Crystal surfaces typically melt into a thin liquid layer at temperatures slightly below the melting point of the crystal. Such surface premelting is prevalent in all classes of solids and is important in a variety of metallurgical, geological and meteorological phenomena1. Premelting has been studied using X-ray diffraction2 and differential scanning calorimetry3, but the lack of single-particle resolution makes it hard to elucidate the underlying mechanisms. Colloids are good model systems for studying phase transitions4 because the thermal motions of individual micrometre-sized particles can be tracked directly using optical microscopy5. Here we use colloidal spheres with tunable attractions to form equilibrium crystal–vapour interfaces, and study their surface premelting behaviour at the single-particle level. We find that monolayer colloidal crystals exhibit incomplete premelting at their perimeter, with a constant liquid-layer thickness. In contrast, two- and three-layer crystals exhibit conventional complete melting, with the thickness of the surface liquid diverging as the melting point is approached. The microstructures of the surface liquids differ in certain aspects from what would be predicted by conventional premelting theories. Incomplete premelting in the monolayer crystals is triggered by a bulk isostructural solid–solid transition and truncated by a mechanical instability that separately induces homogeneous melting within the bulk. This finding is in contrast to the conventional assumption that two-dimensional crystals melt heterogeneously from their free surfaces3,6 (that is, at the solid–vapour interface). The unexpected bulk melting that we observe for the monolayer crystals is accompanied by the formation of grain boundaries, which supports a previously proposed grain-boundary-mediated two-dimensional melting theory7. The observed interplay between surface premelting, bulk melting and solid–solid transitions challenges existing theories of surface premelting and two-dimensional melting.
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We thank X. Cao and M. Liao for the discussions and Chromatech Inc. for the dye sample. This study was supported by RGC grants GRF601911, GRF16301514 and C6004-14G (Y.H.) and an NWO VENI grant 680-47-441 and the Start-Up Grant from Nanyang Technological University (R.N.).
The authors declare no competing financial interests.
This file contains Supplementary Text and Data, Supplementary Figures 1-18 and additional references. (PDF 9576 kb)
Surface premelting process of an equilibrium bilayer crystal at four temperatures (×3 real time). (MP4 7813 kb)
In the final temperature step, we shifted the field of view containing two domains with different surface lattice orientations and a grain boundary to show that they exerted little effect on the surface liquid thickness l (×3 real time). (MP4 4200 kb)
Non-quasistatic surface premelting process of a monolayer crystal accompanied with a bulk lattice expansion
The temperature of the sample dropped rapidly from 26.5°C to 22.8°C in 30 s, making the solid–solid transition easier to visualize (×50 real time). (MP4 8621 kb)
The white spots are smaller than the physical size of the sphere because of the high-contrast CCD camera setting (×3 real time). (MP4 6628 kb)
Homogeneous melting of a monolayer crystal inside the bulk (×3 real time). (MP4 17051 kb)
The melted monolayer is a structural liquid with polycrystalline patches (×40 real time). (MP4 20410 kb)
Premelted monolayer crystal at T = 23.1°C under equilibrium for 20 min shows no drift flow (×50 real time). (MP4 14927 kb)
Epitaxial growth of a two-layer crystal under a temperature gradient of 15.0°C/mm at 32.0°C (×30 real time). (MP4 9252 kb)
Image of the particles becomes sharper by increasing the contrast of the controlling software of the CCD camera under a constant brightness.
Image of the particles becomes sharper by increasing the contrast of the controlling software of the CCD camera under a constant brightness. (MP4 5947 kb)
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Li, B., Wang, F., Zhou, D. et al. Modes of surface premelting in colloidal crystals composed of attractive particles. Nature 531, 485–488 (2016). https://doi.org/10.1038/nature16987
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