New Worlds: The very thin line between solids and liquids

Discovery means solid-liquid interfaces have to be viewed in a totally different way from now on.

microscope 88 (photo credit: )
microscope 88
(photo credit: )
Israeli and German researchers who caught the first glimpse of nanometer-scale structures at the boundary between droplets of aluminum and solid sapphire say their discovery means solid-liquid interfaces have to be viewed in a totally different way from now on. The detailed view provides direct evidence that the sapphire's crystal structure induces the liquid aluminum atoms to line up in an orderly fashion, says Prof. Wayne Kaplan of the Technion Institute in Haifa, who led the research along with doctoral student Yaron Kauffmann and colleagues from the Max Planck Institute in Stuttgart. Their work was reported recently on the online version of Science. The findings have fundamental implications for a variety of processes, including lubrication, the growth of thin crystal layers and "wetting" (how well liquids spread over a solid surface). Rethinking the nature of the liquid-solid boundary also has other implications, since so many current processes depend on understanding such phenomena, Kaplan adds. For example, the processes play a key role in building semiconductor chips and other microelectronic devices, soldering materials, and maneuvering liquids through small spaces. Using a special high-resolution transmission electron microscope located at the German institute, the researchers were able to glimpse the dynamics of the liquid-solid interface. These microscopes work by passing a high-voltage beam of electrons through a thin slice of material; the beam scatters due to interactions with atoms in the sample. Kaplan and colleagues used a device on the microscope to heat a thin slice of pure, single crystal sapphire (also known as aluminum oxide) above aluminum's melting point. The combination of heat and electron irradiation knocked oxygen atoms out of the sapphire, causing aluminum atoms to drift to the surface and form liquid droplets. They used images and real-time movies to capture a dynamically evolving interface between the sapphire and the aluminum drops. Due to the interface with the crystal, atoms in the liquid adopted an ordered structure not normally characteristic of liquids. As a result, atoms in the liquid adjacent to the interface have properties differing from both those in the liquid and those in the solid. In addition, due to the presence of a small amount of oxygen in the microscope, oxygen combined with the ordered aluminum atoms within a split second to make the crystal grow layer by layer into the liquid. The researchers were then able to determine the mechanism of crystal growth for sapphire, an important engineering material. "The approach we developed for analysis of the data has not been applied to data from solid-liquid interfaces in the past, and without these efforts the data would only be pretty pictures and videos, and not be convincing to the scientific community," Kaplan adds. The team will continue to work on the liquid-solid interface with the help of a new transmission electron microscope that has a unique, image-correcting lens and extremely high-resolution capabilities that will be installed at the Technion in January. FINGERPRINTS NOT INFALLIBLE Although it is said that no two fingerprints in the world are alike, an Israeli psychologist working at the University of Southampton in England says fingerprint evidence can never be 100%. Dr. Itiel Dror has worked with more than 70 fingerprint experts from the UK, US, Netherlands, Australia and Israel over the past two years to test his theory that mistakes can occur because of the way the human brain processes information. The issue hit the headlines when an American Muslim from Oregon was wrongly identified as one of the Madrid bombers after his prints appeared to match those taken at the crime scene. The findings were presented at the Biometrics 2005 conference last week. "The mind is not a camera," says Dror. "It's a dynamic machine that can distort what it sees, not a passive recorder of visual information. Perception is far from perfection." He argues that forensic scientists, along with everyone else, can't avoid the risk of subjective bias. Frequently they have to use their skill and expertise to decide whether a fragment of a fingerprint at a crime scene matches a precise impression given in a police station. Research by Dror and his Southampton students Dave Charlton and Ailsa Peron has tested whether experts can be affected by outside influences. Five examiners were shown examples of prints they had identified as positive matches five years earlier. In the new external context only one of the experts agreed with his or her previous decision; three contradicted their previous judgement and decided the prints didn't match. The fourth wasn't sure. THE BIRDS, THE FISH AND THE BEES Believe it or not, fish can take the role of supporting actors alongside bees in pollinating plants. Flowering plants near ponds may owe their pollination not only to winged creatures, but also to finned ones, according to University of Florida and Washington University in St. Louis researchers, who published their findings recently in Nature. Fish help spread pollen among flowers near the water by eating dragonfly larva, which live in ponds. Adult dragonflies are major predators of bees, butterflies and other pollinators. The result is a simple but unexpected cascade: The more fish, the fewer dragonflies, the more bees and butterflies, the more plant pollination. "Much of the science of ecology is elucidating the surprising and counterintuitive connections among species," said zoology Prof. Robert Holt, one of the five authors. The research is significant because it shows how organisms from one ecosystem can change those in a seemingly separate one - with something such as a dragonfly tying them together. But it's also notable because it highlights an unusual example of how humans can shape nature. For example, it's well known that habitat destruction has ecological consequences, from limiting food for grizzlies and other predators to reducing the nutrient levels of mountain streams. The fish-flower research shows the same potential at a far more local and everyday level. People may never notice, but when they create, fill or drain a pond, or stock it, the ecological ripples lap onto shore.