Robin Berkelaar

Nanobubble-like objects at solid-liquid interfaces

Promotion date: September 19.

Promotors: Prof.dr.Detlef Lohse Harold Zandvliet

Assistant Promotor: Dr. Stefan Kooij

Bubbles of nanoscopic size are expected to dissolve in milliseconds, while these surface nanobubbles have been observed with lifetimes of several days. A number of applications have been suggested for which the use of surface nanobubbles could be beneficial. It might enhance the cleaning efficiency of particle or protein contaminated surfaces, for example.

This thesis is mainly on the procedures and systems which cause the unwanted formation of nanobubble-like objects and their properties. Atomic force microscope (AFM) is the most commonly used technique to study nanobubbles. The soft nature of surface nanobubbles results easily in an apparent nanobubble height and radius which is smaller than their actual dimensions.

The “bubbles” were exposed to a flow of degassed water or to a lowered ambient pressure in order to degas the nanobubble-like objects. Also the substrates for the nucleation of surface nanobubbles were studied in detail. The results reveal that non-gaseous features can easily be mistaken for surface nanobubbles.

In the past, disposable needles which leech PDMS contamination into the water, were often used to deposit water onto the substrates. The stability in thin liquid films of surface nanobubbles is reinterpreted, using the knowledge that these objects are not gaseous surface nanobubbles but PDMS droplets. Experimental techniques are reviewed. Here, the emphasis lies on the ability to distinguish between gaseous surface nanobubbles and nanobubble-like objects. Unfortunately, it appears no standalone experimental technique is suited to make this distinction unambiguously.

Your research was in some respects contra-intuitive.

Yes, it was. My research offers a new way of looking at nanobubbles, which have shown unexpected long and stable lifetimes. For about fifteen years already, researchers have been looking for explanations to account for this phenomenon. At the same time the techniques to nucleate these nanobubbles were never straightforward, as it is difficult to work with liquids and surfaces and keep everything during the process nanoscopically clean.

Investigating this problem in detail - beginning from the fabrication procedures used to create these bubbles - we found that using disposable needles contaminated our surfaces with PDMS. This resulted in the conclusion that the alleged nanobubbles were in fact PDMS nanodroplets when disposable syringes were used. I was able to make this statement plausible by performing degassing and low-pressure experiments. Furthermore, it is not straightforward to distinguish between these droplets and bubbles by AFM techniques nor by X-ray spectroscopy techniques.

When confronted with these findings two distinctive responses from researchers around the world were prevalent. Some of them look at the results in a serious manner and carefully reinterpret the results, investigating their own fabrication procedures in a critical way. This view I value greatly, as some of their experimental results stem from periods more than ten years back. Unfortunately, in other cases, scientists were unwilling to think of the possibility nanobubbles could, in some cases, be an artifact. They opposed my insights from the mere beginning.

In the PhD-project I learned to perform conclusive experiments and subsequently convince others of my findings, surprising as they may seem. Also I learned to present results like this in a political correct way: draw attention to them by concentrating on arguments alone.

Can you recall some special moments during your PhD period?

In the beginning some phenomena were elusive. For example, more nanobubbles - or nanodroplets, as they turned out to be - originated at the early hours of the day. In the end this phenomenon could be attributed to the needles used, as after frequent use of one needle the PDMS concentration weakened, leading to a decrease of formed droplets.

Although this result was not expected (or hoped for) by TNO, as one of the funders, this subsidiary organization was nevertheless content with the results obtained. One of the expected applications might have been the cleaning potential of nanobubbles in fabrication environments of next generation chip machines. My results give rise to more realistic expectations for procedures like this.

In what leading magazines did you publish your results?

Articles were published in Langmuir and in Soft Matter. The issue of polymer contamination led to quite some citations, averaging one each month.

What are your future plans?

I am not planning an academic career as pursuing this would involve a somewhat migrant way of life for the years to come, in order to broaden my knowledge level and academic skills at different universities all over the world. Right now I am looking for some permanency, be it in industry or in another kind of institution, like a patent office or even a financial institution. Here some vast interest exists for people being able to work on an academic thinking level coming from a quite different background, for example a mathematical or technological one. I am convinced to be able to build up an economic or juridical knowledge base, and combine this with my background as a physicist. By doing so I might be of added value to this kind of organizations. That seems a good challenge to me.

In what way did you develop personally, as a researcher and scientist?

During the PhD period I became more on par with my supervisors and colleagues, feeling much more free to bring in my own ideas and act like a sparring partner during discussions. Also I learned to be more critical towards scientific articles. Though published in renowned papers, or stemming from research groups with great reputation, errors and mistakes are still possible. Therefore the reproducibility of experiments is an essential part of scientific research.