Surface freezing

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Definition

A representation of surface freezing. The left images from bottom to top show a normal surface melting. The right images from bottom to top show surface freezing. During surface freezing there is ordering of the alkanes on the surface frozen material, while the bulk is liquid. The brick structure denotes a bulk solid crystalline phase, while the blue structure denotes a bulk liquid phase. The image was created by Philip Ash and has been reprinted from the following website: http://www.colinbain.net/group/ash/


Surface Freezing is long range crystalline ordering at the surface of a liquid. It usually occurs in alkanes and long chained hydrocarbons. In most cases the hydrocarbons orient normal to the plane of the surface, and pack in the hexagonally close packed structure. Also, it has only been observed in alkanes that have a degree of polymerization between 14 and 50. It is the opposite of the more common effect known as surface melting. Although it was discovered by John Earnshaw at Queens University in the early 1990s, it is still not well understood.

Theory

It is not well understood what causes surface freezing. It seems there should be massive entropic penalties for having it. The leading theory that tries to explain these entropic penalties states that the hydrocarbon chains, when normal to the surface, can experience fluctuations along their long axis. These fluctuations are enough to prevent the rotation of the chains down into the bulk liquid. Furthermore, the molecules are tightly packed, again preventing the rotation.

Applications

There were no applications that I could find for surface freezing.

References

A. V. Tokachenko and Y. Rabin, Phys. Rev. E 55, 778 (1997).

Keyword in References

Crystalline monolayer surface of liquid Au–Cu–Si–Ag–Pd: Metallic glass former