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Stefan von Kann (promotion date: 21 December 2012)

Dense suspensions: Force response and jamming

Promotion date: December 21.

Promotor: Prof. dr. Deveraj van der Meer

Assistant promotor: Prof. dr. Detlef Lohse

Assistant promotor: Dr. ir. Jacco Snoeijer

The response of dense suspensions to an external force was studied using two different experiments.

In the first experiment objects were settled in a deep bath of a dense cornstarch suspension. This is the only suspension to result in two unexpected phenomena: Velocity oscillations in the bulk, and stop-go-cycles near the bottom. Both are believed to be caused by a change in the particle configuration within the suspension.

In the second experiment, vertical vibrations were exerted to a thin layer of dense suspensions. Depending on suspension and shaking parameters, a lot of different unexpected phenomena were found: Stable and growing holes and fingerlike protrusions, to name a few.

All have in common that hydrostatic pressure is overcome, which would normally cause a hole to collapse in any Newtonian liquid, even when vibrated. A convection roll was found on the edge of the holes in the suspensions, which might be explained by a break in symmetry in the oscillations of the hole edge, which in turn can cause a change in particle configuration in the suspension.

Your work as a part of a collaborated project.

My work was part of an encompassing national project consisting of ten parallel projects spread over several different universities in Holland, funded by FOM. Right from the beginning the experimental character of my project was very clear.

The results in the shaking experiments were rather difficult to reproduce because the phenomenon occurring, is dependable on the exact composition of the suspension. Some common characteristics were found, which gave rise to a new idea about how these phenomena can exist.

The cornstarch we used was found to be unique, consisting of sharp-edged particles of five till twenty micrometers in diameters. No comparable particles were found, that are commonly available, and other particles showed different phenomenology.

In which journals were the findings published?

Two publications appeared in Physical Review Letters E. A third one is underway at the moment. The shaking experiment will be submitted to Physics of Fluids. Right now I am writing on the dynamics involved in these processes. I hope to get this paper submitted on short notice as well

What are your future plans?

I would like to go and work at a R&D division in a commercial setting, as I don’t feel attracted to going abroad and work on my skills in various temporary post-doc jobs. Although I believe fundamental research will not be part of my job there, I hope the projects will be similar and equally challenging, and it will be nice to see more direct practical use of the results. Another big difference will be the pressure to perform, which is likely higher in industry.

Did you collaborate with other groups during your thesis work?

The cooperation and help needed, were provided mostly by attending the project-meetings that were planned during the past years. Here the researchers from the other FOM-groups presented their progress and we could discuss about new ideas.

At Mesa+, expertise is available in many areas and up-to-date facilities can be shared. Scientists and technicians are very willing to help the PhDs. I guess that is a matter of strength of the great institution Mesa+ for many years now.