Surface melting of ice significantly governs a wide variety of phenomena in nature. We recently succeeded in directly observing the surface melting on ice basal faces (Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 1052-1055) by our advanced optical microscopy, which can detect 0.37-nm-thick elementary steps on basal faces. However, the direct observation of surface melting on the other prism and high-index faces remains an experimental challenge. To fully obtain a comprehensive understanding, we need to examine the surface melting on these faces. Here we show the appearance of two types of quasi-liquid layers (QLLs) on prism and high-index faces just below the melting point. We quickly raised the temperature of ice crystals and then observed prism and high-index faces during a roughening process. We found that with increasing temperature, round liquid-like droplets (alpha-QLLs) first appeared at temperatures higher than -1.4 to -0.5 degrees C, and then thin liquid-like layers (beta-QLLs) also appeared at temperatures higher than -0.8 to -0.3 degrees C, as in the case of basal faces. This result demonstrates that the presence of two types of QLL phases with different morphologies plays an intrinsically important role in the surface melting of ice crystals, irrespective of face indexes. We also revealed that the roughening process was caused thermally.
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