Controlling the Morphology and Phase of Dried Aerosol Particles

Abstract

Understanding aerosol droplet drying behaviour and its influence on resultant dried particle morphology is
crucial for reducing uncertainties in climate models, mitigating airborne disease transmission and improving
efficiency in food/pharmaceutical powder production. The fundamentals of droplet drying are well
understood and there has been significant research into the relationship between aerosol droplet drying and
the morphology of resultant particles for simple systems (e.g. aqueous single salt droplets). However,
investigations into more complex systems that better represent real-world aerosols are still needed.
In this thesis, I first explore the effect of introducing additional soluble inorganic salts to an aqueous NaCl
droplet. Atmospheric aerosols contain complex mixtures of multiple components, but previous research has
focused on single component systems. I show in this work that the effect of evaporation rate on particle
morphology for complex mixtures follows the behaviour of single component salt droplets and the
homogeneity of the resultant particles was shown to be compositionally dependent. This has implications
for improving the accuracy of climate models where understanding the relationship between environmental
relative humidity (RH) and temperature, and particle phase, morphology, optical properties and Cloud
Condensation Nuclei (CCN) behaviour is essential.
I then move to focusing on insoluble components in aerosol droplets, looking at the effect of included species
size on drying behaviour and morphological development of aerosol droplets. Diblock co-polymer
nanoparticles were used, with the polymer type and nanoparticle size being shown to have no effect on the
evaporation rate of the drying droplets at the solution concentrations investigated. However, increasing the
nanoparticle size and/or increasing the RH, was shown to increase the sphericity of the resultant dried
particle. Indeed the microstructure of the resultant dried particle was shown to be predictable using
evaporation kinetic data and included particle type. This discovery has implications for the spray drying
industry where control of generated particle properties is key.
Finally, the applicability of single drying droplet techniques for predicting spray dried particle properties
was assessed by recreating the particle morphologies of spray dried dairy powders in a falling droplet
column. Determination of the drying regime at temperatures found in a spray dryer (using the dimensionless
Péclet number) allowed said drying regime to be recreated at ambient temperatures by manipulating the
solution concentration and humidity. The effect of carbohydrate content, protein type and fibre content on
the drying behaviour of dairy-based droplets was also investigated. The success in recreating spray dried
powder morphologies has implications for industrial spray drying processes, where using single droplet
drying techniques can save significant time and money.

Date of Award	17 Mar 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jonathan P Reid (Supervisor) & Rachael E H Miles (Supervisor)