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Sissie de Beer (promotion date: 27 May 2011)

Probing the properties of confined liquids

Promotion date: 27. May 2011

Promotor: Prof. dr. Frieder Mugele

Co-promotor: Dr. Dirk van den Ende

In this thesis is described: Atomic Force Microscopy (AFM) measurements and Molecular Dynamics (MD) simulation of the static and dynamic properties of layered liquids confined between two solids surfaces.

Next to average forces on the tip, force-fluctuations were monitored. MD-simulations confirmed the variations in the dissipative forces very similar to the experimental results, at the transition regions of 3-2, 2-1 and 1-0 layers. Spectral analysis suggests that the elastic and viscous response of the confined liquid is best described as either a gel or a soft glassy material.

During your thesis project, were there some special moments that you can recall?

Actually observing the oscillations at the molecular layers was a fantastic moment. This result was really difficult to achieve.

Using chain-molecules to further understand the AFM measurements performed, also was of major significance.

At last, we were able to understand the measurements in great detail by coupling the experimental results to the MD-simulations.

Although discussions in the field remain open, I believe we made some major internationally renowned contributions on a fundamental topic, existing from the early eighties on. My professor, Frieder Mugele, just received a FOM-grant to continue this field of research within the Physics of Complex Fluids group. That’s very rewarding.

Did your work lead to some nice publications?

At the first stage of the project my work already led to a publication in Applied Physical Letters, in 2008: Atomic Force Microscopy cantilever dynamics in liquid in the presence of tip sample interaction.

Langmuir, Nanotechnology and Microfluidics & Nanofluidics, published in the following years. In 2011 Journal of Physics/Condensed Matter, published: Confinement-dependent damping in a layered liquid.

Now we are working on publications with regard to the Molecular Dynamics part, which seem very promising.

How did you develop personally, as a researcher and scientist?

For example, I learned that simulations and experiments can be different means to reach the same goal, so to say. They can lead to comparable and complementary results. Perhaps one could say simulations can serve as a tool to discover relevant parameters on which to perform real great experiments.

More in general, a scientist has to learn to conduct his or her experiments and scientific work in an independent manner. I learned to set my own goals as clear as possible, trying to find ways to reach these. Perseverance is the key characteristic in reaching scientific success, I believe. It is more important than being smart, although that helps a lot, of course.

What are your future plans?

I am going to continue academic research, as I like the freedom to choose my own direction and daily routines to reach my goals.

An important next step in my career is joining the research group, led by Professor Martin Müser at the Forschungszentrum Jülich in a few months. Also theoretical work and simulations on liquids are important subjects there. The topic will have nice new features as well, for example the simulation of ‘real’ liquids and the interaction of liquids and solid materials.

Did you feel part of the Mesa+ community during your thesis project?

Not performing my experiments in the cleanroom, I didn’t always feel a Mesa+ member as other people might be. My experiments took place at the labs of the Physics of Complex Fluids group. Nevertheless, it was great that the facilities were there. For example, I used the Scanning Electron Microscope (SEM) together with Mark Smithers and the Focussed Ion Beam (FIB) together with Willem Tjerkstra and Frans Segerink. Also, close collaboration took place with the Computational BioPhysics group, led by Wim Briels and the Physics of Fluids group, led by Professor Detlef Lohse.

The Mesa+ days, I liked very much so. Discussing my poster with colleagues from various groups led to nice insights and new approaches. The Mesa+ days were a great joy and very refreshing.