Everything is dirty, and getting things clean is often an uphill climb that not even Sisyphus would perform. But clean surfaces are important. While researchers have found substances that can be used to make self-cleaning surfaces, they aren't really clear on the mechanisms behind them. To discover more, a research team had to prepare the world’s cleanest water droplet.
The international team investigated the self-cleaning properties of titanium dioxide, an extremely versatile substance that is used in mirrors that don’t fog up, making paper whiter, sunscreen, and in protecting food. As reported in Science, the team discovered that titanium dioxide forms a single layer of acids on its surface and these are likely crucial to how it cleans itself.
Still, the researchers needed to understand more about these acids and how they form on the titanium dioxide. Water was believed to be a crucial player in the process, as titanium dioxide can both attract and repel water depending on the amount of light they are exposed to. And that’s when the purest droplet of water came into play.
If the goal is to understand cleanliness, you can’t introduce an impurity into the system. So they had to clean titanium dioxide by scrubbing it all the way to the atomic scale. To do this, they created an uncontaminated water droplet by making sure that it had never been in contact with air. The whole experiment was conducted in a vacuum, where water is mostly in vapor form. First, they froze water on a metal, finger-like structure at -140°C (-220°F). The metal finger was then heated and the ultraclean water dropped on the titanium dioxide.
Yet studying the surface after it came in contact with water didn’t show any peculiar molecules. The use of soda water, another potential culprit, also didn't produce the layer observed in previous observations. Only when the sample was exposed to air did the curious acids appear.
These molecules, acetic acid (found in vinegar) and formic acid, are formed by plants and found in small traces in the air. While they are only found in minute amounts in the air (a few molecules per billion), it is enough for them to stick to titanium oxide.
"This result shows us how careful we need to be when conducting experiments of this kind," senior author Ulrike Diebold, from Vienna University of Technology, said in a statement. "Even tiny traces in the air, which could actually be considered insignificant, are sometimes decisive."