Sonoluminescence and sonochemiluminescence from a microreactor

Micromachined pits on a substrate can be used to nucleate and stabilize microbubbles in a liquid exposed to an ultrasonic field. Under suitable conditions, the collapse of these bubbles can result in light emission (sonoluminescence, SL). Hydroxyl radicals (OH*) generated during bubble collapse can react with luminol to produce light (sonochemiluminescence, SCL). SL and SCL intensities were recorded for several regimes related to the pressure amplitude (low and high acoustic power levels) at a given ultrasonic frequency (200 kHz) for pure water, and aqueous luminol and propanol solutions. Various arrangements of pits were studied, with the number of pits ranging from no pits (comparable to a classic ultrasound reactor), to three-pits. Where there was more than one pit present, in the high pressure regime the ejected microbubbles combined into linear (two-pits) or triangular (three-pits) bubble clouds (streamers). In all situations where a pit was present on the substrate, the SL was intensified and increased with the number of pits at both low and high power levels. For imaging SL emitting regions, Argon (Ar) saturated water was used under similar conditions. SL emission from aqueous propanol solution (50 mM) provided evidence of transient bubble cavitation. Solutions containing 0.1 mM luminol were also used to demonstrate the radical production by attaining the SCL emission regions.Comment: http://www.sciencedirect.com/science/article/pii/S1350417712000855; ISSN 1350-417

Efficient Sonochemistry through Microbubbles Generated with Micromachined Surfaces

Sonochemical reactors are used in water treatment, the synthesis of fine chemicals, pharmaceutics and others. The low efficiency of sonoreactors have prevented its massive usage at industrial scales. Controlling the appearance of bubbles in place and time is the most limiting factor. A novel type of sonochemical reactor was designed making use of micro-fabrication techniques to control the nucleation sites of micro-bubbles. The efficiency was increased first by locating the nucleation sites in the most active region of a micro-chamber; additionally the desired chemical effect was significantly higher at the same powers than when not controlled. Silicon substrates were micromachined with "artificial nucleation sites" or pits, and placed at the bottom of the micro-chamber. The pits entrap gas which, upon ultrasonic excitation, sheds off a stream of microbubbles. The gas content of the pits is not depleted but is replenished by diffusion and the emission of microbubbles can continue for hours

Photocatalytic, sonocatalytic and sonophotocatalytic degradation of Rhodamine B using ZnO/CNTs composites photocatalysts

A series of ZnO nanoparticles decorated on multi-walled carbon nanotubes (ZnO/CNTs composites) was synthesized using a facile sol method. The intrinsic characteristics of as-prepared nanocomposites were studied using a variety of techniques including powder X-ray diffraction (XRD), high resolution transmission electron microscope (HR-TEM), transmission electron microscope (TEM), scanning electron microscope (SEM) with energy dispersive X-ray analysis (EDX), Brunauer Emmett Teller (BET) surface area analyzer and X-ray photoelectron spectroscopy (XPS). Optical properties studied using UV–Vis diffuse reflectance spectroscopy confirmed that the absorbance of ZnO increased in the visible-light region with the incorporation of CNTs. In this study, degradation of Rhodamine B (RhB) as a dye pollutant was investigated in the presence of pristine ZnO nanoparticles and ZnO/CNTs composites using photocatalysis and sonocatalysis systems separately and simultaneously. The adsorption was found to be an essential factor in the degradation of the dye. The linear transform of the Langmuir isotherm curve was further used to determine the characteristic parameters for ZnO and ZCC-5 samples which were: maximum absorbable dye quantity and adsorption equilibrium constant. The natural sunlight and low power ultrasound were used as an irradiation source. The experimental kinetic data followed the pseudo-first order model in photocatalytic, sonocatalytic and sonophotocatalytic processes but the rate constant of sonophotocatalysis is higher than the sum of it at photocatalysis and sonocatalysis process. The sonophotocatalysis was always faster than the respective individual processes due to the more formation of reactive radicals as well as the increase of the active surface area of ZnO/CNTs photocatalyst. Chemical oxygen demand (COD) of textile wastewater was measured at regular intervals to evaluate the mineralization of wastewater

Decrease in the Surface Tension of Nanobubble Dispersion in Water: Results of Surface Excess of Bulk Nanobubbles at Interfaces

The effect of nanobubbles (NBs) on the surface tension of liquid was investigated by three methods of different measuring principles, pendant drop (PD), Wilhelmy, and du Noüy methods, over a wide range of number concentration of bulk NBs (BNBs). In all of the three methods, the surface tension decreased in proportion to the number concentration of BNBs and the proportional constant was different among the three methods. Such behavior was inferred to be caused by the surface excess of BNBs at the gas–liquid or solid–liquid interface. In the PD method, the hydrophobic interaction between BNBs and air around a drop seems to cause the surface excess of BNBs along the surface of water drops. It brings about a subtle change in its profile, resulting in the decrease in surface tension, which takes a time of hundreds of seconds. Meanwhile, in the Wilhelmy and du Noüy methods, electrostatic attractive force between BNBs and a plate or ring is a likely cause of surface excess at the solid–liquid interface, resulting in the instantaneous decrease in surface tension. This study also provides a practical methodology of comparison for surface tension of NB dispersion: surface tension shall be compared among different samples with the same measurement method. Especially in the PD method, retention time of droplets before measurement shall be the same among samples

Fabrication of silver nanoparticles deposited on boehmite sol for surface enhanced Raman scattering

The composite consisting of silver nanoparticles deposited on boehmite hybrid was synthesized by NaBH4 reduction technique. The morphology of the composite was studied by TEM, UV/Vis spectrophotometer and particle sizer. The size of the silver nanoparticles deposited on the surface of the boehmite ranged from 10 nm to 100 nm. The contact of silver nanoparticles increased by means of deposition of silver nanoparticles on the boehmite sol and the aggregation of the composites. This leads to the appearance of a shoulder at 450 nm in the UV–Vis absorption spectra with the addition of 0.15 mg and 1.5 mg boehmite. It was found that the intensity of the SERS in the case of the composite was higher than for silver colloids consisting of a concentration of silver greater than 3.2 mM

Mechanism of enhancement of sonochemical-reaction efficiency by pulsed ultrasound

The enhancement of sonochemical-reaction efficiency by pulsed ultrasound at 152 kHz has been studied experimentally through absorbance measurements of triiodide ions from sonochemical oxidation of potassium iodide at different liquid volumes to determine sonochemical efficiency defined by reacted molecules per input ultrasonic energy. The mechanism for enhancement of the reaction efficiency by pulsed ultrasound is discussed using captured images of sonochemiluminescence (SCL), and measured time-resolved signals of the SCL pulses and pressure amplitudes. The high sonochemical-reaction efficiency by pulsed ultrasound, compared with that by continuous-wave ultrasound, is attributed both to the residual pressure amplitude during the pulse-off time and to the spatial enlargement of active reaction sites

Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles

Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) [26]. The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. "Transient" cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, "stable" cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble-bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both "stable" and "transient" cavitation bubbles which pulsate nonlinearly with the period of ultrasound

Optical cavitation probe using light scattering from bubble clouds

To understand the behaviour of systems containing clouds of bubbles (multibubble system) in real sonochemical reactors, a new diagnosis method, i.e., optical cavitation probe (OCP), has been proposed. When a laser beam is introduced into the cavitation bubble cloud, the scattered light intensity changes by the collective oscillation of cavitation bubbles. The frequency domain spectrum of the scattered light contains rich information on the cavitation bubble clouds, comparable with the acoustic emission spectra detected by a hydrophone. The significant merits of OCP, such as capability for spatially resolved, non-invasive measurement of the cavitation bubble clouds, robustness even in a violent cavitation field have been experimentally demonstrated. © 2008 Elsevier B.V. All rights reserved

Study of an acoustic field in a microchannel

Using a standing-wave field, it is possible to trap small objects at nodes of a sound pressure distribution. In the present study, a sound wave was generated by a transducer outside of a microchannel, and propagated into a microchannel on a glass plate, where it generated a standing wave field. When water containing alumina particles was injected into the microchannel, several layers of particles were formed in the sound field. Moreover, when the ultrasound driving frequency was swept, it was possible to control the direction of the particle flow. The sound field was numerically calculated and the experimental results are discussed

Fabrication of silver nanoparticles deposited on boehmite sol for surface enhanced Raman scattering

The composite consisting of silver nanoparticles deposited on boehmite hybrid was synthesized by NaBH4 reduction technique. The morphology of the composite was studied by TEM, UV/Vis spectrophotometer and particle sizer. The size of the silver nanoparticles deposited on the surface of the boehmite ranged from 10 nm to 100 nm. The contact of silver nanoparticles increased by means of deposition of silver nanoparticles on the boehmite sol and the aggregation of the composites. This leads to the appearance of a shoulder at 450 nm in the UV–Vis absorption spectra with the addition of 0.15 mg and 1.5 mg boehmite. It was found that the intensity of the SERS in the case of the composite was higher than for silver colloids consisting of a concentration of silver greater than 3.2 mM