Aerosols consist of liquid droplets or solid particles dispersed within a gas phase (typically air). Such droplets and particles can range in size from nanometres to millimetres. Aerosols are widely used to treat asthma via inhalation of therapeutic drugs and, in principle, enable the treatment of systemic diseases and the delivery of vaccines. They also find widespread application in consumer and agrochemical products, are prevalent in the atmosphere as particulate matter (PM) affecting air quality and human health, and are vehicles for the transmission of respiratory pathogens such as SARS-CoV-2, the virus responsible for COVID-19, and the bacterium responsible for tuberculosis. In all cases, the dispersed phase is dynamic, changing rapidly in moisture content and particle/droplet size during transport in the atmosphere, and often interchanging phase. Further complexity arises in most real-world systems: the droplets/particles can be multiphase consisting, for example, of dispersed solid nanoparticles within a liquid host droplet. Understanding such complex multiphase systems is crucial for designing pharmaceutical formulations to deliver drugs to the lungs, controlling the drying kinetics and engineered final particle structure in industrial processes such as spray-drying, and rationalising the airborne survival of viruses and bacteria in exhaled respiratory aerosol. Despite the importance of this broad range of problems, there are very few relevant studies of the dynamic transformation of aerosol droplets containing dispersed nanoparticles. We will integrate complementary expertise at the Universities of Bristol, Manchester and Sheffield to investigate the many physicochemical parameters that control the stability and structure of dried microparticles formed from solution aerosol droplets containing nanoparticles. The Bristol team has developed an array of state-of-the-art experimental methods to study the evaporation and drying of aerosol droplets in real time by monitoring their evolving size, composition, phase state and structure, while also capturing the final dried microparticles for post-mortem analysis. At Manchester, the team has extensive modelling capabilities to simulate the drying kinetics of evaporating aerosol droplets to account for changes in fluid viscosity, composition and temperature. The Sheffield team has developed synthetic routes to produce tailored polymer nanoparticles of varying size, shape, and surface chemistry in water, polar solvents or non-polar solvents, including the bio-inspired synthesis of several virus mimics. This combined expertise will enable us to examine a wide range of nanoparticles of selected size and character at known concentrations within host liquid droplets. Such nanoparticle-loaded droplets will be generated with reproducible size in a controlled environment of known temperature and gas phase composition, and their evaporation will be studied in real time (on timescales ranging from milliseconds to hours) through to the point of solidification. The structure of the final dried microparticles will be examined by scanning electron microscopy. These experiments will be compared with model predictions of evolving particle size and composition, and the structure and moisture stability of the microparticles will be evaluated. Ultimately, these observations will enable us to develop a framework for predicting how the various microphysical processes that occur during drying and the character of the nanoparticles within the host droplets affect the final microparticles. Working closely with industrial partners with expertise in the pharmaceutical, consumer product and aerobiology sectors, we will establish robust physical principles for understanding the dynamics occurring in aerosols of complex composition and phase in domains extending from drug delivery to the lungs to spray-drying of commercial products to mechanisms of disease transmission.

'Watching paint dry' is a metaphor for a boring and pointless activity. In reality, the drying of liquids is a complex process and the imperturbable appearance to the eye can hide a wealth of dynamics occurring inside the liquid. The effect of these internal processes is to change the distribution of materials in the deposit left after drying. We are all familiar with the coffee-ring effect, where split coffee dries to form a ring of solids at the edge of the spill - of little use if you are trying to coat a surface uniformly. This project is all about the drying of droplets, either in air or on a surface; one isolated droplet, two droplets merging or many droplets in a spray. We seek to understand how drops dry and how to control where the particles or molecules in the drop end up after the drop evaporates. When do you get a solid particle or a hollow particle? A round one or a spiky one? A uniform particle or one with shells? Or on a surface: a coffee-ring or a pancake? A uniform deposit, a layered one or a bull's eye? Are particles crystalline or amorphous, are different components mixed or separated? There are a myriad of possibilities for controlling the microstructure and properties of the final particle or film. Drying is complicated for three main reasons. First, many transport processes (evaporation, heat flow, diffusion, convection) occur simultaneously and are strongly coupled. For example, in a small droplet of alcohol and water evaporating on a surface, the liquid inside the drop will flow around in a doughnut pattern tens of times each second. Second, the conditions in a drying droplet are often far from equilibrium. For example, a small water droplet in air or on a smooth clean surface can be cooled to -35 degrees C without freezing. So to understand drying one needs to understand the properties of fluids far from equilibrium. It is generally not possible to predict the final outcome of drying from the properties of simple solutions near equilibrium. Third, drops do not dry in isolation. They may merge or bounce, coalesce or chase each other across a surface. The evaporation of one droplet affects its neighbours. Moving droplets change the flow of air around other droplets, coupling the motion of droplets. Why does anyone care, beyond the intellectual fascination with the bizarre outcomes of droplet drying? Drying of droplets turns out to be a rather important process in practical applications: spray painting, graphics printing, inkjet manufacturing, crop spraying, coating of seeds or tablets, spray cooling, spray drying (widely used in food, pharmaceutical and personal care products), drug inhalers and disinfection, to give a few examples. The physics and chemistry underlying all these applications is the same, but if manifests itself in different ways and the desired outcome varies between applications. The first challenge addressed by this project is one of measurement: how do you work out what is going on in a droplet that is less than a tenth of a millimetre across and may dry in less than a second? We have already developed sophisticated measurement tools but will need to extend these further. Another challenge is one of modelling: to understand the drying process we need a theoretical framework and computer models to explain - and predict - experimental observations. We will begin looking at the fundamental processes occurring in single drops in air and on a surface and then explore what happens when drops interact or coalesce. This fundamental understanding will be fed into improved models of arrays, clouds or sprays of droplets that are encountered in most practical applications (such as spray coating, spray drying, inhalers or inkjet manufacturing). We will use an Industry Club to engage with companies from a range of different sectors. This Club will provide a forum for sharing problems, ideas and solutions and for disseminating the knowledge generated in the project.