Light emission of luminol excited by acoustic cavitation

Images of luminol emission caused by acoustic cavitation for different geometries and operating conditions. These data pertain to a general project on characterization of sono-reactors. The latter term refers to a vessel filled with a liquid undergoing acoustic cavitation, which is produced by one or several ultrasonic transducers. Acoustic cavitation is a complex process involving micron-sized bubbles undergoing strong radial oscillations under an acoustic field. These bubbles typically undergo expansion/collapse phases on each acoustic period. The collapse, occurring on a timescale of about 1 ns, yields large temperatures in the bubble (5000-10000 K), large internal pressures (0.1-1 GPa), and solvent homolytic cleavage, termed as "sonolysis". The radicals emerging from the latter process can be profitably used to perform an unusual chemistry called "sonochemistry" . When water is used as solvent, cleavage yields H° and OH° radicals. Cavitation / sonochemistry zones can be assessed by using luminol, which upon oxydation by OH° radicals form an excited species emitting blue light, a process known as "sonochemical luminescence" (SCL) [1]. The latter can be recorded by long-time exposure imaging with a standard camera, and reveal the sonochemically active zones in the liquid. This procedure is known as "luminol mapping". Our main objective was to examine the effects of geometry (vessel volume and shape, liquid level, tranducer immersion depth) and driving amplitude (RMS current injected in the transducer) on the cavitation fields. In the present work, luminol solutions of1 g/L luminol (3-aminophthalhydrazide) and 1 g/L NaOH in distilled water were submitted to ultrasound produced by a 20 kHz transducer, located in the center of cylindrical vessels. Both the transducer immersion depth and the liquid level could be varied. A digital exposure-controlled camera (Sony alpha 7 III equipped with a Sony FE of focal distance f = 50mm) was used to capture sonochemiluminescence images under ultrasonic irradiation in a completely dark room. The exposure time was 20 seconds and the aperture to f/8. After each shot, the luminol solution was refreshed. Three different vessels have been used : 1L 2L narrow 2L large For each vessel, three operating parameters have been varied : Immersion depth (labelled as "HFT", the latter representing the distance from transducer to vessel bottom) Liquid level (labelled as "HW") RMS current injected in the transducer (labelled as "Ivar") The images corresponding to a given set of experiments are stored in directory : SWEEP_TYPE/VESSEL_TYPE/PHOTOS. For example : HW/VESSEL_2L_NARROW/PHOTOS. The filenames are self-explanatory and contain the values of parameters HW, HFT and Ivar with the following exceptions : When Ivar is unspecified, its value is 0.6 A When HW is unspecified (for HFT experiments), it should be computed from Eq. (2) and Tab 2 in the associated publication For each of the 9 sets of experiments, a reference image displaying the vessel in ambient light is provided. References : [1] A. Henglein, R. Ulrich, J. Lilie, Journal of the American Chemical Society 111 (6) (1989) 1974–1979.