Premelting

Premelting shows its effects in frost heave, and, taking grain boundary interfaces into account, maybe even in the movement of glaciers.

, in the case of complete premelting, the solid melts homogeneously from the outside to the inside; in the case of incomplete premelting, the liquid film stays very thin during the beginning of the melting process, but droplets start to form on the interface.

[1] He compared the effect which holds a snowball together to that which makes buildings from moistured sand stable.

[2][3] Frenkel strengthened this by noting that, in contrast to liquids, no overheating can be found for solids.

[4] After extensive studies on many materials, it can be concluded that it is a common attribute of the solid state that the melting process begins at the surface.

A more detailed or abstract view on what physics is important for premelting is given by the Lifshitz and the Landau theories.

In the case of the second phase being a solid of the same chemical material one speaks of grain boundaries.

Written in interfacial energies this would mean: If this is the case then it is more efficient for the system to form a separating phase (3).

The only possibility for the system to form such a layer is to take material of the solid and "melt" it to a quasi-liquid.

[5] With the help of the Lifshitz Theory on Casimir, respectively van der Waals, interactions of macroscopic bodies premelting can be viewed from an electrodynamical perspective.

Hence the attraction between the liquid molecules themselves will predominate and they will start forming droplets instead of thickening the film further.

This makes it a question of solid and surface free energies whether complete premelting occurs.

For van der Waals interactions Lifshitz theory can now calculate which type of premelting should occur for a special system.

For solid–solid interfaces it cannot be predicted in general whether the premelting is complete or incomplete when only considering van der Waals interactions.

[5] Most insight in the problem probably emerges when approaching the effect form Landau Theory.

Hence one can see that the order parameter in the liquid film can undergo a continuous phase transition for large enough extrapolation length.

what corresponds to the result of the thermodynamical model in the case of short range interactions.

Landau Theory does not consider fluctuations like capillary waves, this could change the results qualitatively.

Basically it is all about showing that there is a phase on top of the solid which has hardly any order (quasi-liquid, see fig.

One possibility was done by Frenken and van der Veen using proton scattering on a lead (Pb) single crystal (110) surface.

An ideal shadowing and blocking measurements results in an energy spectrum of the scattered protons that shows only a peak for the first surface layer and nothing else.

Due to the non ideality of the experiment the spectrum also shows effects of the underlying layers.

So the effects of shadowing and blocking vanish what means all the liquid film contributes the same amount of scattered electrons to the signal.

During their measurements Frenken and van der Veen raised the temperature to the melting point and hence could show that with increasing temperature a disordered film formed on the surface in equilibrium with a still well ordered Pb crystal.

[8] A comparable friction coefficient is that of rubber or bitumen (roughly 0.8), which would be very difficult to ice skate on.

For several decades it was common to explain the low friction of the skates on ice by pressure melting, but there are several recent arguments that contradict this hypothesis.

[10] The strongest argument against pressure melting is that ice skating is still possible below temperatures under -20 °C (253K).

[11] The thickness of the water film due to premelting is also limited at low temperatures.

Although, De Koning et al. found in their measurements that the adding of impurities to the ice can lower the friction coefficient up to 15%.

[9] While the pressure melting hypothesis may have been put to rest, the debate between premelting and friction as the dominant mechanism still rages on.