Carlos Cairós

Effects of argon sparging rate, ultrasonic power, and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions.

The sonoluminescence spectra from acoustic cavitation in aqueous NaCl solutions are systematically studied in a large range of ultrasonic frequencies under variation of electrical power and argon sparging. At the same time, bubble dynamics are analysed by high-speed imaging. Sodium line and continuum emission are evaluated for acoustic driving at 34.5, 90, 150, 365, and 945kHz in the same reactor vessel. The results show that the ratio of sodium line to continuum emission can be shifted by the experimental parameters: an increase in the argon flow increases the ratio, while an increase in power leads to a decrease. At 945kHz, the sodium line is drastically reduced, while the continuum stays at elevated level. Bubble observations reveal a remarkable effect of argon in terms of bubble distribution and stability: larger bubbles of non-spherical shapes form and eject small daughter bubbles which in turn populate the whole liquid. As a consequence, the bubble interactions (splitting, merging) appear enhanced which supports a link between non-spherical bubble dynamics and sodium line emission.

Sonochemistry and bubble dynamics.

The details of bubble behaviour in chemically active cavitation are still not sufficiently well understood. Here we report on experimental high-speed observations of acoustically driven single-bubble and few-bubble systems with the aim of clarification of the connection of their dynamics with chemical activity. Our experiment realises the sonochemical isomerization reaction of maleic acid to fumaric acid, mediated by bromine radicals, in a bubble trap set-up. The main result is that the reaction product can only be observed in a parameter regime where a small bubble cluster occurs, while a single trapped bubble stays passive. Evaluations of individual bubble dynamics for both cases are given in form of radius-time data and numerical fits to a bubble model. A conclusion is that a sufficiently strong collapse has to be accompanied by non-spherical bubble dynamics for the reaction to occur, and that the reason appears to be an efficient mixing of liquid and gas phase. This finding corroborates previous observations and literature reports on high liquid phase sonochemical activity under distinct parameter conditions than strong sonoluminescence emissions.

Visualization and optimization of cavitation activity at a solid surface in high frequency ultrasound fields

Despite the increasing use of high frequency ultrasound in heterogeneous reactions, knowledge about the spatial distribution of cavitation bubbles at the irradiated solid surface is still lacking. This gap hinders controllable surface sonoreactions. Here we present an optimization study of the cavitation bubble distribution at a solid sample using sonoluminescence and sonochemiluminescence imaging. The experiments were performed at three ultrasound frequencies, namely 580, 860 and 1142kHz. We found that position and orientation of the sample to the transducer, as well as its material properties influence the distribution of active cavitation bubbles at the sample surface in the reactor. The reason is a significant modification of the acoustic field due to reflections and absorption of the ultrasonic wave by the solid. This is retraced by numerical simulations employing the Finite Element Method, yielding reasonable agreement of luminescent zones and high acoustic pressure amplitudes in 2D simulations. A homogeneous coverage of the test sample surface with cavitation is finally reached at nearly vertical inclination with respect to the incident wave.

Attached cavitation at a small diameter ultrasonic horn tip

Ultrasonic horn transducers are frequently used in applications of acoustic cavitation in liquids, for instance, for cell disruption or sonochemical reactions. They are operated typically in the frequency range up to about 50 kHz and have tip diameters from some mm to several cm. It has been observed that if the horn tip is sufficiently small and driven at high amplitude, cavitation is very strong, and the tip can be covered entirely by the gas/vapor phase for longer time intervals. A peculiar dynamics of the attached cavity can emerge with expansion and collapse at a self-generated frequency in the subharmonic range, i.e., below the acoustic driving frequency. Here, we present a systematic study of the cavitation dynamics in water at a 20 kHz horn tip of 3 mm diameter. The system was investigated by high-speed imaging with simultaneous recording of the acoustic emissions. Measurements were performed under variation of acoustic power, air saturation, viscosity, surface tension, and temperature of the liqu...

Micropatterning for the Control of Surface Cavitation: Visualization through High-Speed Imaging

For the first time, we apply a high-speed imaging technique to record the activity of acoustically driven cavitation bubbles (86 kHz) on micropatterned surfaces with hydrophobic and hydrophilic stripes. The width of the hydrophobic stripes lies between 3.5 and 115 μm. This work provides the first direct visualization of the preferential location of bubbles on the hydrophobic areas of the patterns. The results confirm our previous prediction that surface cavitation strongly depends on the surface energy of the irradiated substrate. The observations show a remarkable effect of the stripe width on the size, movement, growth, splitting, and multiplying of the bubbles. The high-speed imaging also reveals that there is a minimal width of the hydrophobic stripes that allows bubble attraction and formation. Our observations are supported by a theoretical approach based on the forces acting on the bubbles.

Vortex dynamics of collapsing bubbles: Impact on the boundary layer measured by chronoamperometry

Cavitation bubbles collapsing in the vicinity to a solid substrate induce intense micro-convection at the solid. Here we study the transient near-wall flows generated by single collapsing bubbles by chronoamperometric measurements synchronously coupled with high-speed imaging. The individual bubbles are created at confined positions by a focused laser pulse. They reach a maximum expansion radius of approximately 425μm. Several stand-off distances to the flat solid boundary are investigated and all distances are chosen sufficiently large that no gas phase of the expanding and collapsing bubble touches the solid directly. With a microelectrode embedded into the substrate, the time-resolved perturbations in the liquid shear layer are probed by means of a chronoamperometric technique. The measurements of electric current are synchronized with high-speed imaging of the bubble dynamics. The perturbations of the near-wall layer are found to result mainly from ring vortices created by the jetting bubble. Other bubble induced flows, such as the jet and flows following the radial bubble oscillations are perceptible with this technique, but show a minor influence at the stand-off distances investigated.

Acoustic streaming and bubble translation at a cavitating ultrasonic horn

Acoustic cavitation at a 20 kHz ultrasonic horn is investigated by means of high-speed imaging and particle image velocimetry. In one experimental set-up, bubble dynamics is visualized synchronously with the acoustic streaming liquid flow to reveal their connection. By switching an elevated static pressure, cavitation can be turned off and on for otherwise identical conditions. If cavitation is present, an average increase of liquid streaming velocities by a factor of 30 is found as compared to the non-cavitating case, and high flow velocities are well confined to the bubbly regions. Further results show that individual bubble trajectories do not always coincide with the liquid flow direction, but can even run in opposite direction. This is highlighted in a second set-up where the periodic back-and-forth translation of a single bubble near the horn tip in phosphoric acid is analyzed. It is concluded that translation of larger cavitation bubbles is mainly determined by acoustic forces, even in the presence...

High-speed imaging of ultrasound driven cavitation bubbles in blind and through holes

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5 kHz) cavitation bubbles inside the blind and through holes of PMMA plates by using high-speed imaging. Various diameters of blind (150, 200, 250 and 1000 µm) and through holes (200 and 1000 µm) were investigated. Gas bubbles are usually trapped in the holes during substrate immersion in the liquid thus preventing their complete wetting. We demonstrate that trapped gas can be successfully removed from the holes under ultrasound agitation. Besides the primary Bjerknes force and acoustic streaming, the shape oscillations of the trapped gas bubble seem to be a driving force for bubble removal out of the holes. We further discuss the bubble dynamics inside microholes for water and Cu2+ salt solution. It is found that the hole diameter and partly the type of liquid media influences the number, size and dynamics of the cavitation bubbles. The experiments also showed that a large amount of the liquid volume inside the holes can be displaced within one acoustic cycle by the expansion of the cavitation bubbles. This confirmed that ultrasound is a very effective tool to intensify liquid exchange processes, and it might significantly improve micro mixing in small structures. The investigation of the effect of ultrasound power on the bubble density distribution revealed the possibility to control the cavitation bubble distribution inside the microholes. At a high ultrasound power (31.5 W) we observed the highest bubble density at the hole entrances, while reducing the ultrasound power by a factor of ten shifted the bubble locations to the inner end of the blind holes or to the middle of the through holes.