Claas Willem Visser


Promotion date: October 31.

Promotor: Prof. Dr. rer. nat. Detlef Lohse

Assistant Promotor: Prof. Dr. Chao Sun

The dynamics of mm-sized droplets is compared

to µm-sized droplets. First, the generation and controlled impact of individual microdroplets is pursued. Three methods have been studied: generation of a laser-induced micro-jet; deflection of a single droplet from a droplet train, and laser-induced forward transfer.

High-speed microjets were generated by laser-induced vaporization of water in a micro-tube. A single droplet could be deflected away from a droplet train. Placing an impact surface in the path of this droplet resulted in highly reproducible impact events. By carefully optimizing the setup, impact of droplets

with a diameter of 50 µm and velocities of 50 m/s within a time window of 1 µs was achieved: sufficiently short to visualize the impact using flash-illumination. Reproducible impacts could be generated. The fastest and smallest droplets were generated using laser-induced forward transfer. However, these droplets were too small to visualize their impact.

Microdroplet impact appears to be equivalent to mm-sized droplet impact. The implication is that mm-sized droplet impact experiments can be tailored to make predictions for µm-sized droplets. For the first time, laser-induced forward transfer was applied for the fabrication of three-dimensional metal structures. A pulsed laser was focused on a thin metal film, which resulted in the ejection of small copper droplets. By repeating the ejection of copper micro-droplets at the same location, deposition occurs on the same position and pillars are manufactured.

Biofabrication is another novel application of microdroplets. Here, cells do not always survive impact events, which is a severe problem in several mainstream biofabrication technologies. Therefore, the cell deformation during impact was modeled. Predictions and experiments show improved cell survival for larger surrounding droplets, slower impact, lower droplet viscosity, and soft-surface impact,.

Was your research fundamental in nature or was it also application oriented?

By designing and performing clever and highly reproducible experiments, fundamental knowledge was gained concerning impact events of µm-sized droplets. Using our method we could go beyond the experimental performances of high-speed cameras, being able to analyze up to ten million high resolution frames per second.

The moments I remember best during my PhD period, were related to the potential for application of my experimental and theoretical work. Using experimental laser-induced set-ups for fabricating gold and copper pillars for example, proved to be very rewarding and really innovative.

Also I was able to combine my work in the first phases of the PhD-project with already existing models, in order to predict the survival rate of living cells within droplets during impact. Stretching of the cell membrane appeared to be vital in this, which was predicted quite well by including the droplet’s size and impact conditions in the model. The approach and modeling chosen, proved to work out very well. We were able to test the effectiveness of the model in several experiments.

Before starting your PhD you were working in industry. Was this an advantageous route?

I worked in the Rolling Metal Strip department at Tata Steel for four years. I was convinced I could further develop my knowledge and skills in research, and take my research substantively further. In my PhD project, which had both fundamental and applied aspects, this worked out very well.

The fact that the PhD position was my second job was advantageous as I already experienced important aspects of any first job, such as dealing with set-backs in complex projects and then to proceed from there. So, I started my PhD with a clear start ahead.

During my PhD I have learned about several new disciplines, including biofabrication and materials science involving metal fabrication techniques. Also collaborations, both within and external to the University of Twente, proved enriching for me. I collaborated with the Development Bioengineering Group, led by professor Marcel Karperien, and with the Applied Laser Technology Group, led by professor Bert Huis in ‘t Veld. These were vital partners in my research work. Also I collaborated with Göttingen University and LAM Research Corporation in Austria.

What are your future plans?

As a scientist my knowledge is applicable in a wide range of technologies. What I like about academia is that I can apply my knowledge to multiple topics. In a company, the research is usually aligned to the main technology of that company.

As is shown in my PhD-project, my work can be of societal impact on more than one topic. It is a great feeling to show and communicate one’s results to a broader audience. This is a main task in science, I believe.

It would be very satisfying if entrepreneurs picked up my findings in the future and transform them into serious products. Starting a company myself - resulting from my own work - could be a side-activity, but my main goal will always be to take research further in various areas. I better leave commercialization to people more skilled in that type of work, and who enjoy these activities.

One of my ambitions is to be a leader of my own research team one day. Until that time I hope to work passionately as a researcher, although I know teaching and writing grant proposals will be a vast part of the job. My PhD-work provides sufficient clues to be taken further, such as Cell FACS (Flow-assisted cell cytometry) in which we collaborated with the University of Göttingen, in order to ‘print’ and reproduce micro-droplets in a highly reproducible way.

In what journals did you publish your results?

Scientific publications are important, of course, but the main challenge is to be read as an author, preferably in different areas of research. Articles were published in Soft Matter, Biophysical Journal, and I was a third author in PNAS (Proceedings of the National Academy of Sciences) and Physical Review X.

Did you feel part of the Mesa+ community during your PhD research?

The work of our Group is not at the very heart of Mesa+ research. Nevertheless, we benefited considerably by the presence of a very well equipped cleanroom nearby, allowing us to perform advanced SEMS measurements for example. Also in metal printing and fabricating well-defined metal layers we were happy to share some good apparatus and – even the more important – we could approach experts from within Mesa+ to help us out. The working atmosphere within Mesa+ is very pleasant. Also, it was a good thing to see that the former cleanroom was open to companies doing research and to fabricate prototypes and first production series.