at Stockholm, have developed a theory that may change the way we understand the microscopic construction of water. [Huang, et al., PNAS (2009), doi:10.1073/pnas.0904743106]. Pettersson and Nilsson’s work challenges our hitherto understanding of the weird and wonderful properties of water. Water is unique as a liquid in that it already has 67 known anomalies compared to normal liquids. For example, water has a higher density at 4C, an unusual thermal capacity and very high surface tension .Up until fairly recently there hasn’t been a comprehensive explanation for these anomalies; however, Anders Nilsson and Lars Petterson published an article in 2004 that claimed to identify that water has a “dual structure”. Pettersson and Nilsson’s results have prompted considerable discussion amongst the scientific
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well lead to a better understanding of how drugs and proteins interact with water molecules within the body, enabling researchers to develop more effective medicines. An increased understanding of the microscopic structure of water could also lead to the development of better membranes for water purification. “One of the effects of global warming that is often overlooked is the shortage of drinking water in the future,” says Anders Nilsson. “New desalination and purification techniques are going to have enormous significance in guaranteeing pure water for the world’s population. To develop these we need a solid understanding of water at a molecular level and how narrow pores in the water desalination membrane, affect the structure.” For Professor Pettersson striving to understand the physycial chemistry of water remains a challenge.
Jonathan Agbenyega
