The role of polymers in atmospheric ice formation

Contrary to popular perception, ice usually does not form in liquid water at 0°C. Indeed, there is excellent evidence that even ocean-scale volumes of water can measurably supercool to temperatures below the melting point of ice (1). Small droplets of water can be cooled to temperatures below -38°C before freezing. In nature, ice is usually nucleated heterogeneously by substances in contact with supercooled water. This process, heterogeneous ice nucleation, plays a key role in many scientific and technological contexts, notably atmospheric science, where glaciation of mixed-phase clouds impacts cloud radiative effects and therefore climate (2, 3).

Many substances are known to nucleate ice well. These include mineral dusts, biological substances, inorganic crystals, alcohol monolayers and many others (2). Remarkably, we do not have a sound understanding why one substance should nucleate ice more effectively than another. We are forced to determine empirically what substances nucleate ice well. This presents profound difficulties for understanding what species induce ice nucleation in clouds. At present, order of magnitude differences in ice nucleating particle effectiveness are used in these atmospheric clouds models, but are not physically underpinned (4). Resolving this will allow more accurate representation of clouds in weather and climate models.

Very recently, progress has been made towards understanding how polysaccharides produced by pollen nucleate ice effectively (5) and also towards understanding how simple polymers nucleate ice (6, 7). These results suggest that many macromolecules may be able to nucleate ice well. Geopolymerisation in the environment has been shown to produce a wide and chemically varied range of macromolecules (8) yet the ice nucleation ability of such molecules has not yet been studied at all.

In this PhD project the student will answer two questions:

1) How well do macromolecules we expect to be present in clouds nucleate ice?

2) What are the physicochemical factors that cause a macromolecule to nucleate ice effectively?

To do this the student will a) synthesise macromolecules representative of what may be present in clouds using well established geopolymerisation techniques developed in the lab of Prof Caroline Peacock b) chemically and physically characterise the macromolecules using techniques such as Raman spectroscopy, size exclusion chromatography and x-ray photoelectron spectroscopy c) determine the ice nucleation effectiveness of the macromolecules, using droplet freezing assays and high-speed cryomicroscopy d) link the chemical and physical properties of the macromolecules to their ice nucleation ability using a combination of classical nucleation theory and recently-developed ‘toy-models’ of ice nucleation surfaces.

This will be an intrinsically interdisciplinary project, allowing the student to engage with physical chemistry, atmospheric science, and biogeochemistry offering opportunities to collaborate with scientists from a diverse range of backgrounds and develop skills relevant skills for a wide range of scientific careers.