The complex dynamics in a single droplet

One of the challenges for inkjet and 3D printer manufacturers is to ensure that the ink or printing fluid stays liquid in the printing channel, but dries very quickly after drop impact. One of the organizations working closely with the UT on cutting-edge research in this area is Océ, a Canon Company and a world leader in high-end professional printing. Despite not being active in the development of 3D printing technology – its focus is on high-speed printing systems for commercial printing – Océ is an eager participant in fundamental fluid dynamic research projects, because they are important for printer development today and in the future. UT scientist Detlef Lohse explains: ‘One of the challenges is that ink is not usually a pure fluid, but a mixture of substances, each of which may behave very differently. How does the evaporation process take place in such a multi-component material? Any alcohol included in the mixture will evaporate quickly, leaving the water behind. Because evaporation starts at the edge of the droplet before reaching its centre, the fluid comes into motion. Gravity can play a role and thermal effects come into play: evaporation costs energy, which means that the droplet cools down. All in all, the interplay of the forces at work in a multi-component material can be highly complex and difficult to fathom. The ouzo-effect we published on in 2016 – in which the anise oil in ouzo, when you add water, forms micro droplets and becomes milky – is a well-known example of this. If the print mixture contains solid particles, as many 3D printing applications do, things becomes even more complicated.’

Printing technological or biological materials

3D printing materials are by definition multi-component materials. Lohse’s insights into droplet formation and other behavioural properties of such mixtures are paving the way for more accurate analyses and predictions. ‘Droplet formation in multi-component materials is not yet properly understood and can be very complicated, particularly because of interaction with the surface. Ultimately, the research we’re engaged in will not only make printing faster, more accurate and cheaper, it will also enable us to use many different multi-component materials in 3D printing. Consumers are certainly going to notice. Think of printing on textiles, and printing large-scale technological materials or biological material, such as organ printing or printing skin cells onto a burn.’

Printing structures with live cells

At the beginning of this year Lohse was involved in the introduction of ‘in-air microfluidics’, a new technology for printing structures with live cells. ‘With in-air microfluidics we allow micro droplets to merge in the air. As a result, we get printable, viable micro building blocks very quickly. We can use these, for example, to repair damaged tissue. These are revolutionary developments in printing technology. 

‘Our children will benefit’

There are still many challenges, recognizes Lohse, who has been distinguished frequently for his work, for example, with the two most prestigious awards in the world of fluid dynamics: the Fluid Dynamics Prize 2017 of the American Physical Society (APS) and the George Batchelor Prize in 2012. ‘We still need to learn to better control the transition phase substances go through – from liquid to solid. Another challenge will be to apply this knowledge in cheap and effective ways. But the progress we are currently making is remarkable. The whole industrial printing process will change. I am sure that our children will benefit from these technologies.’

PROF. DR. DETLEF LOHSE
prof.dr. D. Lohse (Detlef)
University professor in Physics of Fluids, Faculty of Science and Technology, University of Twente | Studied Physics at the universities of Kiel, Bonn and Marburg

‘What drives me in my work as a scientist is pure curiosity. I see an issue and I get together everything I need to solve it. So the starting point is not a method, but a concrete question. What is the mechanism behind a certain phenomenon, how can I interpret and explain it? I have always been curious, from a young age. My father was an engineer; I wanted to know how things work. When I was younger I wasn’t sure whether I would go into physics, chemistry or mathematics, but it was clear that I wanted to work in science – I never had to think twice about that. And in what I do now all those disciplines come together.’