Incidental Ice: Why purified water freezes and pollen macromolecules nucleate ice

Predicting when water freezes, in the lab or in clouds, hinges on heterogeneous nucleation events that remain difficult to describe across scales. A synthesis of recent “purified” water experiments shows that droplets larger than ~10 nL almost always freeze at temperatures warmer than homogeneous nucleation allows, with the median freezing temperature increasing linearly with log(volume)—as found by Bigg (1953). Our compilation of recent results produces a trend line that closely matches that reported by Langham and Mason (1958). This empirical trend lacks a satisfactory theoretical basis. We advance a “chance nucleator” hypothesis: any somewhat disordered material in contact with supercooled water can, by combinatorial chance, present nanoscale patches that achieve a low effective contact angle with ice and trigger freezing. A simple classical nucleation theory (CNT) treatment captures much of the observed trend and predicts pronounced flattening at larger volumes, implying that carefully isolated millilitre- to litre-scale water volumes might supercool to lower temperatures than is reported in most of the literature.

We then apply the chance nucleator framework to interpret recent results on the nature of ice‑nucleating macromolecules (INMs) produced by pollen (Kinney et al., 2024). In this view, a statistical, non‑adaptive origin naturally explains why ice‑nucleation activity (INA) shows high interspecific variability and no consistent correlation with phylogeny, growth biome, seasonality, or pollination mode, yet still permits exceptional nucleators in which macromolecular composition or aggregation fortuitously produces rare, low‑contact‑angle patches. Thus, pollen INMs can be widespread and diverse despite the lack of an evolutionary driver for ice‑nucleation ability.

References
Bigg, E. K.: The supercooling of water, Proceedings of the Physical Society. Section B, 66, 688, 10.1088/0370-1301/66/8/309, 1953.
Kinney, N. L. H., Hepburn, C. A., Gibson, M. I., Ballesteros, D., and Whale, T. F.: High interspecific variability in ice nucleation activity suggests pollen ice nucleators are incidental, Biogeosciences, 21, 3201–3214, 10.5194/bg-21-3201-2024, 2024.
Langham, E. J. and Mason, B. J.: The Heterogeneous and Homogeneous Nucleation of Supercooled Water, Proceedings of the Royal Society of London Series A-Mathematical and Physical Sciences, 247, 493-&, 10.1098/rspa.1958.0207, 1958.