See Overview 2013

Laura Stricker (promotion date: 16 January 2013)

Acoustic cavitation and sonochemistry

Promotion date: January 16.

Promotor: Prof.dr. Detlef Lohse

Assistant-promotor: Prof. dr. Andrea Prosperetti

Sonochemistry is the use of cavitation for achieving a chemical conversion. Implosion of microbubbles (5-20 μm in size) can generate localized extreme temperatures of 10000 K and pressures up to 1000 bar, the conditions of the surrounding liquid remaining ambient. Therefore high temperature chemical conversions can occur at ambient conditions.

Reaction products can be used for synthesis of fine chemicals, food ingredients or pharmaceuticals, or for break-down of recalcitrant components in water. However, industrial application of sonochemistry is limited by the energy inefficiency of large scale sonochemical reactors, caused by energy losses due to insufficient focus of energy tranfer to the microbubble, as well as wall and bubble interference effects and equipment size.

The challenge therefore is to gain full control over the cavitation process and to improve the energy efficiency of the process, by miniaturizing sonochemical reactors. Design, development and energy testing of efficient sonochemical microreactors are studied both experimentally and numerically. Considering its crucial role in chemical conversions, special attention in analytical modeling is given to the temperature field description.

Was there an important moment that you recall during your PhD working period?

The project was well-designed from the start. So I was able to produce some steady progress along the way.

The project was somewhat divided into two parts, me executing the theoretical work and two colleagues performing crucial experiments. Along the way one of them came up with important results, showing that our line of research was well chosen. I remember him emailing me about this very vividly. His results were very encouraging to me, proving present and future results and applications were within reach. Using sonochemistry can lead to controlled reactions where unstable radicals are involved. However, the energy efficiency and controllability of processes like this, still remain poor and unpredictable.

We succeeded in our main purpose, improving the efficiency by a factor of ten in respect to the pre-existing immersed bath reactors. In this kind of reactors, an oscillating piezo element/plane is glued to the bottom on the outside of a liquid-filled cuvette and the microbubbles are created from the bottom itself. Our big idea was to make use of artificial crevices as nucleation sites for the bubbles, highly increasing the number of the bubbles themselves and the controllability of the process.

The main part of the modelling effort regarded the study of the radical production of these bubbles and the optimization of the whole process. We also theoretically addressed the mechanisms involved in the bubble formation. Although we could gain a lot of knowledge on the optimal working conditions of this kind of sonochemical reactors, still some questions remain open on the fundamental level, about the bubble formation mechanism.

In what way did you develop on a personal level, as a researcher and scientist?

As a theorist I learned to work with my colleagues and collaboration partners, discussing about my results and ideas in an intense way. The input from experimentalists and experimental data I value greatly nowadays. Also, I discovered that, as a theorist, I can provide helpful models for colleagues to design clever experiments out of them. It is a great feeling to discover one can be of help there.

My supervisor, professor Detlef Lohse, always encouraged me to do so. Also Andrea Prosperetti has been of amazing support. They both possess a huge knowledge, not only in a theoretical sense. They show great abilities in reading and explaining the data coming out of the experiments.

Another thing is: I liked the teaching part of the job. This I didn’t expect at all beforehand. It is a very nice side effect, so to say. Working with motivated students is challenging as they prompt you to make clear your own frame of understanding on the main topics.

Did you manage to publish some nice results?

Two publications I am very proud of. One was in JASA (the Journal of the acoustic society of America), and in Ultrasonics Sono-Chemistry. Some manuscripts are underway still. Also, I was a speaker at the Cavitation Conference 2012 in Singapore.

What are your future plans?

I like to stay in academic research, as I am convinced my skills and focus are mainly on the theoretical work. Reading experimental data is important as well, I am fully convinced of now. Performing some experiments is okay with me, as long as it is not the core business of my working activities.

In Europe and America, I think some nice opportunities will arise. In my home country of Italy it will be more difficult to start an academic career. The money for performing research is very badly distributed there, in my opinion.

Did Mesa+ play an important role in your thesis project?

The microfabrication of the probes was really important for my research, as I have to take into account the technical possibilities of this area of research. Mesa+ as an institution is important. The management should follow actual scientific and political developments carefully, making clever strategic decisions for the near and more far away future.