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Details of Grant 

EPSRC Reference: EP/N025245/1
Title: Evaporative Drying of Droplets and the Formation of Micro-structured and Functional Particles and Films
Principal Investigator: Bain, Professor C
Other Investigators:
Reid, Professor JP Bayly, Professor A Wilson, Dr M
Gaskell, Professor PH Veremieiev, Dr S
Researcher Co-Investigators:
Project Partners:
AKZO Nobel Bristol-Myers Squibb Pharm Research UK Centre for Process Innovation Limited
Chiesi Limited Croda (Group) Danone Nutricia
Hovione (International) Inca Digital Printers Ltd Kuecept Ltd
Merck & Co Inc (MSD) Nestle SA Procter & Gamble
Sun Chemical Syngenta
Department: Chemistry
Organisation: Durham, University of
Scheme: Standard Research
Starts: 01 October 2016 Ends: 30 September 2020 Value (£): 2,270,296
EPSRC Research Topic Classifications:
Fluid Dynamics Particle Technology
EPSRC Industrial Sector Classifications:
Chemicals Pharmaceuticals and Biotechnology
Electronics
Related Grants:
Panel History:
Panel DatePanel NameOutcome
19 Feb 2016 Future Formulation FULL Announced
Summary on Grant Application Form
'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.
Key Findings
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