The evaporative drying of droplets is an important process with widespread practical applications including in spray painting, inkjet manufacturing, crop spraying, the coating of seeds or tablets, spray cooling, spray drying (widely used in food, pharmaceutical and personal care products), the use of drug inhalers and disinfection. The physics and chemistry underlying all of these applications, although the same, manifests in different ways with outcome varying between applications.

Numerous experimental and modelling challenges must be addressed to explore the evaporative drying of aerosol droplets. The first challenge is one of measurement: how can such dynamic processes be studied in a droplet that is smaller than the diameter of a human hair which dry in less than a second? A second challenge is one of modelling: to understand the drying process we need a theoretical framework and computer models to explain - and predict - experimental observations. 

The drying of liquid droplets is a complex process with the evaporative flux, internal diffusion and mass transport controlling the distribution of materials in the final dried particle and the morphology. Using a range of single droplet techniques, including an electrodynamic balance and falling droplet column, we aim to investigate how the final dried particle state can be controlled, forming solid or hollow particles, spherical or cubic, uniform or one with shells. In addition, we are exploring the factors that determine whether final particles are crystalline or amorphous, mixed in composition or phase separated, a myriad of possibilities for the final particle microstructure and properties. Many transport processes (evaporation, heat flow, diffusion, convection) occur simultaneously and are strongly coupled. The conditions in a drying droplet are often far from equilibrium. For example, evaporative colling for a small water droplet in air can be as large as -35 degrees C without freezing. As such, it is generally not possible to predict the final outcome of drying from the properties of simple solutions near equilibrium. Further, drying particles may merge or bounce, coalesce or impact on a surface, and the evaporation of one droplet may affect neighbouring droplets.

Our focus is to study fundamental processes occurring in single drops in air 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).