Why Is Ice So Slippery? Mysteries Of The "Invisible" Ice Surface

April 06, 1998

For many years scientists have tried to understand the unique properties of ice in terms of the behavior of the molecules in the topmost layer. However, despite extensive studies the exact structure and dynamical motion of the individual water molecules at the ice surface have remained elusive. An international team of physicists (J. Braun, A. Glebov, A. P. Graham, A. Menzel) in the group of Peter Toennies at the Max Planck Institute for Fluid Dynamics in Göttingen have used the scattering of very low-energy He atoms for the successful analysis of the structural arrangement of water molecules on the ice surface and have also gained direct information on their vibrational motion. The results of these experiments, published in the March 23 issue of the Physical Review Letters [80, 2638 (1998)], indicate that the molecules are surprisingly mobile which explains many peculiarities in the interactions of ice with its environment.

Why do solid ice crystals have a melted surface-layer at temperatures far below the bulk melting point 0°C, that allows us to ski, skate and slide so easily; Why do two pieces of ice, when put together, adhere and become one; Why are different molecules in the earth stratosphere easily trapped on the surface of ice particles, where they can react, with consequences such as depletion of the ozone layer? This wide variety of intriguing questions have made ice one of the most frequently studied materials. However, until now, no definite answers to these and many other questions have been forthcoming since all of the attempts to gain information on the microscopic structure of a single crystal ice surface have failed. Very recently, even the powerful method of electron diffraction, routinely used in surface structure analysis, failed to provide any clear evidence on the structural arrangement of the topmost layer of ice. The group of scientists from the Lawrence Berkeley National Laboratory, Free University in Amsterdam, and the University of Pierre and Marie Curie in Paris [Surface Science 381, 190 (1997); also see report of Charles Seife in Science 274, 2012 (1996)] have suggested, on the basis of theoretical simulations, that the uppermost water molecules vibrate so strongly that a coherent diffraction pattern cannot be observed.

In the attempt to resolve this problem the researchers in Göttingen have employed low-energy helium atom scattering. This technique has the advantage of being completely nondestructive and exclusively sensitive to the topmost layer of crystals. Since the (111) surface of platinum has nearly the same lattice spacing as ice, it was used as a template on which single crystal ice films of 10-100 nm thickness were grown. Only after cooling the surface to 30 K was it possible to observe a sharp intense series of diffraction peaks. These not only provide information on the lattice spacing and arrangement of the first layer molecules but also indicate at least a partial alignment of the hydrogen atoms (ferroelectric ordering) at the surface.

A further advantage of the He atom scattering technique is that with the same equipment high-resolution time-of-flight energy loss and gain spectra can be measured. These spectra provide information on the frequencies and wave-lengths of the collective vibrations (phonons) at the surface. As the crystal was again cooled down to 30 K, a very intense inelastic peak emerged from a strong multiphonon background. This intense inelastic peak was simulated with a theoretical model which allowed its assignment to a special very large amplitude in-plane shearing motion of the surface molecules. At higher temperatures, this motion becomes increasingly enhanced leading to a high density "phonon bath" and ultimately individual molecules will break away from their original sites. This explains the liquid-like topmost layer as well as the difficulties experienced in the electron diffraction experiments.

This vibrational disorder at the ice surface also explains >

Transfer interrupted!

ressed together. The H2O molecules at the ice crystal surface form hydrogen bonds with those of another ice surface when two crystals are brought in contact, thus, increasing their coordination. This results in a stiffening of the soft surface vibrations, making the interface solid. In addition, the high rate of accommodation of molecules on the surface of ice particles in the stratosphere can also be understood in terms of the facile energy transfer of the molecules with the phonon bath available at the surface. The situation is rather similar to the ping-pong ball dropped onto a concrete floor covered with a soft rubber carpet. Without the carpet the ball would bounce back while the soft rubber overlayer allows the ball to lose all its translational energy permitting it to be accommodated on the surface. Many of the other fascinating properties of ice can also be explained in terms of the enhanced vibrations of water molecules at the surface.-end-


Related Water Molecules Articles from Brightsurf:

Transport of water to mars' upper atmosphere dominates planet's water loss to space
Instead of its scarce atmospheric water being confined in Mars' lower atmosphere, a new study finds evidence that water on Mars is directly transported to the upper atmosphere, where it is converted to atomic hydrogen that escapes to space.

Water striders learn from experience how to jump up safely from water surface
Water striders jump upwards from the water surface without breaking it.

Chemistry's Feng Lin Lab is splitting water molecules for a renewable energy future
Feng Lin, an assistant professor of chemistry in the Virginia Tech College of Science, is focusing on energy storage and conversion research.

How a crystalline sponge sheds water molecules
How does water leave a sponge? In a new study, scientists answer this question in detail for a porous, crystalline material made from metal and organic building blocks -- specifically, cobalt(II) sulfate heptahydrate, 5-aminoisophthalic acid and 4,4'-bipyridine.

Water molecules are gold for nanocatalysis
Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals.

Liquid water is more than just H2O molecules
Skoltech scientists in collaboration with researchers from the University of Stuttgart showed that the concentration of short-lived ions (H3O+ and OH-) in pure liquid water is much higher than that assumed to evaluate the pH, hence significantly changing our understanding of the dynamical structure of water.

'Pregnancy test for water' delivers fast, easy results on water quality
A new platform technology can assess water safety and quality with just a single drop and a few minutes.

Water molecules dance in three
An international team of scientists has been able to shed new light on the properties of water at the molecular level.

Unique structural fluctuations at ice surface promote autoionization of water molecules
Hydrated protons at the surface of water ice are of fundamental importance in a variety of physicochemical phenomena on earth and in the universe.

Researchers create new tools to monitor water quality, measure water insecurity
A wife-husband team will present both high-tech and low-tech solutions for improving water security at this year's American Association for the Advancement of Science (AAAS) annual meeting in Seattle on Sunday, Feb.

Read More: Water Molecules News and Water Molecules Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.