Mapping the wetting


When it comes to chemistry, surfaces are the places to be. Where two phases meet, interesting stuff is bound to happen. One of the phase interactions that received increased attention during the last few years is the peculiar meeting of a liquid and a solid. One rather interesting phenomenon encountered here is the lotus effect, where water drops roll off a surface because it is not only hydrophobic, but also nanostructured to reduce affinity further. We call this, for obvious reasons, superhydrophobic. Afterwards, surfaces were constructed that are superoleophobic, and amazingly one that is both at the same time. This tunable property of a liquid-solid interface is called wettability, and it is probably the prototypal liquid-solid interaction.

ResearchBlogging.orgFrom a chemists point of view, however, the concept of wettability has a major flaw: It is entirely macroscopic. You define superhydrophobicity by measuring the contact angle between a drop of water and the surface. On the molecular level the concept of wettability is poorly defined. And that’s not the only example for this problem. There is a general lack of information about molecular and atomic details of liquid-solid interactions, especially when compared to the knowledge to be gained. Just recently a paper about supercooled liquid gold, for example, revealed astonishing details about the way the surface influences the freezing point of a liquid.

Now a novel Atomic Force Microscopy technique sets out to improve our ability to explose these interactions dramatically. A team led by Kislon Voitchovski from MIT devised a method to determine the adhesion energy between liquid and solid by immersing the AFM tip into a thin liquid layer. The adhesion energy manifests itself as a resistance to the re-ordering of the liquid layer closest to the surface, which leads to a change in energy dissipation and hence cantilever frequency. They present a few surfaces mapped by this technique, and the images look quite good. Of course there is nothing spectacular in water adsorbed on mica, but they already tried out DMSO as an alternative solvent and several interesting surfaces, and it looks like this method will eventually work with every conceivable system, apart from probably liquid tungsten.

Curiously the authors claim that unlike other AFM techniques the novel method doesn’t depend on tip sharpness for maximum resolution. Instead they claim the performance depends on the minimum distance between tip and surface: As close as possible without actually interacting. The reason appears to be that damping decreases very quickly with increasing distance from the surface, so only a very small part of the tip contributes to measurement, no matter what the rest of the tip looks like. I find this hard to believe, but it seems to work: The resolution of the images presented in the paper is rather impressive.

However, the MIT press release cites one Arvind Raman, who appears to have similar doubts and mentions other possible mechanisms how the image really forms. I’d like to know what they are.

Voïtchovsky, K., Kuna, J., Contera, S., Tosatti, E., & Stellacci, F. (2010). Direct mapping of the solid–liquid adhesion energy with subnanometre resolution Nature Nanotechnology DOI: 10.1038/NNANO.2010.67

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