Mechanism of the Decrease in Surface Tension by Bulk Nanobubbles (Ultrafine Bubbles).

The mechanism of the decrease in the surface tension of water containing bulk nanobubbles (ultrafine bubbles) is studied theoretically by numerical simulations of the adsorption of bulk nanobubbles at the liquid's surface based on the dynamic equilibrium model for the stability of a bulk nanobubble under the conditions of the Tuziuti experiment (Tuziuti, T., et al., Langmuir, 2023, 39, 5771-5778). It is predicted that the concentration of bulk nanobubbles in the bulk liquid decreases considerably with time, as many bulk nanobubbles are gradually adsorbed at the liquid's surface. A part of the decrease in surface tension is due to the Janus-like structure of a bulk nanobubble that could partly break the hydrogen bond network of water molecules at the liquid's surface because more than 50% of the bubble's surface is covered with hydrophobic impurities, according to the dynamic equilibrium model. The theoretically estimated decrease in surface tension due to the Janus-like structure of a bulk nanobubble agrees with the experimental data of the decrease in surface tension solely by bulk nanobubbles obtained by the comparison of before and after the elimination of bulk nanobubbles by the freeze-thaw process. This effect cannot be explained by the electric charge stabilization model widely discussed for the stability of a bulk nanobubble, although the present model is only applicable to the solution containing hydrophobic impurities. Another part of the decrease in surface tension should be due to impurities produced from a nanobubble generator, such as a mechanical seal, which was partly confirmed by the TOC measurements.

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.

Experimental investigation on the ultrasonic impregnation of wood through measurements of the intensity of sonoluminescence.

Ultrasonic impregnation is thought to be an effective way of permeation of liquid into material through the material-surface reforming with the attack by an ultrasonic cavitation jet or by the shock wave emitted from a collapsing bubble, or through dynamic transformation of material like a sponge. The action of a cavitation bubble can also provide penetration of liquid into the interior of the material. This paper investigates whether there is a correlation between the intensity of sonoluminescence (SL) measured at different positions and the increment in the mass of the wood material (cedar) after sonication with immersion into water in order to clarify the role of cavitation bubbles for ultrasonic impregnation. It was found that a high mass change was obtained for the material located at the position for high (the maximum) SL intensity. The number density of ultrasonic cavitation bubbles that are able to collapse leading to the emission of SL is correlated with the degree of ultrasonic impregnation.

Influence of bulk nanobubble concentration on the intensity of sonoluminescence.

The present study mainly examined the effects of the volumetric concentration of nanobubbles (ultrafine bubbles) on the intensity of sonoluminescence (SL). The addition of nanobubbles at high acoustic amplitude enhanced the SL intensity for various bubble concentrations in comparison with that in pure water. This probably means that the resulting high amplitude is over the Blake threshold, and accordingly nanobubbles expand to some extent, leading to higher SL intensity. Therefore, nanobubbles have the potential to provide nucleation sites for sonochemistry. The influence of bubble size on the intensity of SL was also evaluated.

Interaction of Bulk Nanobubbles (Ultrafine Bubbles) with a Solid Surface.

The experimental results [Kanematsu, W. Chem. Eng. Sci. 2020, 219, 115594] on the temporal variations of number concentrations of bulk nanobubbles (ultrafine bubbles) in contact with polymer materials are theoretically analyzed based on the dynamic equilibrium model of bulk nanobubbles partly covered with hydrophobic materials (impurities). It is suggested that bulk nanobubbles are adsorbed on a polymer surface by attractive hydrophobic interaction between a polymer surface and a hydrophobic material partly covering the bubble surface, overcoming the repulsive double-layer interaction. There are two mysteries. One is that the maximum surface number concentration of bulk nanobubbles of about 70 nm in diameter adsorbed on a hydrophobic polymer surface is more than an order of magnitude lower than the typical value for colloid particles of a similar or larger size. The other is that the experimental adsorption rate of bulk nanobubbles on hydrophobic polymer surface is several orders of magnitude lower than the theoretically estimated one. The mysteries are resolved if many of the bulk nanobubbles adsorbed on a hydrophobic polymer surface change to surface nanobubbles with a footprint diameter of about 1 µm.

Mechanism of OH radical production from ozone bubbles in water after stopping cavitation.

It has been experimentally reported that OH radicals are produced from ozone microbubbles even after stopping cavitation. Microbubbles are mostly produced using acoustic or hydrodynamic cavitation. In the present paper, numerical simulations of chemical reactions inside an ozone bubble and an oxygen bubble are performed during and after acoustic cavitation in order to study the mechanism of OH radical production. The results have indicated that less than one molecule of OH radicals is produced from a dissolving ozone bubble after stopping cavitation when local pH near the bubble wall is <8. On the other hand, more than 108 or 106 molecules of H2O2 are produced inside an ozone or oxygen microbubble, respectively, in water during acoustic cavitation per violent collapse. It suggests that OH radicals are possibly produced by the chemical reaction of H2O2 with O3 in the liquid phase even after stopping cavitation. Furthermore, in strongly acidic conditions (pH < 5), the reaction of H2O2 with O3 in the liquid phase is relatively slow, and OH production considerably continues after stopping cavitation. This may be the reason for the enhanced OH signals in strongly acidic conditions observed experimentally after stopping cavitation.

High temperature and pressure inside a dissolving oxygen nanobubble.

Numerical simulations of dissolution of an oxygen (O2) nanobubble into water without dynamic stimuli have been performed in order to study the possibility of OH radical formation from oxygen nanobubbles experimentally reported by Liu et al. (2016). The dissolution of an oxygen nanobubble is much faster than that of an air nanobubble due to higher solubility of oxygen in water. However, the temperature and pressure inside an oxygen nanobubble at the final moment of the bubble dissolution are about 2800 K and 4.5 GPa, respectively, which are slightly lower than those inside an air nanobubble due to higher thermal conductivity of oxygen. A few molecules of OH radicals may be formed per 107 bubbles according to the numerical simulation. The estimated production rate of OH radicals is 13 orders of magnitude smaller than the experimentally reported one.

Is surface tension reduced by nanobubbles (ultrafine bubbles) generated by cavitation?

Nanobubbles (ultrafine bubbles) are produced by hydrodynamic or acoustic cavitation. They work as cavitation nuclei. Is the experimentally reported considerable reduction of surface tension of liquid water by nanobubbles real? It is theoretically suggested that nanobubbles partly covered with hydrophobic materials are concentrated at a surface of liquid water. A hydrophobic cap is directed toward a gas phase above a liquid surface. Uncovered surface of a nanobubble is directed into liquid water underneath the liquid surface. It is suggested that a liquid film is more easily ruptured by the presence of nanobubbles at the liquid surface, which reduces the value of surface tension in du Noüy ring method. A role of ionic surfactants on accumulation of nanobubbles at a liquid surface is also discussed.

Mysteries of bulk nanobubbles (ultrafine bubbles); stability and radical formation.

There are two main mysteries in bulk nanobubbles which are cavitation nuclei. One is the mechanism of stability of a bulk nanobubble. The other is the problem whether OH radicals are produced from bulk nanobubbles without a dynamic stimulus. For the former problem, several proposed models are briefly reviewed. The dynamic equilibrium model is discussed in details that a bulk nanobubble is stabilized by a partial coverage of the bubble surface by a hydrophobic material. The TEM images of bulk nanobubbles seem to support the dynamic equilibrium model. For the latter problem, numerical simulations of dissolution of an air nanobubble are reviewed, which suggest that no OH radical is produced from a dissolving nanobubble. A possible role of H2O2 generated during bulk nanobubble production using hydrodynamic cavitation is briefly discussed in relation to the experimental results of "OH radical" detection.

Influence of addition of degassed water on bulk nanobubbles.

The effects of the addition of degassed water on bulk nanobubbles (ultrafine bubbles) of air in liquid water were investigated by measuring the volumetric concentration and size distribution at different dissolved air degree of saturation (DOS) values. The proportion of degassed water mixed with water containing bulk nanobubbles was increased to prepare samples having lower DOS values. It was found that the volumetric concentration of nanobubbles mostly decreased and the average nanobubble size became larger as the DOS was decreased. In our proposed mechanism, smaller nanobubbles are selectively dissolved into the surrounding liquid by Laplace pressure due to surface tension as the DOS is reduced. These results demonstrate that stable bulk nanobubbles are present even in water undersaturated with gas. The role of nanobubble under an ultrasound is also discussed.

Influence of increase in static pressure on bulk nanobubbles.

The concentration and size distribution of bulk nanobubbles (ultrafine bubbles) of air in liquid was measured before and after increasing the static pressure. It was found that, after pressurization, the number concentration decreased and the size increased. The effect of pressurization is to compress the bubbles and decrease the distance between solids on the bubble surface, which act as nucleation sites for bubble growth and coalescence when the static pressure is returned to atmospheric pressure.

Extreme conditions in a dissolving air nanobubble.

Numerical simulations of the dissolution of an air nanobubble in water have been performed taking into account the effect of bubble dynamics (inertia of the surrounding liquid). The presence of stable bulk nanobubbles is not assumed in the present study because the bubble radius inevitably passes the nanoscale in the complete dissolution of a bubble. The bubble surface is assumed to be clean because attachment of hydrophobic materials on the bubble surface could considerably change the gas diffusion rate. The speed of the bubble collapse (the bubble wall speed) increases to about 90 m/s or less. The shape of a bubble is kept nearly spherical because the amplitude of the nonspherical component of the bubble shape is negligible compared to the instantaneous bubble radius. In other words, a bubble never disintegrates into daughter bubbles during the dissolution. At the final moment of the dissolution, the temperature inside a bubble increases to about 3000 K due to the quasiadiabatic compression. The bubble temperature is higher than 1000 K only for the final 19 ps. However, the Knudsen number is more than 0.2 for this moment, and the error associated with the continuum model should be considerable. In the final 2.3 ns, only nitrogen molecules are present inside a bubble as the solubility of nitrogen is the lowest among the gas species. The radical formation inside a bubble is negligible because the probability of nitrogen dissociation is only on the order of 10^{-15}. The pressure inside a bubble, as well as the liquid pressure at the bubble wall, increases to about 5 GPa at the final moment of dissolution. The pressure is higher than 1 GPa for the final 0.7 ns inside a bubble and for the final 0.6 ns in the liquid at the bubble wall. The liquid temperature at the bubble wall increases to about 360 K from 293 K at the final stage of the complete dissolution.

Dynamic Equilibrium Model for a Bulk Nanobubble and a Microbubble Partly Covered with Hydrophobic Material.

The dynamic equilibrium model for a bulk nanobubble partly covered with hydrophobic material in water is theoretically and numerically studied. The gas diffusion into a bubble near the peripheral edge of the hydrophobic material on the bubble surface balances that out of the bubble from the other part of the uncovered bubble surface. In the present model, gas diffusion in quiescent liquid is assumed and there is no liquid flow. The total changes of energy and entropy are both zero as it is a kind of equilibrium state. The main origin of the dynamic equilibrium state is the gradient of chemical potential of gas near the peripheral edge of the hydrophobic material. It is caused by the permanent attractive potential of a hydrophobic material to gas molecules dissolved in liquid water as there is permanent repulsion of a hydrophobic material against liquid water. Thus, the gas supply will not terminate. It is numerically shown that stable nanobubble could be present when the fraction of surface coverage by hydrophobic material is from about 0.5 to 1. The stable size of a nanobubble changes with the liquid temperature as well as the degree of gas saturation of water. In slightly degassed water, not only a nanobubble but also a microbubble could be stable in mass balance when the fraction of surface coverage for a microbubble is on the order of 10-4 or less. For hydrophilic materials, however, a bubble could not be stable unless the fraction of the surface coverage is exactly 1. It is suggested that in many experiments of bulk nanobubbles there could be aggregates of nanobubbles.

Influence of sonication conditions on the efficiency of ultrasonic cleaning with flowing micrometer-sized air bubbles.

This paper describes the sizes of cleaned areas under different sonication conditions with the addition of flowing micrometer-sized air bubbles. The differences in the cleaned area of a glass plate pasted with silicon grease as a dirty material under different sonication conditions were investigated after tiny bubbles were blown on the dirty plate placed in an underwater sound field. The ultrasound was applied perpendicular to the bubble flow direction. The shape of the cleaned areas was nearly elliptical, so the lengths of the minor and major axes were measured. The length of the minor axis under sweep conditions (amplitude modulation), for which the average power was lower than that for continuous wave (CW) irradiation, was comparable to that for CW irradiation and was slightly larger than under bubble flow only. Not only the relatively high power for CW irradiation, but also the larger angular change of the bubble flow direction under sweep conditions contributed to the enlargement of the cleaned area in the direction of the minor axis. The combination of bubble flow and sonication under sweep or CW conditions produced a larger cleaned area compared with bubble flow only, although the increase was not higher than 20%. A rapid change from an air to water interface caused by the bubble flow and water jets caused by the collapse of bubbles due to violent pulsation is the main cleaning mechanism under a combination of ultrasound and bubble flow.

Advanced dynamic-equilibrium model for a nanobubble and a micropancake on a hydrophobic or hydrophilic surface.

The dynamic-equilibrium model for stabilization of a nanobubble on a hydrophobic surface by Brenner and Lohse [M. P. Brenner and D. Lohse, Phys. Rev. Lett. 101, 214505 (2008)] has been modified taking into account the van der Waals attractive force between gas molecules inside a nanobubble and solid surface. The present model is also applicable to a nanobubble on a hydrophilic surface. According to the model, the pressure inside a nanobubble is not spatially uniform and is relatively higher near the solid surface. As a result, there is gas outflux near a hydrophilic surface, while near a hydrophobic surface there is gas influx which has been already suggested. In the present model, the radius of curvature for a nanobubble depends on the distance from the solid surface because the pressure depends on it. The shape of the micropancake, which is a nearly-two-dimensional bubble, is reproduced by the present model due to the strong dependence of the radius of curvature on the distance from the solid surface. The effect of temperature on the stability of a nanobubble or micropancake is also discussed.

Measurement of speed of sound in poly(lactic acid)-clay composite.

We measured longitudinal speed of sound for matrix[poly(lactic acid)]-additive(clay particles) composite rectangular-solid specimen prepared by injection molding. It was found that the speed of sound measured in the direction along the longer side of the specimen was the highest at the middle of the specimen. This trend corresponded with that for crystallinity determined through differential scanning calorimetry (DSC). A cross section view of the specimen parallel to its longer side showed that there was a transverse flow trace of resin in the vicinity of the injection gate while the flow trace along the direction of the longer side spread wider as getting far from the gate toward the middle of the specimen. The high crystallinity appeared in the middle of the specimen was inferred to come from the promotion of crystallization by molecular orientation induced with the above flow trace parallel to the direction along the longer side of the specimen.

Influence of degree of gas saturation on sonochemiluminescence intensity resulting from microfluidic reactions.

This work examined the effects of dissolved gas degree of saturation (DOS) on sonochemical reaction yields in both a one-dimensional (1D) microspace and a three-dimensional (3D) millimeter-sized space. The extent of each reaction was monitored by measuring sonochemiluminescence (SCL) intensity at 213 kHz. The results demonstrated that, at relatively low levels of power density, selecting a solution DOS in the supersaturation range at atmospheric pressure resulted in higher yields per unit volume in the 1D space compared to that obtained from the 3D space. This effect is attributed to a decrease in the cavitation threshold of the 1D reaction system since, at low power density, the 1D space represents a more homogeneous reaction volume. Comparing the highest SCL intensity levels obtained from the 3D and 1D reactions shows that enhancing the reaction yield in the 1D space requires higher DOS values than are required to generate elevated yields in the 3D space. The 3D space contains a greater concentration of bubbles than the 1D space, but many of these are ineffective at promoting the reaction. Thus, reactions in the 3D environment require not only the application of higher power density levels but also a lower DOS, so as to allow the bubbles to undergo the violent pulsations necessary to facilitate the sonochemical reaction.

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.

Mist separation and sonochemiluminescence under pulsed ultrasound.

Differences in the amount of water-mist separation and the intensity of luminol chemiluminescence for pulsed and continuous-wave (CW) ultrasound at 135 kHz have been investigated. The amount of mist generated is estimated using the cooling rate of a copper plate sprayed with the mist. For pulsed operation with an appropriate duty cycle, the cooling rate and the cooling rate per input power to the transducer are higher by 4 and 12 times compared to CW operation, respectively. This is due to the amplitude of the pulsed ultrasound being higher than that for CW ultrasound. Relatively low power pulsed operation can successfully produce both a higher sonochemiluminescence (SCL) intensity and cooling rate than those for CW ultrasound. The sonochemical reaction for pulsed ultrasound occurs at the same input power threshold as that for mist separation, whereas for CW ultrasound, the former threshold is lower than the latter. A higher number of large bubbles is produced with CW ultrasound than that with pulsed ultrasound. To achieve a sound pressure amplitude sufficient for mist separation near the surface of a liquid, it is necessary to expel these bubbles by changing the sound field from resonant standing waves to progressive waves that give rise to capillary waves on the liquid surface.

Ultrasonic cavitation induced water in vegetable oil emulsion droplets--a simple and easy technique to synthesize manganese zinc ferrite nanocrystals with improved magnetization.

In the present investigation, synthesis of manganese zinc ferrite (Mn(0.5)Zn(0.5)Fe(2)O(4)) nanoparticles with narrow size distribution have been prepared using ultrasound assisted emulsion (consisting of rapeseed oil as an oil phase and aqueous solution of Mn(2+), Zn(2+) and Fe(2+) acetates) and evaporation processes. The as-prepared ferrite was nanocrystalline. In order to remove the small amount of oil present on the surface of the ferrite, it was subjected to heat treatment at 300 °C for 3h. Both the as-prepared and heat treated ferrites have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), TGA/DTA, transmission electron microscopy (TEM) and energy dispersion X-ray spectroscopy (EDS) techniques. As-prepared ferrite is of 20 nm, whereas the heat treated ferrite shows the size of 33 nm. In addition, magnetic properties of the as-prepared as well as the heat treated ferrites have also been carried out and the results of which show that the spontaneous magnetization (σ(s)) of the heat treated sample (24.1 emu/g) is significantly higher than that of the as-synthesized sample (1.81 emu/g). The key features of this method are avoiding (a) the cumbersome conditions that exist in the conventional methods; (b) usage of necessary additive components (stabilizers or surfactants, precipitants) and (c) calcination requirements. In addition, rapeseed oil as an oil phase has been used for the first time, replacing the toxic and troublesome organic nonpolar solvents. As a whole, this simple straightforward sonochemical approach results in more phase pure system with improved magnetization.