The droplets are composed of a mixture of platinum and germanium and move on a heated substrate in the direction of the heat source. But as soon as the temperature lowers, the droplets start their special behaviour. Like professional skiers, they suddenly change their direction and make a slalom.
“Using a photo-emission electron microscope, we were able to film the skiing and show the whole process of solidifying”, explains Arie van Houselt, corresponding author of the publication (Physical Review Letters, "Presolidification in Eutectic Droplets").
A Skiing Act Unlike Any Other
The skiing droplets form at surprisingly high temperatures. “At ninety degrees above their eutectic point, which is the temperature at which these types of mixtures freeze. The droplets don’t solidify all at once. They first elongate and then the solidifying process starts at the bottom. On their interface with the substrate”, explains Van Houselt.
This first solid layer also explains the skiing. When the material solidifies, it gains a nanostructure which acts as a grid on which the droplet can move. The nanostructure lowers the resistance of the droplets in another direction. The droplets make use of this lowered resistance and make a sharp turn. They start moving in this direction.
Invaluable Insights
This remarkable display isn't just an entertaining performance at the nanoscale. The conditions under which these droplets display their extraordinary skiing are close to those found in the growth of many (nano)structures, such as nanowires and germanene.
Van Houselt: “Discoveries like this one provide invaluable insights into the mechanisms of these transformations, potentially opening the doors to the creation of flawlessly engineered computer chips.”