Electro-wetting driven contact line dynamics
Promotion date: September 5.
Promotor: Prof.dr. Frieder Mugele
Assistant promotor: Dr. Dirk van den Ende
The research presented is part of the FOM Industrial Partnership Program “Contact Line Control during Wetting and Dewetting” (CLC) in collaboration with ASML and Océ.
In this work, the focus was primarily on the behavior of drops and liquid-air-solid contact lines under the influence of electric fields, and how these fields can be used to answer fundamental and industrial questions about contact line dynamics. Our focus was especially on the immersion lithography system, and how electric fields can be used to minimize defects: either by stabilizing the contact line against the pearling instability, or by better understanding of how to remove drops, created by this instability, without leaving defects.
A spatially homogeneous but time-varying (AC) electric field can make a pinned or slow-moving drop more mobile. The resulting decrease in driving force needed to achieve drop motion, is exactly described by the decrease of the static contact angle hysteresis, even for drops sliding at several tens of cm/s. This could be applied to detach drops on car or airplane windows, removing the need for windscreen wipers.
Defects created by spatially inhomogeneous electric fields can serve as model systems for trapping on geometric and chemical defects, but also as a tool for controlling drop motion. Further, the possibilities of steering drops was shown. These could be used, for example, to steer condensate or raindrops (perhaps mobilized as described above) to a specific drainage point. Contact lines at high velocities were also observed. We found that the combination of high velocities with AC electrowetting does lead to new effects. Another important observation was a very strong correlation between the drop geometry and contact angle as function of velocity.
Was your research application driven or more fundamental in nature?
Quantifying, modelling and controlling pinned, slow-moving and even fast-moving drops was the main problem solved in my thesis work. We succeeded to come up with a good predictive model on this. Showing that fast moving drops were more easily removed by applying AC fields was the most surprising feature of this research project.
A lot of experimental work was involved. Though several fundamental issues were tackled, the main goal of research was application driven, as the steering of droplets without leaving mechanical defects is of prime importance in future generation applications. Trying to avoid capture of air bubbles by applying electric fields at the right moment might also be helpful to solve problems caused by drops stuck on these surfaces. However, further research is needed to find new successful strategies to do so, for example by applying the fields at exactly the right moment.
Furthermore, showing that electrical defects can create arrays of droplets might open new possibilities to study detection sensors in the future. Electrical defects would be a great advantage as they can be controlled accurately, so drops could be stopped, analysed, and discarded in a continuous process, as one drop is analysed while another is just coming in and a third is just being discarded.
Also horizontal defects and the new possibilities of the steering of droplets, can be of use in de-wetting critical surfaces, for example in aviation or in cleaning car windows. A case on this in which I supervised student researchers, led to nice publications cited several times already.
Did some nice publications appear as a result of the various topics studied?
One publication appeared in Applied Physics Letters already, hopefully one more can be added in the near future. A publication is now submitted to Langmuir with another to follow. Finally an article was published in the Journal of Colloid and Polymer Science.
How did you develop personally as a scientist and researcher during this four year period?
I was pleasantly surprised, noticing that my writing and presentational skills improved a great deal. For example, I am now much more sober in using texts in my slides and poster presentations. Winning two poster prizes was very rewarding, and served as proof of these improved abilities.
I am much more effective in constructing explanation models these days, filling in the details in a much later stage now. My intuition has grown considerably, as to which factors are vital for a simple but yet sound first theoretical model. Here I admire the skills of my promotors, as they are able to find these vital factors on short notice. They remember a large amount of theoretical knowledge by heart, which is necessary to speed up the scientific output of one’s work as a scientist in this field.
What are your future plans?
I was applying already at knowledge institutes like Deltares and TNO. Also a post-doc in England is appealing to me, perhaps offering the possibility to work on de-electrowetting processes in oil environments which is similar to my expertise, yet in a different direction. Going abroad is not a necessity in my view but when chances are there, for example in England or in Germany, I might go for them.
A pure scientific career doesn’t suit me very well, I believe. Though I like to work with students in education projects or supervising them on a daily basis, visiting international conferences very regularly and writing funding proposals, seems a pretty tough task to me.
What, in your opinion, is important for Mesa+ to stay successful in future?
In everyday research I was more concentrated in collaborating with the technical experts of our own group than as a member of Mesa+.
The annual Mesa+ Day is of prime importance in my view. Here, I came into contact with colleagues working on issues related to my work, sometimes in a very surprising way, for example regarding contact angles of droplets on special surfaces. I think facilitating these contacts could be organized even more. The expertise of Mesa+ is combining physical, biological and chemical processes on different nanotechnology topics and areas. So, exchanging ideas is a vital part of its success.