Orderly world found at liquid-solid boundary


Researchers have caught the first glimpse of nanometer-scale structures at the boundary between droplets of liquid aluminum and the solid face of sapphire. The detailed view provides direct evidence that the sapphire's crystal structure induces the liquid aluminum atoms to line up in an orderly fashion, which is not normally characteristic of liquids. These findings were published online by the journal Science at the Science Express Web site.

"Basically, this means we need to think about liquid-solid interfaces in a totally different way," says Professor Wayne D. Kaplan of the Technion-Israel Institute of Technology, who co-authored the study with Technion Ph.D. student Yaron Kauffmann and colleagues from the Max Planck Institute in Germany.

"The findings have fundamental implications for a variety of processes, including lubrication, the growth of thin crystal layers and 'wetting,' or how well liquids spread over a solid surface. So many current technological processes depend on our understanding of such phenomena," Kaplan adds. "For instance, the processes play a key role in building semiconductor chips and other microelectronic devices, soldering materials, and maneuvering liquids through small spaces." Other applications could include "labs-on-chips," where chemicals or biological fluids are moved through microchannels on a small glass plate, and printing (on fabrics and paper), which also involves the wetting process.

The researchers were able to glimpse the dynamics of the liquid-solid interface using a special high-resolution transmission electron microscope. Transmission electron microscopes work by passing a high voltage beam of electrons through a thin slice of material. The electron beam scatters due to interactions with atoms in the sample, and researchers can construct atomic-scale images of the sample based on measurements of this scattering effect.

Magnified area from a movie snapshot acquired at ~750C showing the contrast perturbations in the liquid parallel to the (0006) planes in alumina. The atom positions in the Al2O3 (red for oxygen and yellow for aluminum) were determined by contrast matching between simulated and experimental images at different objective lens defocus and specimen thickness values. The first layer of liquid atoms is shown schematically. (Photo: TIIT)
Kaplan and colleagues used an in-situ heating stage on the microscope to heat a thin slice of pure, single crystal sapphire (also known as aluminum oxide) above the melting point of aluminum. The combination of heat and electron irradiation knocked oxygen atoms out of the sapphire crystal and allowed aluminum atoms to drift to the crystal's surface to form liquid droplets.

The researchers used images and real-time movies to capture a dynamically evolving interface between the sapphire and aluminum drops. Due to the interface with the crystal, atoms in the liquid formed an ordered structure. As a result, the atoms in the liquid adjacent to the interface have properties differing from those in the liquid and from those in the solid. In addition, due to the presence of a small amount of oxygen in the microscope, over a few hundredths of a second, oxygen combined with the ordered aluminum atoms and the crystal grew layer by layer into the liquid. This allowed the researchers to determine the mechanism of crystal growth for sapphire, an important engineering material. The images show structures at the extremely small scale of less than one nanometer. (A single human hair, by comparison, is 10,000 nanometers wide.)

In some ways, producing a liquid-solid interface was one of the easier tasks for the research team, according to Kaplan.

"Once one side of the interface is liquid, there are a slew of experimental challenges that follow," Kaplan says, including preventing the liquid's evaporation, steadying the sample for measurement and most importantly, analyzing the images to pinpoint the actual atoms amongst the visual "artifacts" created by the microscopy process.

"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 would not be convincing to the scientific community," he adds.

Kaplan says that "one of the nice things about the present study" is that he and his students had predicted order among liquid atoms induced by contact with a crystal from computer simulations and indirect experiments conducted previously.

Kaplan and colleagues will continue their work on the liquid-solid interface with the help of a new transmission electron microscope with a unique, image-correcting lens and extremely high-resolution capabilities. The new microscope will be installed at the Technion in January 2006.

The study was supported in part by the Russell Berrie Nanotechnology Institute at the Technion, the German-Israel Fund and the German Science Foundation.

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