New Frontiers in Aerosol Particle Measurements

Aerosol particles play a critical role in a broad range of disciplines, extending from their impact on atmospheric chemistry and physics, to their use in the delivery of drugs to the lungs and their impact on human health, through to their use in the delivery of fuels for combustion and for fabricating functionalised micropaticles in spray-drying. Aerosols are highly dynamic, variable in size and heterogeneous in composition. Physical and chemical processes can span timescales from the nanosecond for molecular processes at the surface of a particle, extending through to oxidative aging in the atmosphere over a time period of many hours. Particle sizes extend from the nanometre size of nucleation clusters to the millimetre size of rain drops. The size of a particle evolves through the condensation or evaporation of volatile and semi-volatile gas phase components or through particle-particle interaction and collision leading to coalescence. The coupling of mass and heat transfer during condensation and evaporation in aerosol has been widely studied. However, the process of coalescence has been much less well characterised despite its importance in dense aerosol plumes (eg. in fuel delivery and spray drying), in the scavenging of charged aerosol particles by cloud droplets in the atmosphere, and in the agglomeration of solid particles in the delivery of drugs using dry powders. Indeed, the mechanisms of particle-particle interactions and the actual process of coalescence are of considerable fundamental scientific interest. Experiments will provide an opportunity to test models of the interactions of dielectric charged particles and the theory underpinning the shape of inviscid or viscous liquid droplets developed by eminent scientists such as Lord Rayleigh, Lamb and Chadrashekar.

We have developed optical tweezers as a versatile platform for studying either individual particles or arrays of aerosol. Using light to capture and manipulate particles, we can change the separation of particles and bring them to close approach and even coalescence. From Raman spectroscopy, we can determine the size of spherical particles with nanometre accuracy and can determine their refractive index with high accuracy (<0.03 %). Particles can be loaded one at a time into an optical trap using droplet-on-demand generators and can be given a chosen amount of charge. The gas phase composition, temperature and pressure around the trapped particles can be controlled. We will use this platform to investigate in detail the interactions between particles and the coalescence process.

More specifically, by varying the composition of the aerosol and the gas phase relative humidity, we will be able to both measure the rate of the molecular diffusion of water within a particle and the viscosity of a particle by following the relaxation in particle shape to a sphere after coalescence. Indeed, in preliminary work we have shown that we can measure the viscosity over an unprecedented range of more than 12 orders of magnitude. These measurements will provide invaluable insights into the relationship between the molecular and macroscopic scales, exploring the phase behaviour of materials through the competition of nucleation and crystallisation with the slow kinetics of diffusion. We will also investigate the interplay of attractive and repulsive forces between particles of like charge, comparing measurements with recently developed theories. Finally, important processes such as contact freezing (thought to be important for ice nucleation in the atmosphere) will be studied in a controlled way for the first time.

In summary, using versatile optical techniques, we will investigate at a fundamental level and in applied contexts a new frontier in aerosol research that has largely been unexplored to date, providing us with an opportunity to study problems in fundamental physical chemistry and in applied aerosol science.

Aerosols play important roles in the atmosphere, in the delivery of drugs to the lungs and fuels for combustion and in industrial processes such as spray drying. Understanding the factors that control the size distributions of particles in these settings is crucial. Coalescence is a fundamental process that has received little attention because of a lack of robust methods for addressing it. A fuller characterisation of interparticle interactions and particle coalescence will result from this work and will improve predictions of evolving size distributions, not only in the aerosol science community but more widely in colloid science.

In the atmospheric context, indirect effect of aerosols on clouds through changing droplet size distributions, cloud albedo and lifetime were recognised in the 2007 report by the Intergovernmental Panel on Climate Change as representing one of the largest uncertainties in quantifying and predicting climate change. Better characterising the kinetic factors governing, for example, the scavenging of charged aerosol by cloud droplets will improve our understanding of interstitial aerosol in clouds, aerosol particle lifetimes and cloud droplet size distributions. An improved understanding of the mechanisms of phase change on particle coalescence (contact freezing) leading to ice particle formation will contribute to reducing the uncertainty in the radiative forcing by ice clouds, omitted entirely from the last IPCC report because the uncertainties were assessed as being too large to quantify. Thus, the atmospheric science community will be wider beneficiaries of this work.

In the delivery of drugs to the lungs, the particle size distribution is considered to be key in determining the depositional patter of drugs in the respiratory tract. Better understanding the coalescence of particles in the dense spray formed from medical nebulisers and metered dose inhalers through quantifying (potentially controlling) interparticle interactions will provide long term benefits to understanding drug efficacy. A further problem in the use of dry powder inhalers is the separation and suspension of small active pharmaceutical ingredient particles from the much larger excipient carrier particles, impacting on the dose delivered to the respiratory tract. This process is the reverse of the coalescence studies described in this proposal and will benefit from the studies of interparticle interactions between dielectric particles. In the long term, the fundamental work described in this proposal will lead to greater understanding of the aerosol processes at the core of the delivery of drugs to the lungs.

Further, the fabrication of microparticles by spray drying finds widespread application in the food, pharmaceutical, paints, ceramics and catalysis sectors. At the heart of the process is the rapid evaporation of solvent leading to solute rich particles in which molecular diffusion can become slow and even arrested, and phase transitions controlled. The measurements that will be performed here will, as for the earlier areas of impact, provide fundamental insights into the processes occurring in single particles and, thus, in particles in the ensemble. Better control of spray drying processes from an improved understanding of the aerosol dynamics occurring, will in the long term permit improvements and efficiencies to be made in this industrially important processes.

The potential impact of the work in the areas of atmospheric science and the delivery of drugs to the lungs could lead to improved quality of life through an improved understanding of the role of aerosols in governing climate and air quality, and improved efficacy of drugs for treating respiratory and systemic illness. Improvements to industrial processes such as spray drying would lead to economic competitiveness and efficiency, and increases in the functionalities of the materials that can be fabricated and produced.