New interferometric technique to evaluate the electric charge of gas bubbles in liquids

We report a new interferometric technique to measure the electric charge at the gas-liquid interface of a bubble in a liquid. The bubble rests by buoyancy against an electrode, and an alternating electric field excites its capillary oscillations. The oscillation amplitude of the quadrupolar mode frequency is measured by the interferometer, and it is used to evaluate the electric charge. The mode frequency scales with the square root of the interfacial tension and with a -(3)/(2) power law as a function of the bubble radius. For bubbles in the millimeter diameter range in pure water, the measured negative charge scales with the square of the radius, hence, giving a constant surface charge density on the order of 1.8 × 10(-5) C m(-2), which is rather consistent with the electrophoretic values reported in the literature.     

Long-lived submicrometric bubbles in very diluted alkali halide water solutions

Solutions of LiCl and of NaCl in ultrapure water were studied through Rayleigh/Brillouin scattering as a function of the concentration (molarity, M) of dissolved salt from 0.2 M to extremely low concentration (2 × 10(-17) M). The Landau-Placzek ratio, R/B, of the Rayleigh scattering intensity over the total Brillouin was measured thanks to the dynamically controlled stability of the used Fabry-Perot interferometer. It was observed that the R/B ratio follows two stages as a function of increasing dilution rate: after a strong decrease between 0.2 M and 2 × 10(-5) M, it increases to reach a maximum between 10(-9) M and 10(-16) M. The first stage corresponds to the decrease of the Rayleigh scattering by the ion concentration fluctuations with the decrease of salt concentration. The second stage, at lower concentrations, is consistent with the increase of the Rayleigh scattering by long-lived sub-microscopic bubbles with the decrease of ion concentration. The origin of these sub-microscopic bubbles is the shaking of the solutions, which was carried out after each centesimal dilution. The very long lifetime of the sub-microscopic bubbles and the effects of aging originate in the electric charge of bubbles. The increase of R/B with the decrease of the low salt concentration corresponds to the increase of the sub-microscopic bubble size with the decrease of concentration, which is imposed by the bubble stability due to the covering of the surface bubble by negative ions.     

Adsorption of heavy metal ions from aqueous solution by polyrhodanine-encapsulated magnetic nanoparticles

The influence of salt concentration on the terminal velocities of gravity-driven single bubbles sliding along an inclined glass wall has been investigated, in an effort to establish whether surface forces acting between the wall and the bubble influence the latter's mobility. A simple sliding bubble apparatus was employed to measure the terminal velocities of air bubbles with radii ranging from 0.3 to 1.5 mm sliding along the interior wall of an inclined Pyrex glass cylinder with inclination angles between 0.6 and 40.1°. Experiments were performed in pure water, 10 mM and 100 mM KCl solutions. We compared our experimental results with a theory by Hodges et al. which considers hydrodynamic forces only, and with a theory developed by two of us which considers surface forces to play a significant role. Our experimental results demonstrate that the terminal velocity of the bubble not only varies with the angle of inclination and the bubble size but also with the salt concentration, particularly at low inclination angles of ～1-5°, indicating that double-layer forces between the bubble and the wall influence the sliding behavior. This is the first demonstration that terminal velocities of sliding bubbles are affected by disjoining pressure.     

Bubble colloidal AFM probes formed from ultrasonically generated bubbles

Here we introduce a simple and effective experimental approach to measuring the interaction forces between two small bubbles (approximately 80-140 microm) in aqueous solution during controlled collisions on the scale of micrometers to nanometers. The colloidal probe technique using atomic force microscopy (AFM) was extended to measure interaction forces between a cantilever-attached bubble and surface-attached bubbles of various sizes. By using an ultrasonic source, we generated numerous small bubbles on a mildly hydrophobic surface of a glass slide. A single bubble picked up with a strongly hydrophobized V-shaped cantilever was used as the colloidal probe. Sample force measurements were used to evaluate the pure water bubble cleanliness and the general consistency of the measurements.     

Electrostatic attraction between a hydrophilic solid and a bubble

The contact between fine hydrophilic α-Al(2)O(3) particles and nitrogen bubbles was studied as a function of solution composition in single bubble capture experiments, where the bubble collection efficiency was measured. The surface charges of both bubble and particle were controlled by varying the electrolyte concentration and pH of the solution. In all experiments the bubbles were negatively charged while the α-Al(2)O(3) particles were either negatively (above pH of the isoelectric point, pH(IEP)) or positively (below pH(IEP)) charged. The collection efficiency was found to be strongly influenced by the surface charge of the particles. The maximum collection efficiency occurred when the bubble and particle were oppositely charged (at low pH values) and at low salt concentration, i.e. when a long range attractive electrostatic interaction is present. In the case where both bubble and particle were of the same charge, the collection efficiency was near to zero within experimental error and was not influenced by either salt concentration or pH. This is the first experimental proof of the concept of 'contactless flotation', first proposed by Derjaguin and Dukhin in 1960, with far reaching implications from minerals processing to biology.     

Kinetics of disproportionation of air bubbles beneath a planar air-water interface stabilized by food proteins

The rate of shrinkage of air bubbles of initial radii, r, from 50 to 150 microm injected beneath a planar air-water interface has been measured. Bubbles were stabilized by 0.05 wt% protein in approximately 0.1 mol dm(-3) ionic strength buffer at pH 7.0 and at room temperature. Four proteins were studied: commercial whey protein isolate (WPI), sodium caseinate, gelatin, and pure beta-lactoglobulin. Bubbles in all systems showed shrinkage due to diffusion of gas from the bubbles, which accelerated as the bubbles got smaller. Within approximately 1 h all bubbles had disappeared, having shrunk to below approximately 1 microm, so that in no cases was there evidence of stabilization via a surface rheological mechanism. The rates of shrinkage with the different proteins were not significantly different except in the case of gelatin, which at any given bubble size appeared to give a slightly higher rate, probably because the surface tension is higher for this system. A new theoretical analysis of the dissolution kinetics for the case of a bubble close to a planar interface has been developed. For caseinate and WPI a simple model incorporating a constant surface tension and a constant bubble-interface separation appears to account for the kinetics. Interestingly, the model predicts a linear dependence of r(n) versus time when n is closest to 3, in contrast to n = 2 expected from previous work. For gelatin and pure beta-lactoglobulin, the introduction of modest dilatational elasticities of approximately 2.3 and 7 mN m(-1), respectively, gives good agreement between theory and experiment. This is particularly the case for beta-lactoglobulin, where there is a noticeable slowing, but not cessation, of the shrinkage as the bubbles get smaller. In the light of these findings the practical significance of surface rheology with respect to stability to disproportionation is discussed. Finally, we present experimental evidence that a bubble stabilized by beta-lactoglobulin shrinks to a nonspherical protein particle consisting of the completely collapsed protein film.     

聚醚型表面活性剂在界面上的分子取向 Ⅱ. 环氧乙烷环氧丙烷共聚多元醇在水面上的表面膜

测定了水面上六种环氧乙烷环氧丙烷共聚多元醇所成表面膜的π-A关系, 其中三种是Plaronie系列ABA型嵌段共聚物(分子量在2×10~3-8×10~3之间), 三种是无规共聚的超商分于(分子量约为2×10~6)。结果表明, 在水相中有46％NaNO_3时, 成膜分子中的聚氧乙烯(EO)链将以一部分留在界面上其众部分伸入水中的方式取向;π越大, 挤入水面下的EO链节越多。聚氧丙烯(PO)链的情形也相似, 但远不如EO链那样易于挤离水面。根据这一模型, 并假定EO链和PO链的行为是相互独立的, 即可将Triton型表面活性剂和聚丙二醇的表面膜的实验结果定量地与Pluronic共聚物和超高分子无规共聚物的表面膜性质联系起来。但当基底水相中无盐时, 成膜分于的EO链极易进入水相, 而且很可能拖带一部分PO链伸进水相; 也就是说, 这时EO链和PO链的行为不再是相互独立的了, 这可以解释实验的结果。     

Rupture of wetting films caused by nanobubbles

It is now widely accepted that nanometer sized bubbles, attached at a hydrophobic silica surface, can cause rupture of aqueous wetting films due to the so-called nucleation mechanism. But the knowledge of the existence of such nanobubbles does not give an answer to how the subprocesses of this rupture mechanism operate. The aim of this paper is to describe the steps of the rupture process in detail: (1) During drainage of the wetting film, the apex of the largest nanobubble comes to a distance from the wetting film surface, where surface forces are acting. (2) An aqueous "foam film" in nanoscale size is formed between the bubble and the wetting film surface; in this foam film different Derjaguin-Landau-Verwey-Overbeek (DLVO) forces are acting than in the surrounding wetting film. In the investigated system, hydrophobized silica/water/air, all DLVO forces in the wetting film are repulsive, whereas in the foam film the van der Waals force becomes attractive. (3) The surface forces over and around the apex of the nanobubble lead to a deformation of the film surfaces, which causes an additional capillary pressure in the foam film. An analysis of the pressure balance in the system shows that this additional capillary pressure can destabilize the foam film and leads to rupture of the foam film. (4) If the newly formed hole in the wetting film has a sufficient diameter, the whole wetting film is destabilized and the solid becomes dewetted. Experimental data of rupture thickness and lifetime of wetting films of pure electrolyte and surfactant solutions show that the stabilization of the foam film by surfactants has a crucial effect on the stability of the wetting film.     

End effects for a rodlike polyelectrolyte molecule in salt solution

Electrostatic potentials around a single rodlike polyelectrolyte molecule are calculated by solving the nonlinear Poisson–Boltzmann equation numerically in the presence of externally added salt. The polyion is regarded as a cylinder with a finite length whose side surface is uniformly charged and end surfaces uncharged. The calculations show that the distance to which end effects extend is about half the Debye screening length and is almost independent of the surface charge density and concentration of added salt. For a long polyion whose length is much greater than the Debye length, the end effects can be neglected even for a polyelectrolyte with high surface charges, whereas they play an important role for a short polyion with a length of the same order as the Debye length. In addition, a strong charge condensation is found in the direction of the axis of the cylinder for a long polyion.     

海盐颗粒物表面的NO_2非均相反应

为了深入理解沿海城市大气环境中NO2和海盐颗粒物的非均相反应规律,本研究使用漫反射红外傅立叶变换光谱(DRIFTS)比较研究了0%和20%相对湿度(relative humidty,RH)下NO2在湿海盐颗粒物表面的非均相反应.动力学测量表明硝酸盐的生成对NO2是二级反应,并且0%和20%相对湿度条件下,NO2分子浓度为1.96×1015molcules·cm-3时,反应增长阶段反应摄取系数分别为(5.51±0.19)×10-7和1.26×10-6.结果还显示相对湿度在30%以下时,海盐表面MgCl2·6H2O、CaCl2·2H2O所在点位通过释放结合水和吸附水汽,在海盐表面形成液态水的斑点,增强了反应持续能力.因此氯化钠表面非均相反应的研究可能会低估海盐颗粒物的非均相反应活性.     

Proximal adsorption at glass surfaces: ionic strength, pH, chain length effects

The adsorption and desorption of pyridinium chloride surfactants on borosilicate glass are studied as a function of the separation between two glass-solution interfaces. Both the adsorption and desorption change exponentially with the separation; the decay is equal to the solution Debye length. Changes in adsorption are smaller at pH 1.8 (near the point of zero charge of glass) than at pH 6. These results are consistent with an electrostatic cause for the changes in adsorption. The magnitude of the adsorption regulation, however, depends on the length of the alkyl chain and the surface excess of the surfactant. Therefore, proximal adsorption in this system depends on the coupling between the long-range electrostatic forces and the short-range chain-chain interactions. The equation of state for the surfactant on a regulating surface is discussed with respect to changes in intersurface separation.     

Curvature dependence of surface free energy of liquid drops and bubbles: A simulation study

We study the excess free energy due to phase coexistence of fluids by Monte Carlo simulations using successive umbrella sampling in finite L×L×L boxes with periodic boundary conditions. Both the vapor-liquid phase coexistence of a simple Lennard-Jones fluid and the coexistence between A-rich and B-rich phases of a symmetric binary (AB) Lennard-Jones mixture are studied, varying the density ρ in the simple fluid or the relative concentration x(A) of A in the binary mixture, respectively. The character of phase coexistence changes from a spherical droplet (or bubble) of the minority phase (near the coexistence curve) to a cylindrical droplet (or bubble) and finally (in the center of the miscibility gap) to a slablike configuration of two parallel flat interfaces. Extending the analysis of Schrader et al., [Phys. Rev. E 79, 061104 (2009)], we extract the surface free energy γ(R) of both spherical and cylindrical droplets and bubbles in the vapor-liquid case and present evidence that for R→∞ the leading order (Tolman) correction for droplets has sign opposite to the case of bubbles, consistent with the Tolman length being independent on the sign of curvature. For the symmetric binary mixture, the expected nonexistence of the Tolman length is confirmed. In all cases and for a range of radii R relevant for nucleation theory, γ(R) deviates strongly from γ(∞) which can be accounted for by a term of order γ(∞)/γ(R)-1∝R(-2). Our results for the simple Lennard-Jones fluid are also compared to results from density functional theory, and we find qualitative agreement in the behavior of γ(R) as well as in the sign and magnitude of the Tolman length.     

Ultrafast charge separation and recombination dynamics in lead sulfide quantum dot-methylene blue complexes probed by electron and hole intraband transitions

Lead salt quantum dots (QDs) have emerged as attractive materials for solar energy conversion because of their broad spectral response, long exciton lifetime, and efficient multiexciton generation. However, charge separation dynamics from these QDs remain poorly understood. In this study we investigate charge separation and recombination dynamics in PbS-methylene blue (MB(+)) complexes by femtosecond transient absorption spectroscopy. We show that while the 1S electrons and holes in excited PbS QDs lead to overlapping transient absorption features in the visible and near-IR regions, their intraband absorptions in the mid-IR can be monitored independently to directly follow the charge separation and recombination processes. The charge separation and recombination rates in PbS-MB(+) complexes were found to be (2.7 ± 0.2) × 10(12) and (1.1 ± 0.2) × 10(11) s(-1), respectively. The ultrafast charge separation rate suggests the possibility of hot electron injection and multiexciton dissociation from these strongly quantum confined QDs, consistent with recent reports of these phenomena at lead salt QD/TiO(2) interfaces.     

Factors controlling the formation and stability of air bubbles stabilized by partially hydrophobic silica nanoparticles

Air bubbles have been formed using partially hydrophobic silica nanoparticles as the stabilizer. The particles were of primary particle size 20 nm, chemically treated to different degrees with dichlorodimethylsilane to render them partially hydrophobic. Above a certain bubble size range (typically 80-microm diameter), the bubbles seemed to be almost indefinitely stable, while for any size above 20 microm their stability against disproportionation is far better than bubbles stabilized by any protein film investigated in previous studies. A possible theoretical justification for this observation is presented. Bubbles could be formed by shaking water with the particles, but a much higher volume fraction of bubbles was obtained by pressurizing the aqueous phase to 5 atm overnight followed by suddenly releasing the pressure to nucleate bubbles within the silica dispersion. Sonicating the silica dispersion before nucleation also gave more bubbles, which were also found to be more stable. There appeared to be an optimum degree of surface hydrophobicity that gave maximum foamability and foam stability, where around 20-33% of the silanol groups on the silica surface had been converted to dimethylsilane groups. However, a sharp increase in stability occurred when between 1.8 and 2 mol dm(-3) NaCl was also included in the aqueous phase. The change in stability due to inclusion of salt can be rationalized in terms of changes occurring in the value of the particle contact angle. The effects of increasing sonication and an optimum surface chemical treatment can be explained by the need to make the particles sufficiently hydrophobic so that they adsorb strongly enough, while at the same time minimizing their tendency to aggregate in the bulk aqueous phase, which hinders their adsorption. Furthermore, confocal laser scanning microscopy of the bubble dispersions suggests that a large volume fraction of stable bubbles is only formed when the particles adsorbed to the bubbles are also part of a spanning silica particle network in the bulk aqueous solution, forming a weak gel with a finite yield stress.     

Electron bubbles in helium clusters. II. Probing superfluidity

In this paper we present calculations of electron tunneling times from the ground electronic state of excess electron bubbles in ((4)He)(N) clusters (N=6500-10(7), cluster radius R=41.5-478 A), where the equilibrium bubble radius varies in the range R(b)=13.5-17.0 A. For the bubble center located at a radial distance d from the cluster surface, the tunneling transition probability was expressed as A(0)phi(d,R)exp(-betad), where beta approximately 1 A(-1) is the exponential parameter, A(0) is the preexponential factor for the bubble located at the cluster center, and phi(d,R) is a correction factor which accounts for cluster curvature effects. Electron tunneling dynamics is grossly affected by the distinct mode of motion of the electron bubble in the image potential within the cluster, which is dissipative (i.e., tau(D)tau(0)) in superfluid ((4)He)(N) clusters, where tau(D) is the bubble motional damping time (tau(D) approximately 4 x 10(-12) s for normal fluid clusters and tau(D) approximately 10 s for superfluid clusters), while tau(0) approximately 10(-9)-10(-10) s is the bubble oscillatory time. Exceedingly long tunneling lifetimes, which cannot be experimentally observed, are manifested from bubbles damped to the center of the normal fluid cluster, while for superfluid clusters electron tunneling occurs from bubbles located in the vicinity of the initial distance d near the cluster boundary. Model calculations of the cluster size dependence of the electron tunneling time (for a fixed value of d=38-39 A), with lifetimes increasing in the range of 10(-3)-0.3 s for N=10(4)-10(7), account well for the experimental data [M. Farnik and J. P. Toennies, J. Chem. Phys. 118, 4176 (2003)], manifesting cluster curvature effects on electron tunneling dynamics. The minimal cluster size for the dynamic stability of the bubble was estimated to be N=3800, which represents the threshold cluster size for which the excess electron bubble in ((4)He)(N) (-) clusters is amenable to experimental observation.     

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.     

Ion-specific influence of electrolytes on bubble coalescence in nonaqueous solvents

We report the effects of electrolytes on bubble coalescence in nonaqueous solvents methanol, formamide, propylene carbonate, and dimethylsulfoxide (DMSO). Results in these solvents are compared to the ion-specific bubble coalescence inhibition observed in aqueous electrolyte solutions, which is predicted by simple, empirical ion combining rules. Coalescence inhibition by electrolytes is observed in all solvents, at a lower concentration range (0.01 M to 0.1M) to that observed in water. Formamide shows ion-specific salt effects dependent upon ion combinations in a way analogous to the combining rules observed in water. Bubble coalescence in propylene carbonate is also consistent with ion-combining rules, but the ion assignments differ to those for water. In both methanol and DMSO all salts used are found to inhibit bubble coalescence. Our results show that electrolytes influence bubble coalescence in a rich and complex way, but with notable similarities across all solvents tested. Coalescence is influenced by the drainage of fluid between two bubbles to form a film and then the rupture of the film and one might expect that these processes will vary dramatically between solvents. The similarities in behavior we observe show that coalescence inhibition is unlikely to be related to the surface forces present but is perhaps related to the dynamic thinning and rupture of the liquid film through the hydrodynamic boundary condition.     

The thermocapillary migrations of two bubbles in microgravity environment

The thermocapillary motion of two bubbles along their line of centers in a uniform temperature gradient is investigated theoretically. The bubbles are moving in the direction of the temperature gradient. And the interaction between the leading bubble and the trailing one becomes significant as the separation distance between them is decreased greatly so that the bubble interaction is considered in this case. The appropriate equations of momentum and energy are solved using the method of reflections. In order to proceed analytically, sets of transformations between two coordinates are obtained. By using these transformations and the reflection process, accurate migration velocities of these two bubbles in the microgravity environment are derived for the limit of small Marangoni and Reynolds numbers. These results are employed to describe the thermocapillary motion of two bubbles and to estimate the effects of bubble size and the thermal gradient on the interaction between two bubbles. All of our results for the migration of the two bubbles demonstrate that the approach of the second bubble to the first one intensifies the mutual interaction between these two bubbles and yields some interesting thermocapillary motions.     

Stabilization and characterization of colloidal gas aphron dispersions

Colloidal gas aphrons (CGAs) are finding increasing application in water processing, bioseparation, bubble-entrained floc flotation, separation of oil from sand, etc. This article proposes an effective method of encapsulation to stabilize the CGA bubbles with silicic sol solution for their characterization. The stabilized CGA bubbles can keep shapes and sizes for at least 12 h; even the bubbles smaller than 25 mum can also be stabilized and exist for very long times. Effects of concentration and pH of silicic sol solution on CGA stabilization were studied. The optimal ranges of concentration and pH of silicic sol solution are 0.15-0.25 mol/dm(3) and 7-10, respectively. The bubble distortion behavior in disturbance and size distribution of CGAs were examined by using the stabilization method and photographic techniques.     

The boundary condition at the air–liquid interface and its effect on film drainage between colliding bubbles

The drainage of thin liquid films between colliding bubbles is strongly influenced by the boundary conditions at the air–liquid interface. Theoretically, the interface should not resist any tangential stress (fully mobile) in a clean water system, resulting in very fast film drainage and coalescence between bubbles within milliseconds. In reality, under most experimental and industrial conditions, the presence of impurities or surfactants can immobilize the interface and significantly hinder bubble coalescence by several orders of magnitude. In this opinion, we introduce the recent progress on understanding the boundary conditions at the air–water interface, and how they may affect the outcome of bubble collisions. The transition from mobile to immobile boundary conditions in the presence of contaminations is discussed. Despite the considerable recent progress, there are still experimental and theoretical challenges remaining on this topic, for example, finding the mechanism for hindered bubble coalescence by high salt concentrations.