Industrial inkjet technology involves the generation, manipulation and deposition of very small drops of liquid (typically 20-50 um diameter) under digital control. The UK is recognised internationally as a leader in this area. Inkjet technology already dominates the desktop printing market. In commercial printing, it is rapidly becoming established for short-run applications and has, in only a few years, conquered a market previously occupied by conventional screen-printing equipment, where its great flexibility and inherent scalability give significant advantages. If higher printing speeds and greater quality can be achieved, then it will also be able to move into large-volume commercial printing. Apart from these printing applications, novel opportunities for inkjet deposition are also beginning to be exploited commercially in the manufacturing of high-value, high precision products (e.g. flat-panel displays, printed/plastic electronics, photovoltaic cells for power generation). By extending the existing benefits of inkjet methods (e.g. flexible, digital, non-contact, additive) to attain the speed, coverage and material diversity of conventional printing and manufacturing systems, we can transform inkjet from its present status as a niche technology into a group of mainstream processes, with the UK as a major player. But in order for this transformation to happen, we need a much better understanding of the science underlying the formation and behaviour of very small liquid drops at very short timescales, and to widen the range of materials which can be manipulated in this way, especially to allow fluids with high solids content (i.e. colloidal fluids) to be deposited. This cross-disciplinary programme of research is strongly supported by a consortium of nine UK-based companies and will bring together established research groups from three major UK universities to study three themes focused on key aspects of the industrial inkjet process: the formulation, rheology and jetting behaviour of colloidal printing fluids; understanding and controlling dynamic micro-scale drop impact, spreading and fixing; and development and validation of an advanced process model for industrial inkjet. Within these themes we aim to: develop a theoretical and practical understanding of how to make stable high solid-content colloids suitable for inkjet deposition, and how they behave in an inkjet system and on the substrate; explore post-impact processes that determine the structure and functionality of the printed features, including surface morphology, chemistry and surface treatment, fluid dynamics of wetting and the interaction of successively printed materials; and develop a set of models, validated by precise measurements and underlying physical theory, to describe all aspects of the formation and ultimate fate of ink drops. Industrial beneficiaries will include companies in the fields of inkjet printing and digital manufacturing, as well as other companies involved in the precise manipulation of small liquid droplets: examples of sectors include pharmaceuticals, agrochemicals, combustion, coating application, materials processing, and particle technology. Academic beneficiaries, apart from researchers working directly on inkjet technology, will include those in the fields of rheology, fluid mechanics, microfluidics, materials science and surface engineering.

'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.