YES-27-A-29: Testing classical nucleation theory through chemical and isotopic effects on heterogeneous ice nucleation

Ice nucleation is important in atmospheric clouds and hence influences weather and climate, yet our ability to predict whether a particular material will nucleate ice remains extremely limited. Classical nucleation theory (CNT) provides the usual framework for interpreting nucleation experiments, but its application to heterogeneous ice nucleation requires various rather dubious assumptions about interfacial energies, nucleus geometry and the representation of complex, spatially variable surfaces using a small number of macroscopic parameters. It remains unclear whether CNT captures the essential physics of heterogeneous ice nucleation or is more a flexible description of observed freezing behaviour.

This project will use controlled chemical and isotopic perturbations to test predictions derived from CNT for immersion-mode heterogeneous ice nucleation. Experiments will examine several well-established ice-nucleating materials, including feldspars and other minerals, silver iodide, Snomax and pollen-derived ice-nucleating macromolecules. These nucleators span a wide range of chemical compositions, structures and characteristic nucleation abilities, allowing us to test whether common theoretical principles can explain their behaviour or whether apparently similar freezing curves arise from fundamentally different processes.

The student will vary the identity and concentration of dissolved solutes, including ions with similar bulk effects on water activity but different interactions with water, ice and nucleating surfaces. Previous work suggests that some solutes may alter heterogeneous ice nucleation by changing the configurational entropy of ice-like clusters at charged surfaces. Such effects would modify the nucleation barrier through mechanisms not captured by considering water activity alone. Experiments will also use heavy water and mixtures of heavy and normal water to perturb hydrogen bonding, molecular dynamics and the thermodynamic driving force for ice formation. Measurements will combine microfluidic techniques with microlitre droplet-freezing assays. Together, these approaches will cover a large proportion of the accessible nucleation-temperature range for each material and test whether different regions of a nucleation curve respond differently to chemical and isotopic perturbations.

The resulting freezing distributions and nucleation rates will be analysed using a mixture of statistical inference and explicit hypothesis testing. Particular attention will be given to whether changes can be explained using consistent, physically meaningful CNT parameters across nucleators, droplet sizes, solute conditions and isotopic compositions. Alternatively, fitted parameters may prove to encompass molecular-scale processes, including entropic effects, or variations between nucleation sites that are absent from the theory.

The project will combine laboratory experimentation with old-school pen-and-paper theoretical development and quantitative statistical analysis. It will determine which observations support classical descriptions of heterogeneous nucleation, identify where those descriptions become inadequate, and provide new mechanistic insight into how solution composition, isotopic substitution and interfacial chemistry control the formation of ice. By establishing how these effects vary between atmospherically relevant ice-nucleating materials, the project will also help determine how solution chemistry influences ice formation in supercooled cloud droplets. The results will therefore provide a stronger physical basis for representing heterogeneous ice nucleation in cloud and climate models.