Scratching the surface of ice

Water holds many mysteries. In this thesis, we reopen a cold case as we explore some of its frozen secrets, focusing on the remarkable properties of the surface of ice. As early as the 1860s, Michael Faraday proposed that this surface is always covered by a thin liquid layer. His claim was inspired by the observation that pieces of ice can merge at temperatures below the freezing point. Although based on qualitative evidence, his idea sparked a scientific debate that continues to this day.
Today, the outermost layer of an ice crystal is often described as ‘liquid-like’ or ‘quasi-liquid’. However, depending on the experimental or computational technique, the proposed thickness varies by orders of magnitude: there is no consensus. Moreover, our experiments provide evidence that the behavior of this layer is not as liquid as its name suggests.
By repeating Faraday’s experiment under controlled humidity and temperature conditions, we quantitatively conclude that ice merges through the vapor phase rather than as a liquid. This same vapor driven transport mechanism also causes the spontaneous disappearance of scratches induced in the surface of the ice crystal.
Furthermore, our experiments and calculations on the wetting behavior of ice reveal that water does not wet its own crystalline surface. In other words, a droplet does not fully spread when placed on ice. This is not the behavior one would expect if the surface were truly liquid.
In the second part of the thesis, we discuss how these findings influence our understanding of everyday phenomena such as freezing damage, slipperiness during ice skating, and the growth of icicles.