Brief Overview of Ice Nucleation

Abstract

The nucleation of ice is vital in cloud physics and impacts on a broad range of matters from the cryopreservation of food, tissues, organs, and stem cells to the prevention of icing on aircraft wings, bridge cables, wind turbines, and other structures. Ice nucleation thus has broad implications in medicine, food engineering, mineralogy, biology, and other fields. Nowadays, the growing threat of global warming has led to intense research activities on the feasibility of artificially modifying clouds to shift the Earth's radiation balance. For these reasons, nucleation of ice has been extensively studied over many decades and rightfully so. It is thus not quite possible to cover the whole subject of ice nucleation in a single review. Rather, this feature article provides a brief overview of ice nucleation that focuses on several major outstanding fundamental issues. The author's wish is to aid early researchers in ice nucleation and those who wish to get into the field of ice nucleation from other disciplines by concisely summarizing the outstanding issues in this important field. Two unresolved challenges stood out from the review, namely the lack of a molecular-level picture of ice nucleation at an interface and the limitations of classical nucleation theory.

Keywords: classical nucleation theory; contact nucleation; heterogeneous nucleation; ice; immersion nucleation; memory effect; multi-step nucle-ation; nucleation; organic ice nucleator; surface nucleation.

Figure 1

Schematic illustration of the activation barrier. The figure is shown for a constant temperature/pressure for which the prevailing phase is metastable.

Figure 2

The Wulff theorem. The size of the area that a facet exposes to its surrounding medium is inversely proportional to the specific interfacial free energy value between that facet and the surrounding medium. Image reproduced from [41], with permission from Royal Society of Chemistry.

Figure 3

Ice forms on an organic nucleator, like cholesterol shown in the picture, at very small supercoolings in the form of discrete hexagonal crystals, as opposed to continuous two-dimensional films or layers. Image reproduced from [67], with permission from Elsevier.

Figure 4

A schematic illustration of condensation of a wetting liquid in a very small wedge when the thermodynamically stable phase of the same macroscopic substance is solid. The very tip of a narrow wedge is effectively one-dimensional and as such condensation can proceed without nucleation (or surmounting of an activation barrier). Presence of a metastable liquid phase could cast doubt as to the applicability of classical nucleation theory to the nucleation of the solid phase.

Figure 5

A schematic illustration of the crystal structures of hexagonal I_h (top) and cubic I_c (bottom). Image reproduced from [107], with permission from The Royal Society.

Figure 6

A schematic illustration of surface nucleation vs. homogeneous nucleation in the absence of a foreign solid wall (top) and contact nucleation vs. immersion nucleation in the presence of a foreign solid wall (bottom).