For the first time, scientists have learned a bit more about surface premelting phenomena. Using a new colloidal crystal model system, they've examined interplays among surface premelting, bulk melting and solid-solid transitions.
Understanding melting is important. For example, it can help us learn more about glacier movement, frost heave and snowflake growth. Surface premelting is the phenomenon in which the surface of a solid can melt into a thin layer of liquid, even below its melting point. This allows two pieces of ice to fuse together once ice melts slightly and then freezes again.
Until now, researchers had been unable to figure out the premelting mechanisms at the molecular or atomic level. This is particularly true for two dimensional (2-D) melting under the presence of free surfaces.
In this latest study, however, the researchers used a model system that consisted of microscopic plastic spheres confined in a parallel-plate glass cell. The researchers then lowered the temperature in the experiment, weakening the attraction between the spheres. Then, the researchers analyzed the behavior of the system by using single-particle dynamics.
What did they find? The new colloidal model system is well-suited to study surface physics of crystals. In fact, the researchers were able to figure out that the surface premelting is related to bulk melting and solid-solid transitions.
During the experiment, the researchers saw that a crystal with free surfaces melted homogenously from both surfaces within the bulk.
"These findings are in contrast to the commonly assumed heterogeneous melting from surfaces," said Ran Ni, one of the researchers. "In particular we have established that the bulk melting was accompanied with the formation of grain boundaries, demonstrating a grain-boundary-mediated 2D melting scenario. These novel interplays among surface premelting, bulk melting and solid-solid transitions present new challenges on theories on surface premelting and 2D melting."
The findings reveal a bit more about surface premelting. This, in turn, may help researchers better understand how melting in nature occurs and how that impacts larger bodies, such as glaciers. This could help researchers more accurately understand the melting that occurs.
The findings are published in the March 2016 journal Nature.
