Written by Carmelo Amalfi Friday, 13 June 2008 15:38
Project leader Klaas Wynne, professor of chemical physics at the University of Strathclyde in Glasgow, said debate had raged for decades over the scientific reasons behind why substances such as honey are so sticky.
The consensus had been that sugar and salt molecules in water would bind to the water molecules and thereby alter the structure of water, he explains at http://bcp.phys.strath.ac.uk/pr/08pr1.php.
“When salts or sugars are added to water, the resulting mixture is very ‘sticky’ and flows with great difficulty or, to use the scientific term, it has a high viscosity.
“This effect is probably most familiar in mixtures of sugars and water such as in syrup and honey.”
The study, published in the Journal of Chemical Physics, used complimentary techniques to make it possible, for the first time, to understand the motions of water molecules in solution on time scales of about one-trillionth of a second or 1/1,000,000,000,000 seconds.
The team studied solutions of metal salts, rather than those of sugars, because salt ions are simpler, appearing as smooth spheres rather than knobbly molecules.
“The metal ions we have studied hold on tightly to a few neighbouring water molecules forming tacky spheres that randomly jam together at high enough concentration like cars in a traffic jam,” Professor Wynne explained.
This type of jamming has been observed in the movement of desert dunes and flow of grains in a silo.
Murdoch’s Associate Professor Glenn Hefter said the behaviour of concentrated electrolyte solutions has been a mystery for a long time.
"Such solutions are very commonly encountered in, for example, sea water, blood and other body fluids," he told ScienceNetwork WA.
"One of the strange things about these electrolyte solutions and other concentrated non-electrolyte solutions, such as honey, is that they remain so much like water, even though there is still so much dissolved material."
He said the research involved a combination of two techniques to show there is a separation of the two main types of motion of water molecules in these solutions - the translational and rotational motions.
Water is free to rotate, but the translational motions (movement in a straight line) become blocked by the presence of the dissolved materials or solutes in its path.
So when the solutions become too highly concentrated translational movement of the water molecules becomes "jammed", just like on the freeway when too many cars get on it at the same time.
"So although the molecules might be free to move in one direction (rotationally), they cannot 'flow' along in one direction," he explained.
"This is analogous to the situation of cars along a freeway - you can change lanes all you like, but if the movement is restricted in the direction you want to go you will not get there any faster.
He said substances such as honey (a small amount of water with a high amount of dissolved substances, mostly sugars) will always remain thick, sticky and viscous, as they experience this phenomenon.
"One of the interesting things about our research is that as the number of dissolved particles increases, the flow becomes increasingly difficult; there is no critical point that stops the flow," he said.
"This has clear implications for systems such as the human body as it shows, for example, that the likelihood of a blockage will increase gradually as the concentration of dissolved particles increases."
