Latent heat induced rotation limited aggregation in 2D ice nanocrystals

Abstract

The basic science responsible for the fascinating shapes of ice crystals and snowflakes is still not understood. Insufficient knowledge of the interaction potentials and the lack of relevant experimental access to the growth process are to blame for this failure. Here, we study the growth of fractal nanostructures in a two-dimensional (2D) system, intercalated between mica and graphene. Based on our scanning tunneling spectroscopy data, we provide compelling evidence that these fractals are 2D ice. They grow while they are in material contact with the atmosphere at 20 °C and without significant thermal contact to the ambient. The growth is studied in situ, in real time and space at the nanoscale. We find that the growing 2D ice nanocrystals assume a fractal shape, which is conventionally attributed to Diffusion Limited Aggregation (DLA). However, DLA requires a low mass density mother phase, in contrast to the actual currently present high mass density mother phase. Latent heat effects and consequent transport of heat and molecules are found to be key ingredients for understanding the evolution of the snow (ice) flakes. We conclude that not the local availability of water molecules (DLA), but rather them having the locally required orientation is the key factor for incorporation into the 2D ice nanocrystal. In combination with the transport of latent heat, we attribute the evolution of fractal 2D ice nanocrystals to local temperature dependent rotation limited aggregation. The ice growth occurs under extreme supersaturation, i.e., the conditions closely resemble the natural ones for the growth of complex 2D snow (ice) flakes and we consider our findings crucial for solving the "perennial" snow (ice) flake enigma.

Structure and Dynamics of Confined Alcohol–Water Mixtures

“…At low humidity 2D ice fractals are formed, see figure 6b and refs. 22,23 The structure and growth of the fractals can be disturbed by contamination of the graphene-mica interface. 43 Typically ice fractals have a well-defined 7 branch thickness, which depends on the graphene thickness and the evaporation rate of the intercalated water molecules.…”

“…The evaporation of the water molecules induces the growth of ice layers at the interior of the graphene-mica pores, as we have described in detail in ref. 23 Meanwhile, due to the increase of the alcohol vapor in the chamber, alcohol molecules creep into the pores (of the graphenemica interface). These alcohol molecules, as we will show later, preferentially adsorb on top of the ice layers.…”

“…The ice growth is induced by the heat extracted from the system by evaporation of water molecules. 23 The fractal shape is a result of a rotation limited growth of the ice crystal.…”

“…Furthermore, the presence of graphene and mica, with their large differences in thermal conductivity along and perpendicular to the interface, warrants a valuable isolation from the environment and allows to study the intrinsic 2 properties of the system. 23 Therefore, it can provide useful information on the behavior of thin films of small molecules at the nanoscale. A number of studies have been performed for confined water molecules.…”

Graphene Visualizes the Ion Distribution on Air-Cleaved Mica

“…Graphene has been shown to be able to accurately stabilize and conform to water layers1314151617181920 and other small molecules21222324 when deposited on a substrate. It has been proven an essential tool to probe small and otherwise dynamic molecules.…”