Evaporative Drying of Droplets and the Formation of Micro-structured and Functional Particles and Films

'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.

Who benefits from this research?

The primary non-academic beneficiaries of this project will be companies who (i) employ evaporative drying in manufacturing processes or (ii) sell products in which evaporative drying occurs during the use of the product. These companies work across a wide-range of sectors including protective coatings, decorative coatings, graphic printing, display technologies, pharmaceutical production (including tablet coating, spray-drying, inhalers and nebulisers), crop protection, wound protection, disinfection, inkjet manufacturing, ceramic production, food production and personal care products.

There is a much wider set of beneficiaries which includes those whose quality of life is enhanced by improved products. There is also the potential for significant health and safety and environmental benefits.

How do they benefit?

Companies benefit from an improved ability to troubleshoot, optimise and control existing processes and from the ability to design new, innovative, products and processes faster and more effectively. We give several illustrative examples. Optimising the distribution of a metallic particles in a conductive ink would give finer lines and lower ohmic losses after sintering. Controlling coalescence of ink drops of different colours would allow high-resolution single-pass inkjet printing, making it competitive in speed with offset lithography. Uniform distribution of an antimicrobial agent in sprayed disinfectant would inhibit the growth of bacteria on hard surfaces in hospitals. Optimising the distribution, morphology and phase of active ingredient and adjuvant in a systemic pesticide would reduce the dosage of pesticide needed, while controlling the evaporation of droplets in flight would reduce drift and adverse environment impact. Control of the drop size distribution on inhalation from metered dose inhalers and nebulisers could lead to the optimal regional delivery of drugs to the respiratory track for enhanced efficacy. Improved scale-up and understanding of spray dried solids dispersion will get new, poorly soluble drugs to the patient faster. Models which predict the build-up of materials on the wall of the spray dryer can enable the formulators to assess manufacturability without costly, time consuming, large scale trials.

The project is designed to allow developmental activities to occur in parallel with the research programme (though separately funded) so that benefits to partners start to accrue before the end of the grant.

Other benefits to companies include sharing of best practice through the Industry Club, employment of trained personnel (> 10) from the project, access to new experimental tools and software. We note that membership of the Industry Club is limited to companies who have R&D in the UK, who manufacture in the UK or who play a significant role in the UK supply chain, thus enhancing the competitiveness of UK industry.

Potential long-term benefits to the consumer are equally diverse (beyond better quality products at lower cost). Examples include improved healthcare outcomes, e.g. reduced infection rates, faster recovery with improved wound dressings, controlled release of drugs, predictive models of manufacturing technologies which rapid development of the large numbers of formulations that personalised medicines will require, more effective personal care products with lower production and environmental/energy impact.

There are also significant health and safety benefits that arise from controlling evaporation, particle size and particle morphology. For example reduction in overspray and drift and suppression of dust generation from coatings. More efficient delivery of products in an outdoor environment (e.g. spray painting, crop spraying) also reduces negative impacts on the natural environment.