14 DAY TRIAL // Two researchers at the U.S. Department of Energy's Brookhaven National Laboratory have elaborated a new theory about the mechanisms of crystallization (freezing) of metals.
They used the world's smallest pipette (laboratory instrument used to transport a measured volume of liquid) made from germanium nanowires with a reservoir of gold-germanium alloy at the tip, to demonstrate that tiny droplets of liquid metal (just a billionth of a trillionth of a liter in size) freeze much differently than their larger counterparts.
Crystallization and its opposite reaction, melting are phase transformations, fundamental processes by which most substances change between a disordered liquid state (such as liquid water) and an ordered solid state (ice). When a liquid droplet is cooled, the motion of its atoms gradually slows until they come to a stop, resulting in a solid. For large droplets, this crystallization usually starts at a small impurity (a speck of dirt), from which it rapidly spreads over the entire droplet.
However, very pure substances lack such crystallization centers and have difficulty starting the phase transformation.
Eli Sutter, a scientist at Brookhaven's Center for Functional Nanomaterials (CFN) and the lead author of the study, said "Our findings could advance the understanding of the freezing process, or crystallization, in many areas of nature and technology. The accepted theory of crystallization, developed in the first half of the previous century, predicts that without impurities, a small solid core generated at random in the interior of the droplet initiates the phase transformation. Our experiments on very small droplets challenge this theory."
Working with an electron microscope, which creates an image of the sample by bombarding it with a beam of electrons, this zeptoliter (zepto - one billionth of a trillionth) pipette suspends the tiny droplets so their phase transformations can be studied with high magnification.
Normally, the metallic alloy must be kept at a temperature above 350 degrees Celsius (662 F) to achieve a liquid state. When the temperature is lowered to about 305 degrees Celsius, the researchers observe a striking phenomenon: The liquid droplets develop surface "facets," which are straight, planar sections on the otherwise spherical-shaped structures. The facets continually form and decay in an "ethereal dance" of the droplet shape. This "dance" can last for hours, but quickly stops if the temperature is lowered any further.
At this point, the droplet solidifies into a structure that is determined by the ending positions of the dancing surface facets.
By contradicting the traditional idea that all crystallization originates in the interior of a liquid droplet and instead showing that the process may differ based on the size of the sample, the new findings are becoming the foundation for a better understanding of freezing processes in the environment as well as in nanotechnology.
Applications can be found in the balance of solid and liquid water in upper-atmosphere clouds - an important factor in climate models - that greatly depends on the exact way water droplets freeze. Such parameters might be predicted more accurately with a better picture of the freezing mechanism.