May 05, 2010
Contact: Tobias Schülli – SP2M

What is the connection between a cloud and a gold droplet?  Answer: Super-cooling, the property of a substance in a liquid state under conditions where it should be solid. Researchers at SP2M have for the first time observed a pentagonal arrangement of atoms at the liquid-solid boundary, which is highly unfavorable to crystallization and thus to solidification.


Under extremely clean conditions, liquids do not freeze at their freezing point. High altitude clouds are a good example: they contain tiny water droplets that do not turn into ice due to the purity of the air. This is due to the fact that the arrangement of atoms is chaotic, and that there is no crystalline germ to trigger the crystallization process. Currently, theoreticians postulate that the internal structure of super-cooled liquids could be incompatible with crystallization. In this case, theoretical models suggest that atoms in the liquid are organized in pentagons. In order to form a crystal, one needs a structure that can be repeated periodically and pave the entire space. This is not possible with pentagons.


We have studied gold droplets on various silicon surfaces. After heating, they melt and form a liquid with the composition of the eutectic alloy Au81Si19. By cooling these droplets we have discovered that their solidification temperature depended upon the symmetry of the studied silicon surface. Super-cooling is observed for the densest surface. Our grazing incidence X-ray diffraction experiments at ESRF have corroborated numerical simulations showing that atoms at the boundary between the solid and the droplet arrange themselves in a pentagonal symmetry. This discovery may have important technological applications concerning the growth of nanowires for which gold droplets are used as catalysts.


Further reading: T. U. Schülli et al., Nature 464 (2010) 1174


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