This Ph.D. thesis presents a quantitative description of the mechanisms that control the aggregation of volcanic particles. Besides representing an important process that controls fine ash dispersal during explosive volcanic eruptions, particle aggregation is of great importance in both natural and industrial contexts. It is, for example, the mechanism that controls the first stages of planet formation, as well as the process that allows for the production of pharmaceutical tablets, and some foodstuffs.
Although aggregation of volcanic particles has already been studied in the last decades, the importance of developing an accurate model became evident during the 2010 Eyjafjallajökull eruption in Iceland, when the European airspace was closed for several days due to the ash-related hazard to aviation. Since ash aggregation exerts a first-order control on ash dispersal, accurate modelling of aggregation processes it is of primary importance to estimate ash concentration through time and space.
Despite field and experimental investigations performed in the last decades have allowed to reach a good overall understanding of aggregation processes, and sophisticated numerical models have been proposed, mechanisms that lead to particle sticking are not yet completely understood. Moreover, the regions in which aggregates are most likely to form are not yet identified.
In this thesis, we provide a theoretical framework for modelling the sticking of volcanic particles in the eruption column, we quantify the role played by all the main parameters for particle aggregation in the eruption column, in the spreading cloud, and for particles settling in the atmosphere. At first, we show how sticking maps can be drawn from thermodynamic profiles of the plume, for both wet and dry aggregation. Then, we compute the sticking maps for a particular plume in order to study how the different parameters affect aggregation processes. The progressive water saturation of the mixture with height, combined with a progressive slowdown of the plume is found to significantly increase particle sticking. Aggregation in the cloud is also studied: the slowdown of the mixture at increasing distances from the eruptive vent leads to an enhancement of sticking for particles and aggregates whose terminal velocity is low enough to stay in the spreading cloud. Finally, the study of aggregation of settling particles shows that the change of the collision mechanism (differential settling velocity instead of differential coupling with the flow), leads to a higher sticking efficiency between objects of similar sizes.
The role of charge on the aggregation of volcanic particles is then investigated theoretically for the case of settling particles. It is found that the collision efficiency is significantly increased for finer particles, while the sticking efficiency is slightly decreased. Moreover, including the effect of electrical forces on the aggregation of volcanic particles leads to a finer grain size distribution than if particles are neutral. Finally, the presented information is put together to determine what types of aggregates are likely to form in different regions, and what is the role of electrical forces in their formation.
Additionally, we carried out a field characterisation of aggregates which provides for the first time measurements of aggregate charge.
