New Frontiers in Aerosol Particle Measurements

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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.