Particle collisions, aggregation & resuspension

The formation of planets and comets, the precipitation of rain or snow from a cloud, the flocculation of particles from an asthma inhaler, the jamming of snacks or pharmaceuticals as they are coated in a plant, the transport by turbidity currents of carbon rich sediments from the continents into the deep ocean all critically depend on the minute details of particle properties and the fluid that mediates them. Particle shape, surface finish and turbulence and structure in the surrounding fluid govern the outcome of the particle collisions that in turn determine whether a planet grows, a rain storm forms, a process plant jams, or a turbidity current deposits its sediment. This Fellowship explores this relationship between particle and fluid properties and collision outcome, by innovating diagnostic technology to allow precision data to be captured in ground-breaking experiments that utilise these new techniques.

This proposed programme follows two complementary threads that examine the particle collisions themselves and the role of the surrounding fluid in mediating and/or promoting those collisions. These threads are bought together to verify aggregation models describing the location, frequency and outcome of collisions, dictating the evolution of particle size and distribution throughout a flow. The first thread exploits the unique properties of ice to create highly bespoke binary collisions to forensically identify the particle properties that influence the energetics of a collision leading to aggregation or rebound. The second thread uses new hydrogel bead technology to attempt to make sub-particle scale turbulence measurements that can start to explain the dramatic synergistic influence particles have on the fluid they are carried in.

Through this research programme the PI aims to deliver significant, fundamental and lasting developments in the understanding of fluid mediated particle collisions. The PI will enhance her growing, global reputation and track-record in this field and attract high-quality graduate students and post-doctoral researchers to form a dedicated group at the forefront of fluid and particle dynamics research. Current international collaborations will be strengthened and exploited to maximise the benefit of the research, and new links with academics and practitioners will be fostered. The Fellowship will also exploit the world-class facilities - the superconducting magnet and the cryogenic atomic force microscope - and expertise at the University of Nottingham helping to create a focal point for future experimental particle technology studies.

The key impact of this Fellowship will be the realisation of the techniques, diagnostics and data to fully validate particulate modelling processes, allowing the genesis of truly predictive tools. Two applications are addressed within the proposal lifetime that will lead to relatively near-term societal and economic benefit. The experimental results will be directly implemented within the UK Met Office's cloud microphysics module of the Unified Model, expected to be launched for operational forecasting in 2018. The quantitative prediction of rainfall is notoriously unreliable, due in large part to the uncertainty surrounding ice clustering processes in clouds. The highly bespoke data sets acquired through this Fellowship will be key in improving our understanding of ice particle behaviour in clouds, allowing the time and quantity of precipitation to be more accurately pinpointed in forecasting models. The benefits of improved quantitative prediction of precipitation are both economic and societal, since they allow more targeted mitigation and response to hazards that is both more economical and more effective.

The improved knowledge of ice particle shapes, clustering behaviour and distribution in clouds also impacts significantly on aviation. For example, at the UKAC Icing Prediction, Design and Quantification working group in October 2013 representatives of Rolls Royce and Airbus acknowledged that ice crystal formation at high altitude is of critical importance regarding impact on aircraft and aeroengine and the extensive damage that can result. The current problem with predicting these ice impacts is the difficulty of describing the possible trajectories of highly non-spherical ice particles that are often composed of clustered crystal fragments. This Fellowship will not only make such fragments, but also provide insight into the clustering process and thus the possible shapes and size distribution of real atmospheric ice. The enhanced knowledge of fluid particle interaction will also provide new insight into the concentration distribution of these clusters in a cloud allowing the impacts to be better modelled and mitigated, avoiding costly remedial repair and improving flight safety.

The resuspension work within this Fellowship links to new theoretical work undertaken in the last few years that starts to clarify the apparently essential role of fluid pore pressures in resuspending particles from a bed. Although motivated by the exceptional speeds and run out distances of powder snow avalanches, that could not be satisfactorily explained by shear-based erosion models, volcanologists and petroleum engineers are just starting to ask if these fluid-particle mechanisms are also at play in pyroclastic flows and ocean turbidity currents. Our experiments will elucidate this, informing and enabling the detailed verification and validation of numerical simulations that in turn will form the basis of a new generation of predictive models that can describe the transport of carbon-rich sediment from the continents into the deep ocean. This is essential information for sub-sea oil and gas engineers who need to locate these sediments for economically viable, targeted exploration and drilling.

In the long term, the expertise and knowledge created within this project can feed into food and pharmaceutical processing problems, where the application of coatings can enhance the propensity for expensive jamming and clustering problems, and astrophysics, where the process of planet-building from ice dust is still an open question.