Evaporation Kinetics and Particle Formation from Aerosol Solution Microdroplets

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The evaporation of liquid droplets is an important problem across a range of industries, such as spray drying and respiratory drug delivery. Droplet drying can be highly complex, and the literature lacks a detailed understanding of how evaporation kinetics can affect the process of morphology development and particle formation, and the resulting dried particle properties.
This thesis presents experimental results of evaporation measurements performed using an electrodynamic balance (EDB). Single aqueous droplets were levitated in the instrument and light scattering methods were used to measure the radius throughout evaporation, as well as to detect crystal nucleation induced by a rising solute concentration during drying.
The evaporation of binary droplets containing two volatile solvents (ethanol and water) under a range of different conditions was studied. By comparing the evaporation profiles of droplet size vs. time to various numerical models, including a modified Maxwell model and a binary Kulmala model, the time-dependent composition and temperature of the droplet was studied. These studies elucidated the impacts of the coupling of heat and mass transfer during rapid evaporation.
The evaporation of aqueous droplets containing a range of inorganic solutions was studied using the EDB. The time that crystal nucleation occurred in an evaporating droplet was recorded. A numerical diffusive transport model was compared to the experimental data to provide insight into the evolving gradients in concentration during drying. Aqueous NaCl and NaNO3 droplets were shown to undergo high levels of solute enrichment at the droplet surface as water evaporated, allowing predictions of homogeneous nucleation rates and reconciliation with the measured phase behaviour. Electron microscopy images of dried particles were recorded, collected from drying streams of droplets in a falling droplet column (FDC) under identical evaporation conditions to the EDB. Thus, experimentally measured drying rates, modelled concentration profiles and microscopy images were compared to demonstrate how varying the drying conditions can control the final particle phase and morphology in inorganic aqueous solutions.
Finally, the evaporation of droplets containing mixtures of solutes was studied. The propensity for an inorganic solute (e.g. NaNO3) to undergo crystal nucleation was shown to vary depending upon the presence of other inorganic solutes, organic solutes that affect the droplet surface composition, and SiO2 nanoparticles that act as heterogeneous nucleation sites. This represents a study into the ability to predict and control crystal nucleation in rapidly evaporating aerosol droplets.
Date of Award23 Jun 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJonathan P Reid (Supervisor) & Bryan R Bzdek (Supervisor)

Keywords

  • Aerosols
  • Evaporation
  • Crystallisation
  • Droplets
  • Electrodynamic Balance
  • Physical chemistry

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