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David Fernández Riva (promotion date: 26 October 2012)

Taming acoustic cavitation

Promotion date: 26. October

Promotor: Prof. dr. ir. Han Gardeniers

Sonochemistry is defined as the chemical effects due to acoustic cavitation: formation, growth and collapse of bubbles in liquids. During these cavitation events high temperatures and pressures are achieved inside the bubbles (more than 5000 K and 1000 atm), which may generate radicals by breaking water moleculs inside the bubble.

Sonochemical effects have been used in the purification of water, special food processing and synthesis of specific chemical compounds like new bio- and nanomaterials. Reactions that require complex and dangerous conditions (high temperature and pressure) can be carried out in microfluidic chips with a considerable saving in reactants, increased efficiency and safer operation conditions.

The main objective of my thesis was the design and fabrication of efficient microreactors for carrying out specific chemical reactions and to understand more of the fundamentals of acoustic cavitation and sonochemical effects.

Was your PhD project application driven?

My work contained both fundamental and application driven aspects and is a result of the collaboration with several groups from Twente, Melbourne, Nagoya, Singapore and Badajoz. The sonochemistry effects produced by the microbubbles we have studied are useful for several potential applications, but we also wanted to get a better understanding and control over its fundamental working mechanisms.

The expansion and collapse of bubbles can be compared to the functioning of car engines, where high temperature and pressures reached inside provide useful energy. Each bubble can be considered as a reactor operating in a safe way without effecting much the surrounding liquid. Sonochemistry has therefore been used for water treatment and the synthesis of new materials, for the removal of contaminants from surfaces, in dentistry and drug delivery, for medical imaging, etc.. The sound frequencies are typically above 20 kHz and up to approximately one MHz; these are frequencies that usually are not audible, except perhaps by small kids and certain animals.

The positive effects of acoustic cavitation can be amplified if one manages to control the phenomena involving the appearances of bubbles. By using microfabrication techniques we managed to control and study clouds of bubbles generated from air trapped in predefined crevices or artificial defects.

In dentistry, the cleaning potential of the ultrasonic generation of bubbles is useful when performing root canal treatments. Every day hundreds of root canal treatments are unsuccessful because of a persistent infection, and repeated treatment is necessary costing daily tens of thousands of Euros nation-wide. Our bubble production technique may be very useful for cleaning the root canal more thoroughly. My colleague Bram Verhaagen (Physics of Fluids) and I, pitched this Business Idea (BµBCLEAN) recently at the MESA+ Day and received the Young Business Award; so we are in a good track as we are filling out a STW-grant to further investigating the potentials of commercializing this.

What are your future plans?

The next two years I plan to work in two post-doc projects. The first involves a method patented by dr. Regina Luttge and her company My Life Technologies, on the use of microneedles, to deliver drugs more efficiently through the skin. The other project involves the use of solar energy to produce green fuels and is a collaboration of my PhD Supervisor Prof. Han Gardeniers and Prof. Jurriaan Huskens.

In the meantime I will be working on the valorisation of our bubble production technique, as explained before. I strongly believe ultrasound cavitation is an apt technique for finding green solutions for several applications; particularly in the cleaning of surfaces, like in the BµBCLEAN business idea.

Going back to Cuba is difficult as the scientific or business opportunities in a developing country with such a political government, and the USA embargo, are hard to fulfil or non-existent. On top of that, Cuba has severe restrictions that never allowed my daughter to visit me or to live as a family abroad, and this in turn prevents me from willingly return and work for the same government that mistreats its nationals. Nevertheless, my love for research keeps me going and I still collaborate scientifically with Cuban institutions.

I am also enjoying my musical career in the Netherlands as a guitar player and bandleader of two music groups: Chilangos Habaneros and Casual Mango.

What, in your opinion, is important for Mesa+ to stay successful in future?

I think that in Twente, next to the scientific and technical expertise, supported by the available equipment and facilities; the freedom to interact with colleagues and researchers, is the key ingredient to harvest good scientific results. The University of Twente supplies freedom of research and also is open to the entrepreneurial ideas of its employees. This is a right mix for success, and it also formed and expanded my personal views of life.