Study of the coalescence of acoustic bubbles as a function of frequency, power, and water-soluble additives

The effect that surface-active solutes, such as aliphatic alcohols and sodium dodecyl sulfate (SDS), have on the extent of bubble coalescence in liquids under different sonication conditions has been investigated by measuring the volume change of the solution following a period of sonication. In general, the adsorption of surface-active solutes onto the bubble surface retards bubble coalescence. Within the limitations of the measurement method and the systems studied, bubble coalescence does not appear to be dependent on the applied acoustic power. Also, varying the applied acoustic frequency has a minimal effect on the extent of bubble coalescence in systems where long-range electrostatic repulsion between bubbles, imparted by the adsorbed surface-active solutes, dominates. However, when short-range steric repulsion (or other short-range repulsive forces) is the primary factor in inhibiting bubble coalescence, the dependence on the applied acoustic frequency becomes apparent, with less coalescence inhibition at higher frequencies. It is also concluded that SDS does not reach an equilibrium adsorption level at the bubble/solution interface under the sonication conditions used. On the basis of this conclusion, a method is proposed for estimating nonequilibrium surface excess values for solutes that do not fully equilibrate with the bubble/solution interface during sonication. For the case of SDS in the presence of excess NaCl, the method was further employed to estimate the maximum lifetime of bubbles in a multibubble field. It was concluded that an acoustic bubble in a multibubble field has a finite lifetime, and that this lifetime decreases with increasing applied frequency, ranging from up to 0.35 +/- 0.05 ms for 213 kHz to 0.10 +/- 0.05 ms for 1062 kHz. These estimated lifetimes equate to a bubble in a multibubble field undergoing an upper limit of 50-200 oscillations over its lifetime for applied ultrasound frequencies between 200 kHz and 1 MHz.     

Sonoluminescence quenching of organic compounds in aqueous solution: frequency effects and implications for sonochemistry

Solute-induced quenching of sonoluminescence (SL) is reported for aqueous solutions of two homologous series of methyl esters and ketones using low (20 kHz) and high (515 kHz) ultrasound frequencies. SL data at 20 kHz from aqueous solutions containing alcohols and carboxylic acids are also presented to compare with previously published results at 515 kHz. In addition to supporting the previous findings on the existence of stable and transient bubbles at 515 and 20 kHz, respectively, the results suggest that the hydrogen-bonding characteristics of the solutes also play a major role in the extent of SL quenching. An increase in the SL intensity at low concentrations for most of the solutes suggests that these solutes increase the number of "active" bubbles by hindering the coalescence of bubbles. It is concluded that the effect of the solutes on the SL signal from aqueous solutions at both frequencies is primarily due to the balance of two factors, namely, the incorporation of solute within the bubble, leading to SL quenching, and the prevention of coalescence of the bubbles, leading to SL enhancement. At the higher frequency, SL quenching by the solutes is the main influence on the emission yield. However, at the lower frequency, hindrance to coalescence by the solutes dominates at lower concentrations and leads to SL enhancement. The implications of these results for optimizing conditions for aqueous sonochemical reactions are discussed.     

Acoustic emission spectra from 515 kHz cavitation in aqueous solutions containing surface-active solutes

The effect of adding surface-active solutes to water being insonated at 515 kHz has been investigated by monitoring the acoustic emission from the solutions. At low concentrations (<3 mM), sodium dodecyl sulfate causes marked changes to the acoustic emission spectrum which can be interpreted in terms of preventing bubble coalescence and declustering of bubbles within a cavitating bubble cloud. By conducting experiments in the presence of background electrolytes and also using non-ionic surfactants, the importance of electrostatic effects has been revealed. The results provide further mechanistic evidence for the interpretation of the effect of surface-active solutes on acoustic cavitation and hence on the mechanism of sonochemistry. The work will be valuable to many researchers in allowing them to optimize reaction and process conditions in sonochemical systems.     

Effect of surfactants on inertial cavitation activity in a pulsed acoustic field

It has previously been reported that the addition of low concentrations of ionic surfactants enhances the steady-state sonoluminescence (SL) intensity relative to water (Ashokkumar; et al. J. Phys. Chem. B 1997, 101, 10845). In the current study, both sonoluminescence and passive cavitation detection (PCD) were used to examine the acoustic cavitation field generated at different acoustic pulse lengths in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A decrease in the SL intensity was observed in the presence of low concentrations of SDS and short acoustic pulse lengths. Under these conditions, the inhibition of bubble coalescence by SDS leads to a population of smaller bubbles, which dissolve during the pulse "off time". As the concentration of surfactant was increased at this pulse length, an increase in the acoustic cavitation activity was observed. This increase is partly attributed to enhanced growth rate of the bubbles by rectified diffusion. Conversely, at long pulse lengths acoustic cavitation activity was enhanced at low SDS concentrations as a larger number of the smaller bubbles could survive the pulse "off time". The effect of reduced acoustic shielding and an increase in the "active" bubble population due to electrostatic repulsion between bubbles are also significant in this case. Finally, as the surfactant concentration was increased further, the effect of electrostatic induced impedance shielding or reclustering dominates, resulting in a decrease in the SL intensity.     

Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions

Bubble coalescence behavior in aqueous electrolyte (MgSO(4), NaCl, KCl, HCl, H(2)SO(4)) solutions exposed to an ultrasound field (213 kHz) has been examined. The extent of coalescence was found to be dependent on electrolyte type and concentration, and could be directly linked to the amount of solubilized gas (He, Ar, air) in solution for the conditions used. No evidence of specific ion effects in acoustic bubble coalescence was found. The results have been compared with several previous coalescence studies on bubbles in aqueous electrolyte and aliphatic alcohol solutions in the absence of an ultrasound field. It is concluded that the impedance of bubble coalescence by electrolytes observed in a number of studies is the result of dynamic processes involving several key steps. First, ions (or more likely, ion-pairs) are required to adsorb at the gas/solution interface, a process that takes longer than 0.5 ms and probably fractions of a second. At a sufficient interfacial loading (estimated to be less than 1-2% monolayer coverage) of the adsorbed species, the hydrodynamic boundary condition at the bubble/solution interface switches from tangentially mobile (with zero shear stress) to tangentially immobile, commensurate with that of a solid-liquid interface. This condition is the result of spatially nonuniform coverage of the surface by solute molecules and the ensuing generation of surface tension gradients. This change reduces the film drainage rate between interacting bubbles, thereby reducing the relative rate of bubble coalescence. We have identified this point of immobilization of tangential interfacial fluid flow with the "critical transition concentration" that has been widely observed for electrolytes and nonelectrolytes. We also present arguments to support the speculation that in aqueous electrolyte solutions the adsorbed surface species responsible for the immobilization of the interface is an ion-pair complex.     

Enlightened sonochemistry

Sonochemistry and photochemistry are initiated by high-energy transient species, which may be prone to mutual interaction. Electronic excitation of solutes by energy transfer from high energy species generated in collapsing bubbles is already supported by experimental evidence. The rates of photochemical reactions can be affected by ultrasound-induced mixing of liquids caused by microstreaming near pulsating cavitation bubbles and shockwaves due to bubble collapse. This may not only improve light absorption but also modify the pathway of reaction by increasing the contact between reagents. Finally, one may speculate about a potentially new chemistry of photoexcited solutes under the extreme conditions inside cavitation microreactors. This work reviews research on the excitation of solutes by sonoluminescence, the combined effects of ultrasound and light on liquid systems and the effect of ultrasound on photocatalytic reactions.     

The detachment of particles from coalescing bubble pairs

This paper is concerned with the detachment of particles from coalescing bubble pairs. Two bubbles were generated at adjacent capillaries and coated with hydrophobic glass particles of mean diameter 66 μm. The bubbles were then positioned next to each other until the thin liquid film between them ruptured. The particles that dropped from the bubble surface during the coalescence process were collected and measured. The coalescence process was very vigorous and observations showed that particles detached from the bubble surfaces as a result of the oscillations caused by coalescence. The attached particles themselves and, to some extent the presence of the surfactant had a damping affect on the bubble oscillation, which played a decisive role on the particle detachment phenomena. The behaviour of particles on the surfaces of the bubbles during coalescence was described, and implications of results for the flotation process were discussed.     

Novel Applications of Non Hofmeister Ion Specificity in Bubble Interactions

The ion specificity of bubble-bubble interactions in water remains unexplained. Whatever their valence all ion pairs either completely inhibit bubble coalescence or have no effect whatever. The phenomenon appears unrelated to Hofmeister specificity. Salts which inhibit coalescence enable the formation of a high density bubble column evaporator (BCE). If hot gas bubbles are injected into the bubble column evaporator at a significantly higher temperature than the water, the hot bubble surfaces can be used to produce thermal effects in dissolved and dispersed solutes. These two properties can be exploited for a wide range of applications. Among these, high temperature aqueous reactions catalyzed at low solution temperatures, measurement of enthalpies of vaporization of concentrated salt solutions, wastewater treatments by sterilization and de-watering and desalination are a few.     

The influence of acoustic power on multibubble sonoluminescence in aqueous solution containing organic solutes

The effect of varying the applied acoustic power on the extent to which the addition of water-soluble solutes affect the intensity of aqueous multibubble sonoluminescence (MBSL) has been investigated. Under most of the experimental conditions used, the addition of aliphatic alcohols to aqueous solutions was found to suppress the MBSL intensity, although an enhancement of the MBSL intensity was also observed under certain conditions. In contrast, the presence of an anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solutions generally enhanced the observed MBSL intensity. For a series of aliphatic alcohols and SDS, a strong dependence of the MBSL intensity on the applied acoustic power (in the range of 0.78-1.61 W/cm(2)) at 358 kHz was observed. The relative SL quenching was significantly higher at higher acoustic powers for the alcohol solutions, whereas the relative SL enhancement was lower at higher acoustic powers in SDS solutions. These observations have been interpreted in terms of a combination of material evaporation into the bubble, rectified diffusion, bubble clustering and bubble-bubble coalescence.     

Selective coalescence of bubbles in simple electrolytes

Simple ions in electrolytes exhibit different degrees of affinity for the approach to the free surface of water. This results in strong ion-specific effects that are particularly dramatic in the selective inhibition of bubble coalescence. I present here the calculation of electrostatic interaction between free surfaces of electrolytes caused by the ion accumulation or depletion near a surface. When both anion and cation are attracted to the surface (like H+ and Cl- in HCl solutions), van der Waals attraction facilitates approach of the surfaces and the coalescence of air bubbles. When only an anion or cation is attracted to the surface (like Cl- in NaCl solutions), an electric double layer forms, resulting in repulsive interaction between free surfaces. I applied the method of effective potentials (evaluated from published ion density profiles obtained in simulations) to calculate the ionic contribution to the surface-surface interaction in NaCl and HCl solutions. In NaCl, but not in HCl, the double-layer interaction creates a repulsive barrier to the approach of bubbles, in agreement with the experiments. Moreover, the concentration where ionic repulsion in NaCl becomes comparable in magnitude to the short-range hydrophobic attraction corresponds to the experimentally found transition region toward the inhibition of coalescence.     

Coalescence map for bubbles in surfactant-free aqueous electrolyte solutions

Factors influencing bubble coalescence in surfactant-free aqueous electrolyte solutions are considered in this compilation of literature results. These factors include viscous and inertial thin film drainage, surface deformation, surface elasticity, mobility or otherwise of the air-water interface, and disjoining pressure. Several models from the literature are discussed, with particular attention paid to predictions of transitions between regions where behaviour is qualitatively different. The transitions are collated onto a single chart with salt concentration and bubble approach speed as the axes. This creates a map of the regions in which different mechanisms operate, giving an overall picture of bubble coalescence behaviour over a wide range of concentration and speed. Only mm-size bubbles in water and NaCl solutions are discussed in this initial effort at creating such a map. Data on bubble coalescence or non-coalescence are collected from the literature and plotted on the same map, generally aligning well with the predicted transitions and thus providing support for the theoretical reasoning that went into creating the coalescence map.     

Effect of ultrasound frequency on pulsed sonolytic degradation of octylbenzene sulfonic acid

It has been shown that pulsed ultrasound can influence the amount of surfactant that can adsorb to and decompose at the surface of cavitation bubbles. However, the effect of ultrasound frequency on this process has not been considered. The current study investigates the effect of ultrasound frequency on the pulsed sonolytic degradation of octyl benzenesulfonate (OBS). Furthermore, the effect of pulsing and ultrasound frequency on the rate of *OH radical formation was determined. OBS degradation rates were compared to the rates of *OH radical formation. In this way, conclusions were made regarding the relative importance of accumulation of OBS at cavitation bubble surfaces versus sonochemical activity to the sonochemical decomposition of OBS under different conditions of sonolysis. Comparisons of the data in this way indicate that sonolytic degradation of OBS depends on both the sonochemical activity (i.e., *OH yield) and the accumulation of OBS on cavitation bubble surfaces. However, under a certain set of pulsing and ultrasound frequency exposure conditions, enhanced accumulation of OBS at the gas/solution interface of cavitation bubbles is the sole mechanism of enhanced degradation due to pulsing. On the basis of this finding, conclusions on how pulsing at various ultrasound frequencies affects cavitation bubbles were made.     

Effects of pulsed ultrasound on the adsorption of n-alkyl anionic surfactants at the gas/solution interface of cavitation bubbles

Sonolysis of argon-saturated aqueous solutions of the nonvolatile surfactants sodium dodecyl sulfate (SDS) and sodium 1-pentanesulfonate (SPSo) was investigated at three ultrasonic frequencies under both continuous wave (CW) and pulsed ultrasound. Secondary carbon-centered radicals were detected by spin trapping using 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and electron paramagnetic resonance (EPR) spectroscopy. Following sonolysis, -*CH- radicals were observed for both surfactants under both sonication modes. Under CW at 354 kHz, the maximum plateau -*CH- radical yield was higher for SPSo than for SDS, indicating that SDS, which is more surface active under equilibrium conditions, accumulates at the gas/solution interface of cavitation bubbles to a lesser degree, compared with the less surface active surfactant, SPSo. However, after sonolysis (354 kHz) under pulsed ultrasound with a pulse length of 100 ms and an interval of 500 ms, the -*CH- radical yield at the plateau concentrations was higher for SDS than for SPSo due to increased amounts of SDS accumulation on the bubble surfaces. In contrast to the findings following sonolysis at 354 kHz, sonolysis of aqueous surfactant solutions at 620 kHz and 803 kHz showed a higher -*CH- radical yield for SDS compared with SPSo under CW but lower -*CH- radical yield with increasing pulsing interval, indicating a frequency dependence on accumulation. Results indicate that pulsing the ultrasonic wave has a significant effect on the relative adsorption ability of n-alkyl surfactants at the gas/solution surface of cavitation bubbles.     

Zeta Potential of Nanobubbles Generated by Ultrasonication in Aqueous Alkyl Polyglycoside Solutions

A simple and convenient method to measure microelectrophoretic mobilities was proposed to determine the zeta potential of nanobubbles generated by ultrasonication. Bubbles in pure water solutions and in aqueous solutions of alkyl polyglycoside (AG) with different alkyl chain lengths and degrees of polymerization in the head group were sonicated with a palladium-coated electrode designed specially by the manufacturer. The zeta potentials of bubbles with ordinary cationic and ionic surfactants are consistent with others' previous results. The average size of the bubbles generated by sonication is in the range of 300 to 500 nm. The zeta potentials of bubbles in both pure water and AG solutions at all pH values are negative. As the chain length of AG increases, zeta potentials significantly decrease at high pH. For nonionic AG, a possible charging mechanism based on known mechanisms is suggested to explain the negative charge, known to be unusual. Even with a very high concentration of H(+) ions in solution the bubbles are charged negatively because the interface is covered with slightly acidic alcohol groups of AGs. At high pH, the less polar the surfactant, the more negative the charge, since nonpolar surfactant molecules induce the adsorption of OH(-) ions, rather than H(+) ions that prefer hydration by water molecules. Copyright 2000 Academic Press.     

Bubble-size dependence of the critical electrolyte concentration for inhibition of coalescence

The interaction of pairs of bubbles with equal diameters grown on adjacent capillaries in aqueous magnesium sulfate solutions is observed for varying electrolyte concentrations and bubble diameters. As in previous investigations, a sharp transition from coalescence to bubble detachment without coalescence is observed with increasing electrolyte concentration. The critical electrolyte concentration for this transition is found to increase with decreasing bubble diameter for bubble diameters of 1.4 to 4.2 mm.     

A new approach to nucleation of cavitation bubbles at chemically modified surfaces

Cavitation at the solid surface normally begins with nucleation, in which defects or assembled molecules located at a liquid-solid interface act as nucleation centers and are actively involved in the evolution of cavitation bubbles. Here, we propose a simple approach to evaluate the behavior of cavitation bubbles formed under high intensity ultrasound (20 kHz, 51.3 W cm(-2)) at solid surfaces, based on sonication of patterned substrates with a small roughness (less than 3 nm) and controllable surface energy. A mixture of octadecylphosphonic acid (ODTA) and octadecanethiol (ODT) was stamped on the Si wafer coated with different thicknesses of an aluminium layer (20-500 nm). We investigated the growth mechanism of cavitation bubble nuclei and the evolution of individual pits (defects) formed under sonication on the modified surface. A new activation behavior as a function of Al thickness, sonication time, ultrasonic power and temperature is reported. In this process cooperativity is introduced, as initially formed pits further reduce the energy to form bubbles. Furthermore, cavitation on the patterns is a controllable process, where up to 40-50 min of sonication time only the hydrophobic areas are active nucleation sites. This study provides a convincing proof of our theoretical approach on nucleation.     

The dynamics of a bubble ensemble in a cavitation field

A system of equations was obtained to describe the dynamics of bubbles in a cavitation cloud taking into account the interaction of pulsating bubbles involved in translational motion. The kinetics of cavitation bubble concentration changes, changes in the compressibility of the liquid, and phase transitions within a cavitation bubble and in the neighboring volume of the liquid were taken into account. The role played by bubble deformation in a cavitation cloud was considered. The Bernoulli pressure effect was shown to be negligible. The interaction of cavitation bubbles was a substantial factor that strongly influenced the dynamics of bubbles. It was suggested that there was at least one more mechanism that reduced sonoluminescence intensity from the multiple-bubble cavitation field, namely, a fairly high efficiency of sonoluminescence quenching could additionally be related to the arrival of a cumulative liquid stream at the central cavitation bubble region, where the concentration of active species was high. The dynamics of bubbles in the cavitation field is not only related to the expansion and compression of cavitation bubbles in the acoustic field, but also governed to a great extent by their interaction, translational motion, deformation, and the influence of cumulative streams penetrating the bubbles.     

Dynamic surface tension measurements of surfactant solutions using the maximum bubble pressure method – limits of applicability

One of the essential differences in the design of bubble pressure tensiometers consists in the geometry of the measuring capillaries. To reach extremely short adsorption times of milliseconds and below, the so-called deadtime of the capillaries must be of the order of some 10 ms. In particular, for concentrated surfactant solutions, such as micellar solutions, short deadtimes are needed to minimize the initial surfactant load of the generated bubbles. A theoretical model is derived and confirmed by experiments performed for a wide range of experimental conditions, mainly in respect to variations in deadtime and bubble volume.     

Flow development at the surfaces of bubbles and droplets in gradient solutions of a surfase-active liquid

The development of solutocapillary flows at the surfaces of air bubbles and chlorobenzene droplets was experimentally studied in nonuniform aqueous solutions of ethanol and isopropanol, which have a low surface tension and, hence, exhibit surface-active properties with respect to water. The experiments demonstrated the retardation of the onset of the development of the Marangoni concentration-induced convection relative to the moment of the contact between an inflowing surfactant (alcohol) and the surface. The critical concentration gradients (the Marangoni diffusion numbers) necessary for the initiation of mass transfer of a liquid along the interface were determined as dependent on the rate of inflow of a tongue of a more concentrated solution and the initial alcohol concentration around the bubble.     

Nonequilibrium bubbles in a flowing langmuir monolayer

We investigate the nonequilibrium behavior of two-dimensional gas bubbles in Langmuir monolayers. A cavitation bubble is induced in liquid expanded phase by locally heating a Langmuir monolayer with an IR-laser. At low IR-laser power the cavitation bubble is immersed in quiescent liquid expanded monolayer. At higher IR-laser power thermo capillary flow around the laser-induced cavitation bubble sets in. The thermo capillary flow is caused by a temperature dependence of the gas/liquid line tension. The slope of the line tension with temperature is determined by measuring the thermo capillary flow velocity. Thermodynamically stable satellite bubbles are generated by increasing the surface area of the monolayer. Those satellite bubbles collide with the cavitation bubble. Upon collision the satellite bubbles either coalesce with the cavitation bubble or slide past the cavitation bubble. Moreover we show that the satellite bubbles can also be produced by the emission from the laser-induced cavitation bubbles.