Exploring the Factors that Govern Particle Resuspension with a 3D Printed Wind Tunnel

Abstract

Particle resuspension, the process of once airborne particles detaching from a surface and being transported back into the atmosphere, is an omnipresent phenomenon. Resuspension is weaved into quotidian activities with walking, driving and cleaning all recognised sources of particle detachment,alongside non-anthropogenic mechanisms that result in the aeolian transport of sand and bioaerosols. Despite the persistence of this secondary aerosol source, models that attempt to simulate resuspension are either constrained to specific conditions or an idealised scenario of hard spherical particles being
removed from smooth surfaces. Neither reflect the true complexity of particle resuspension that is influenced by an extensive list of governing factors. Untangling this complexity and understanding the mechanisms that drive particle resuspension is a long term objective of resuspension research.
A novel method for characterising variations in resuspension with differing particle properties is introduced in this thesis. A quadrupole electrodynamic trapping instrument (the QuadFab) was constructed to fabricate particles of a reproducible morphology that were then resuspended in a small-scale 3D printed wind tunnel. The two novel apparatuses combine to offer an accessible route for experimental resuspension research that is not reliant on cumbersome wind tunnel equipment and delivers greater control of particle properties and surface deposition. This methodology provides the opportunity to accelerate resuspension model development by evaluating the influence of factors such as collisions, particle-surface material combinations and particle morphology; the latter being the focus of this thesis.
The influence of two sodium chloride particle morphologies on resuspension efficiency was investigated with raspberry shaped particles compared to flat platelets. The raspberry shaped particles demonstrated a greater susceptibility for detachment, yet in both cases limited resuspension was observed in the wind tunnel apparatus and it was challenging to characterise any mechanistic changes. Consequently, high optical and time resolution imaging techniques were employed to monitor particle displacement at the moment of detachment. This method highlighted that traditional force balance assumptions applied to spherical particle resuspension are not appropriate for irregular particles and
that area-related shape parameters have the greatest correlation to resuspension likelihood.
This novel insight into the morphological influence on resuspension provides scope for introducing non-spherical particles into idealised models. One such model, the “Rock n’ Roll” model, was adapted to include the predicted increase in adhesion forces when transitioning away from sphericity. Ultimately, this adaptation was not sufficient to completely capture irregular particle resuspension. However, validation experiments comparing the model to idealised empirical data, collected with the novel methodology, demonstrate the opportunity for future model developments.

Date of Award	18 Mar 2025
Original language	English
Awarding Institution	University of Bristol
Sponsors	Defence Science and Technology Laboratory
Supervisor	Jonathan P Reid (Supervisor)