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Background

According to an article from Physics Today, ice is slippery because there is a “liquid or liquid-like layer” on its surface.

There are 3 mechanisms that can cause this layer to exist, each playing a role that depends on the temperature, and other factors. These mechanisms are:

  1. Pressure melting (the least dominant)
  2. Frictional heating
  3. Premelting

Pressure melting and frictional heating can cause a liquid water film to form on top of the ice, while premelting can cause a liquid-like layer to form.

Premelting

Premelting refers to the formation of quasi-liquid layers (QLLs) on the surface of a solid crystal even below its melting point. This quasi-liquid layer forms because molecules on the surface are bonded to fewer other molecules when compared to molecules which are below the surface. Therefore, molecules on the surface are less firmly held in place and can move around more.

Question

If premelting occurs when molecules on the surface of a crystal move around because they are bonded to relatively fewer other molecules, does this happen with other crystals such as sugar at room temperature?

Just like water molecules in ice, sucrose molecules are held in place in a sugar crystal via hydrogen bonds. So premelting at room temperature might be possible, perhaps with the quasi-liquid film having a lower thickness in the case of sugar.

On the other hand, according to this paper, the melting point of sucrose is 185 °C. Perhaps this is too high for premelting to occur at all at room temperature?

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    $\begingroup$ "Perhaps this is too high for premelting to occur at all at room temperature?" This is a good bet. Since the mechanism you're describing appears to be thermally activated and is thus likely exponentially dependent on temperature, it seems appropriate to compare the homologous temperatures. In other words, the QLL (if it's not unique to water) might become detectable for sugar at 170°C, for example. An intriguing research hypothesis to investigate. $\endgroup$ Commented Nov 5, 2020 at 20:48

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If so, I don't think it's a significant effect. Otherwise, sugar would not flow as freely as it does. However, sugar is somewhat hygroscopic, and sugar that has absorbed water vapour from the atmosphere can stick together in lumps. In warm humid climates, that happens fairly quickly to sugar that's kept in an unsealed bowl. In cool dry climates, it takes much longer for lumps to form.

Raw & brown sugar is more clumpy than pure sucrose, due to the presence of molasses, which contains some water.

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  • $\begingroup$ That's a good point about sugar being hygroscopic. However, assuming that there is no humidity in the environment, would there be a quasi-liquid layer? $\endgroup$
    – hb20007
    Commented Nov 5, 2020 at 12:57
  • $\begingroup$ @hb20007 I don't think so, because dry sugar behaves like dry sand or salt (NaCl), which have even higher melting points. $\endgroup$
    – PM 2Ring
    Commented Nov 5, 2020 at 13:06

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