27 July 2014

New model settles long-standing debate

Substances like water can have two liquid phases

twee vloeibare fasen

Physicists of Utrecht University and their Italian colleagues have developed a model for water and similar molecules with which the molecules' 'unnatural behaviour' can be studied. This has settled a long-standing debate about the question of whether such substances can indeed have two liquid phases. The answer is an unequivocal yes, and it can be proven with an experimental model. The researchers' findings have been published in Nature Physics of July 27th.  

"The relevance of our model is that it gives insight into the generally applicable principles of physics that explain the unnatural behaviour of substances such as water, silica and silicon", says Dr Laura Filion of Utrecht University. Water, for instance, expands when you cool it down to below 4°C, while 'normal substances' only shrink at lower temperatures. 

Two liquid phases?

Extremely clean water, without a single dust particle, can remain liquid even under 'supercooling' conditions such as minus 10°C. In 1992, American physicist Gene Stanley suggested that this super-cooled water can occur in two phases. Under normal pressure, there is the normal, 'light' variant, but a unique, 'heavy' variation occurs under extremely high pressure. It is still liquid water, only with a much higher density, as the molecules are in closer proximity to each other.

DNA strands

However, researchers have never been able to prove the existence of these phases with experiments. Nor have theoretical models given a decisive answer. Dr Laura Filion of Utrecht University and her colleagues from Rome therefore wanted to create a simple computer model for water-like substances that could also be realized experimentally. They were inspired by recent experiments on DNA strands.

Transition from 'light' to 'heavy' must occur

Their model proves that such a transition of a 'light' fluid to a 'heavy' fluid is a general occurrence in substances with the molecular structure of water, as long as the links between particles are long and flexible enough. The model also shows other 'deviating' behaviours of water, such as the remarkable changes in density around its freezing point.

Experimental research now possible

Most importantly, it is interesting that the model shows under which conditions the light and heavy variants of the liquid phase can co-exist stably. "This shows that we can really conduct experimental research of substances of water with a model of DNA strands, for instance", explains Filion. "This opens doors to new knowledge about substances like water and silica that that are very important to people. It also gives new insights that can be used in the experimental development of new materials."

Publication

Erasing no-man’s land by thermodynamically stabilizing the liquid-liquid transition in tetrahedral particles
Frank Smallenburg, Laura Filion*, Francesco Sciortino
Nature Physics, July 27th 2014, DOI 10.1038/nphys3030

*Debye Institute for Nanomaterials Science, Utrecht University

This research was co-funded by the European Research Council, the Dutch Sector Plan Physics and Chemistry and an NWO-Veni grant.

More information

Monica van der Garde, Press Officer of the Faculty of Science, m.vandergarde@uu.nl +31 6 13 66 14 38.