Suppression of sonochemiluminescence reduction at high acoustic amplitudes by the addition of particles

The effect of particle addition to a liquid or liquid surface on the sonochemiluminescence (SCL) was investigated using a luminol aqueous solution under ultrasonic treatment at 154 kHz. The acoustic-amplitude dependence of the SCL intensity was measured, in addition to capturing images of luminescent spatial patterns. At higher acoustic amplitudes, the cavitation efficiency dramatically reduces. This behavior is suppressed in the presence of particles. Particle addition provides nucleation sites for cavitation bubbles, lowering the cavitation threshold, and weakening the liquid surface vibration as the pressure amplitude decreases. It is shown that the reduction in SCL is suppressed under the addition of alumina particles into luminol aqueous solution. As the amount of alumina particles increases, the range of acoustic amplitude for suppressing the reduction in SCL is enlarged toward high amplitude, and the intensity of the SCL increases. Simultaneous addition of alumina particles into the solution and hydrophobic polytetrafluoroethylene (Teflon) particles onto the liquid surface is also effective. Examination of SCL images revealed that alumina particles added to the liquid at high acoustic amplitude caused the entire region of the reaction volume to be homogeneously luminous. If hydrophobic particles cover the solution surface, the surface vibration at high acoustic amplitude is fixed and the sound field becomes stable. This is responsible for suppression of the reduction in SCL and leads to a high rate of sonochemical reaction, even at high acoustic amplitude.     

FEM calculation of an acoustic field in a sonochemical reactor

The spatial distribution of the acoustic amplitude in a sonochemical reactor has been numerically calculated using the finite element method (FEM). In the FEM program, the acoustic field in a sonochemical reactor is coupled with the vibration of the reactor's wall. The present calculations have revealed that the thin (thick) glass or stainless steel wall is nearly a free (rigid) boundary and that the glass wall is freer than the stainless steel wall. The influence of the attenuation coefficient of ultrasound on the acoustic field has also been studied in order to see the effect of bubbles on the acoustic field. As the attenuation coefficient increases, the vibration amplitude of the reactor's wall becomes smaller and the acoustic emission from the vibrating wall becomes weaker. The qualitative feature of the spatial pattern of sonochemiluminescence from an aqueous luminol solution has been reproduced by the calculation when the attenuation coefficient is in the range of 0.5-5m(-1). When the attenuation coefficient is less than about 0.05 m(-1), the standing wave pattern of an acoustic field in the liquid is very complex due to the acoustic emission from the vibrating wall. The present calculations have also revealed that some stripes of pressure antinodes have also been disconnected when the radius of the transducer is much smaller than the side length of the vibrating plate. The dependence of the acoustic field on the liquid height is also discussed.     

Production of O Radicals from Cavitation Bubbles under Ultrasound

In the present review, the production of O radicals (oxygen atoms) in acoustic cavitation is focused. According to numerical simulations of chemical reactions inside a bubble using an ODE model which has been validated through studies of single-bubble sonochemistry, not only OH radicals but also appreciable amounts of O radicals are generated inside a heated bubble at the violent collapse by thermal dissociation of water vapor and oxygen molecules. The main oxidant created inside an air bubble is O radicals when the bubble temperature is above about 6500 K for a gaseous bubble. However, the concentration and lifetime of O radicals in the liquid water around the cavitation bubbles are unknown at present. Whether O radicals play some role in sonochemical reactions in the liquid phase, which are usually thought to be dominated by OH radicals and H2O2, should be studied in the future.     

The range of ambient radius for an active bubble in sonoluminescence and sonochemical reactions

Numerical simulations of nonequilibrium chemical reactions inside an air bubble in liquid water irradiated by ultrasound have been performed for various ambient bubble radii. The intensity of sonoluminescence (SL) has also been calculated taking into account electron-atom bremsstrahlung, radiative attachment of electrons to neutral molecules, radiative recombination of electrons and ions, chemiluminescence of OH, molecular emission from nitrogen, etc. The lower bound of ambient radius for an active bubble in SL and sonochemical reactions nearly coincides with the Blake threshold for transient cavitation. The upper bound is in the same order of magnitude as that of the linear resonance radius. In actual experiments, however, the distribution of ambient radius for active bubbles may be narrow at around the threshold ambient radius for the shape instability. The threshold peak temperature inside an air bubble for nitrogen burning is higher than that for oxidant formation. The threshold peak temperatures depend on ultrasonic frequency and acoustic amplitude because chemical reactions inside a bubble are in nonequilibrium. The dominant emission mechanism in SL is electron-atom bremsstrahlung except at a lower bubble temperature than 2000 K, for which molecular emissions may be dominant.     

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 (200kHz) 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 (50mM) provided evidence of transient bubble cavitation. Solutions containing 0.1mM luminol were also used to demonstrate the radical production by attaining the SCL emission regions.     

Bubble population phenomena in sonochemical reactor: II. Estimation of bubble size distribution and its number density by simple coalescence model calculation

A simple bubble population model, with emphasis on the bubble–bubble coalescence, is proposed. In this model, the bubble size distribution is simulated through the iteration of fundamental bubble population process: generation, dissolution, and coalescence. With this simple modelling, the bubble size distribution experimentally observed by the pulsed laser diffraction method and the void rate obtained by the capillary system at 443 kHz were successfully simulated. The experimental results on the bubble population growth by the repetitive pulsed sonication and the effect of pulse width on the bubble population were recreated by the numerical simulation in a semi-quantitative manner. The importance of coalescence of bubbles especially for the effect of addition of surfactant is demonstrated. By decreasing the coalescence frequency by one order of magnitude in the simulation, both the drastic decrease in the total bubble volume as well as the depression of bubble size distribution centring from a few tens of microns in water to a few microns in a dilute surfactant solution can be simultaneously derived.     

Bubble population phenomena in sonochemical reactor: I Estimation of bubble size distribution and its number density with pulsed sonication – Laser diffraction method

To characterize the bubble populations (size and its number distribution) in a sonochemical reactor, a simple but powerful technique based on the Fraunhofer laser diffraction (LD) has been proposed. In this method, the acoustic wave disturbance to the laser probe in the sonochemical reaction field was eliminated by the temporal separation using pulsed sonication (pulsed LD). With this relatively simple strategy, the temporal development of the bubble size distribution could be evaluated by pulsed LD. A number density of bubbles was estimated by using a calibration data obtained with monosized standard particles. In addition, the effect of pulse length and a surfactant on the bubble population phenomena in a multibubble system are discussed.     

Influence of degree of gas saturation on multibubble sonoluminescence intensity

The influence of the degree of saturation (DOS) of a gas in a solution on the intensity of multibubble sonoluminescence (MBSL) excited by ultrasound with a frequency of 261 kHz is investigated at various ultrasonic powers and with different concentrations of ethanol, which is added as a volatile solute. At relatively low powers and a high DOS, low ethanol concentrations give higher sonoluminescence (SL) intensities than those obtained with pure water. This intensity enhancement decreases as sonication proceeds because the SL intensity for pure water increases with time, whereas it remains almost constant or decreases slightly in solutions containing ethanol. At relatively low powers, a partially degassed solution has a higher SL intensity than a solution with a high DOS for both pure water and solutions containing ethanol. The reason why the DOS decreases more when ethanol is added is considered mainly to be the accumulation of hydrocarbon products and the promotion of rectified diffusion. Adding an alcohol to a solution enhances ultrasonic degassing.     

Evaluation of peripheral vasodilative indices in skin tissue of type 1 diabetic rats by use of RGB images

Optical Review - We investigated a method to evaluate the arterial inflow and the venous capacitance in the skin tissue of streptozotocin-induced type 1 diabetic rats from RGB digital color images....     

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.