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722_0284.pdf

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SONOCHEMICAL MICROREACTOR WITH MICROBUBBLES CREATED

ON MICROMACHINED SURFACES

D. Fernandez Rivas1*, A.G. Zijlstra2, A. Prosperetti2,3, D. Lohse2 and J.G.E. Gardeniers1

1Mesoscale Chemical Systems, MESA+ Institute for Nanotechnology, University of Twente, THE NETHERLANDS 2Physics of Fluids, MESA+ Institute for Nanotechnology, University of Twente, THE NETHERLANDS 3Department of Mechanical Engineering, The Johns Hopkins University, USA

ABSTRACT

We present an efficient sonochemical microreactor system based on micromachined surfaces which generate micro- bubbles in a liquid when exposed to an ultrasonic field. Cavitation of these bubbles leads to radical formation, introduc- ing chemical reactions in the solution. The system exhibits a high sonochemical yield at conditions which would other- wise not produce any significant chemical effect, providing higher efficiency than equivalent conventional sonochemical batch reactors. The results show an increase in total energy efficiency (expressed in the amount of radicals generated per unit power injected to the system) of one order of magnitude, compared to an experiment without the surface bubbles.

KEYWORDS: Microreactor, radicals, sonochemistry, ultrasound, microbubbles

INTRODUCTION

Sonochemistry employs cavitation, i.e. the growth and the implosion of gas bubbles in a liquid; a process which can generate extreme temperatures of thousands of Kelvin to achieve chemical conversion [1]. Applications are in the syn- thesis of fine chemicals, food ingredients or pharmaceuticals, or the break-down of contaminants in water [2, 3], but have been limited because of the energy inefficiency of large scale sonochemical reactors, mainly caused by the diffi- culty to focus energy to the microbubble.

THEORY

Bubble nucleation from crevices was described before [4]. We have used micromachined pits to generate microbub- bles aiming at sonochemical effects. This new concept is based on the continuous splitting off of microbubbles from os- cillating larger bubbles entrapped in micromachined pits in a silicon substrate. The ejected microbubbles continue to cavitate as well generating the desired chemical effect due to ultrasound insonication.

EXPERIMENTAL

The microfabrication process consists in one photolithographic and one deep reactive ion etching step, by which cy- lindrical pits of typical dimensions 30 m diameter and depth of 10 m were formed in a silicon substrate (Fig. 1 a)).

Figure 1: a) Micromachined pit (30 m diameter) in silicon substrate.

b) Experimental setup.

A piezo glued to a closed glass container with a volume of 300 served as microreaction chamber (Fig. 1 b)). The temperature was kept at 25 0C, and ultrasound frequency was 200 kHz. The power delivered was measured with an os- cilloscope and current probe.

RESULTS AND DISCUSSION

A visible pattern of bubbles ejecting from a three-pit configuration substrate is seen on (Fig. 2 a and c):

978-0-9798064-3-8/μTAS 2010/$20©2010 CBMS 2123

14th International Conference on Miniaturized Systems for Chemistry and Life Sciences 3 - 7 October 2010, Groningen, The Netherlands

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 Supercritical Fluid Extraction 722_0284.pdf Page 001
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