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Sonochemistry and sonoluminescence in microfluidics

Tandionoa, Siew-Wan Ohla, Dave S. W. Owb, Evert Klaseboera, Victor V. Wongb, Rainer Dumkec, and Claus-Dieter Ohlc,1

aInstitute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore; bBioprocessing Technology Institute, 20 Biopolis Way, #06-01 Centros, Singapore 138668, Singapore; and cDivision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore

Edited by Floyd Dunn, University of Illinois, Urbana, IL, and approved March 2, 2011 (received for review January 4, 2011)

One way to focus the diffuse energy of a sound field in a liquid is by acoustically driving bubbles into nonlinear oscillation. A rapid and nearly adiabatic bubble collapse heats up the bubble interior and produces intense concentration of energy that is able to emit light (sonoluminescence) and to trigger chemical reactions (sono- chemistry). Such phenomena have been extensively studied in bulk liquid. We present here a realization of sonoluminescence and sonochemistry created from bubbles confined within a narrow channel of polydimethylsiloxane-based microfluidic devices. In the microfluidics channels, the bubbles form a planar/pancake shape. During bubble collapse we find the formation of OH radicals and the emission of light. The chemical reactions are closely confined to gas–liquid interfaces that allow for spatial control of sonochemical reactions in lab-on-a-chip devices. The decay time of the light emitted from the sonochemical reaction is several orders faster than that in the bulk liquid. Multibubble sonoluminescence emis- sion in contrast vanishes immediately as the sound field is stopped.

cavitation ∣ ultrasound ∣ capillary waves

Small bubbles in liquids excited with high acoustic pressures can produce enormous energy concentration that manifests itself in chemical reactions and the conversion of sound into light (1, 2). This phenomenon of sonoluminescence is based on rapid bubble collapse heating the gas adiabatically (3–6) reaching ioni- zation temperature (7). Spectroscopic methods have revealed that the temperatures during the last stage of bubble collapse in typical sonochemical reactors are of the order of 5,000 K, whereas pressures of more than 1,000 bars can be achieved (6). In general, the maximum temperature is related to how spherical the bubble collapse is. For a single bubble distant from bound- aries, the well-known single bubble sonoluminescence has been shown to exceed 10,000 K during collapse (5, 7). Yet when brought close to boundaries (for example, in microfluidics), in- stabilities develop into liquid jets that diminish the energy con- centration and hinder light emission (8).

In general, sonochemical reactions are studied in large geome- tries (bulk liquid) (9) because sufficient liquid inertia is needed to compress the gas. There is little control over the chemical re- actions due to the complex bubble–bubble interaction (10). It was only recently that we were able to produce intense cavitation in microfluidics (11) by exciting a capillary wave with an ultrasonic vibration. The cavitation bubbles are confined within microfluidic channels that pose severe challenges to the energy concentration through instabilities and viscous surface forces that slow down the bubble wall speed. A second challenge relates to the small amount of volume available in microchannels, which leads to a significantly lower number of cavitation nuclei available com- pared to standard-sized sonochemical reactors.

Both challenges can be addressed with our microfluidic design presented in Fig. 1A. The acoustic driving is based on a standing wave generated on the surface of a glass plate with an attached piezoelectric transducer. Gas–liquid interfaces are created using a T junction where gas is injected at constant pressure into the liquid flow (Fig. 1B). The acoustic pressure distribution at ap- proximately 50 μm above the glass plate (at low driving amplitude of 50 V) is presented in Fig. 1C. The pressure reaching up to

6 bars is measured. A prominent standing wave pattern is visible, and the individual peaks oscillate with a nearly sinusoidal shape at the driving frequency of 103.6 kHz (Fig. 1D and Fig. S1). At higher driving amplitude, nonlinear standing capillary waves are excited at the gas–liquid interfaces, i.e., Faraday waves at half the driving frequency. These waves entrap small bubbles that then serve as cavitation nuclei.

First we present light emission generated through chemical re- actions caused by the bubble oscillations (chemiluminescence) and then present sonoluminescence, i.e., the light emitted directly from the collapse of the cavitation bubbles.

Results and Discussion

Visualization of Chemiluminescence. Because cavitation initiates at the gas–liquid interfaces, it is expected that sonochemical reactions will be most prominent near these locations. We used the well-known oxidation of luminol in a sodium carbonate base solution to monitor the cavitation-induced production of H and OH radicals (12, 13). They subsequently trigger the formation of an amino phthalate derivative with electrons in an excited state. As the excited states relax to lower energy states, excess energy is emitted as visible bluish light. This light emission is captured with an intensified and cooled CCD camera.

A typical light distribution pattern is shown in Fig. 2A; it is overlaid onto a frame taken with side illumination to resolve the gas–liquid interfaces just before the start of the ultrasound (in gray colors). The green dashed lines in Fig. 2A represent the position of the liquid bodies in the microchannels. Fig. 2A reveals that the luminol emission is occurring only within the liquid phase and always close to the gas–liquid interfaces, precisely at the location where cavitation bubbles are nucleated and driven into large oscillations. Snapshots of bubble oscillations at various stages of the ultrasound driving close to the gas–liquid interface are depicted in Fig. 2B. The bubbles are largely pancake shaped (third frame of Fig. 2B) during maximum expansion, whereas se- vere deformations occur during collapse (last frame of Fig 2B).

Time-Resolved Luminescence. Next we study the time-resolved light emission from pulsed ultrasound excitation with a photomulti- plier (PMT) in combination with a high-capacity sampling oscil- loscope. The piezoelectric transducer was repeatedly excited at 230-V amplitude for 1,000 cycles every 30 ms with an on/off ratio of 0.49 (Fig. 3A). The inverted signal of the PMToutput is plotted in Fig. 3B, and the emission during the first cycle is given enlarged in Fig. 3C. The first light emission is observed several hundred microseconds after the ultrasound is switched on. This can be easily explained by the finite time needed to build up the resonant

Author contributions: T., S.-W.O., and C.-D.O. designed research; T. and C.-D.O. performed research; D.S.O., V.V.W., and R.D. contributed new reagents/analytic tools; T. and C.-D.O. analyzed data; and T., S.-W.O., D.S.O., E.K., and C.-D.O. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

1To whom correspondence should be addressed. E-mail:

This article contains supporting information online at doi:10.1073/pnas.1019623108/-/DCSupplemental.

5996–5998 ∣ PNAS ∣ April 12, 2011 ∣ vol. 108 ∣ no. 15

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