N. F. Bunkin

Nanoscale structure of dissolved air bubbles in water as studied by measuring the elements of the scattering matrix

Results of experiments with laser modulation interference microscopy and the Mueller-matrix scatterometry show that macroscopic scatterers of light waves are present in doubly distilled water free of external solid impurities. The experimental data can be interpreted using a computational model of micron-scale clusters composed of polydisperse air bubbles having effective radii of 70-90 nm. The fractal dimension of such clusters was evaluated as 2.4-2.8 and their concentration appeared to be approximately 10(6) cm(-3).

Structure of the nanobubble clusters of dissolved air in liquid media.

A qualitative model of the nucleation of stable bubbles in water at room temperature is suggested. This model is completely based on the property of the affinity of water at the nanometer scale; it is shown that under certain conditions the extent of disorder in a liquid starts growing, which results in a spontaneous decrease of the local density of the liquid and in the formation of nanometer-sized voids. These voids can serve as nuclei for the following generation of the so-called bubstons (the abbreviation for bubbles, stabilized by ions). The model of charging the bubstons by the ions, which are capable of adsorption, and the screening by a cloud of counter-ions, which are incapable of adsorption, is analyzed. It was shown that, subject to the charge of bubston, two regimes of such screening can be realized. At low charge of bubston the screening is described in the framework of the known linearized Debye–Huckel approach, when the sign of the counter-ion cloud preserves its sign everywhere in the liquid surrounding the bubston, whereas at large charge this sign is changed at some distance from the bubston surface. This effect provides the mechanism of the emergence of two types of compound particles having the opposite polarity, which leads to the aggregation of such compound particles by a ballistic kinetics.

Long-living nanobubbles of dissolved gas in aqueous solutions of salts and erythrocyte suspensions

Results of experiments combining laser modulation interference microscopy and Mueller matrix scatterometry show that macroscopic scatterers of light are present in liquids free of external solid impurities. Experimental data on distilled water and aqueous NaCl solutions of various concentrations as well as physiological saline solution are reported. The experimental data can be interpreted by using a model of micron-scale clusters composed of polydisperse air nanobubbles having effective radii of 70-100 nm. Their concentration increases with the growth of ionic content. We hypothesize that under certain conditions those clusters of nanobubbles can affect the erythrocyte structure.

Existence of charged submicrobubble clusters in polar liquids as revealed by correlation between optical cavitation and electrical conductivity

It has been shown previously that in aqueous systems there exist charged gas-filled submicrobubbles (bubstons) associated in clusters that serve as nuclei for optical cavitation [N.F. Bunkin, A.V. Lobeyev, B.W. Ninham and O.I. Vinogradova, Langmuir, submitted]. Here optical cavitation as a function of temperature for a binary solution (with upper critical point) of polar liquids has been explored. For temperatures above the binodal the cavitation probability is enhanced, and the formation of a macroscopic bubble during the laser pulse is most likely due to coalescence of submicrobubble clusters. For temperatures below the binodal, the appearance of new phase droplets leads to a drastic decrease in the cavitation probability. The effect is not associated with changes in concentration of the dissolved gas or to influence of solvophobicity degree of the walls. The inhibited cavitation appears to be closely connected with the decrease in the concentration of ions that has been confirmed by simultaneous decrease in the electrical conductivity of the system. It has been shown that the results obtained constitute indirect evidence for the existence in polar liquids of clusters formed just by charged submicrobubbles, or bubstons.

Nanobubble clusters of dissolved gas in aqueous solutions of electrolyte. I. Experimental proof

Results of experiments with dynamic light scattering, phase microscopy, and polarimetric scatterometry allow us to claim that long-living gas nanobubbles and the clusters composed of such nanobubbles are generated spontaneously in an aqueous solution of salt, saturated with dissolved gas (say, atmospheric air). The characteristic sizes of both nanobubbles and their clusters are found by solving the inverse problem of optical wave scattering in ionic solutions. These experimental results develop our earlier study reported by Bunkin et al. [J. Chem. Phys. 130, 134308 (2009)] and can be treated as evidence for the special role of ions in the generation and stabilization of gas nanobubbles.

Cluster structure of stable dissolved gas nanobubbles in highly purified water

Experiments using phase-modulation interference microscopy and Mueller-matrix polarimetry show that double-distilled water free of foreign solid matter contains macroscopic light scatterers. Numerical calculations suggest that these scatterers can be represented as micrometer-size clusters of polydisperse air bubbles with effective radii between 70 and 90 nm. The fractal dimension of the clusters varies from 2.4 to 2.8, and their concentration is on the order of 106 cm−3.

Bubston Structure of Water and Aqueous Solutions of Electrolytes

Theoretical concepts of the existence of stable gas bubbles (bubstons) in pure water and aqueous solutions of electrolytes that are in equilibrium with an external gas medium have been developed. A theoretical model of ion adsorption on the water surface is proposed, and the double electric layer forming due to this absorption is quantitatively described within this model. These results allowed us to make a thermodynamic description of the bubston structure in the water-external gas medium system. It is shown that the formation of this structure at certain temperatures and concentrations of dissolved impurity ions is a first-order phase transition. The unique role of helium as an external gas medium in the problem under consideration is established: bubston structures in this case do not arise at any initial ion concentrations, whereas the helium solubility increases with temperature. The mechanism of formation for experimentally observed bubston clusters is considered.

Laser scattering in water and aqueous solutions of salts

An automated laser polarimetric scatterometer, operating at a wavelength of 532nm, has been developed for measuring the total scattering matrix of liquid samples in a cylindrical geometry. The optical scheme of the polarimeter combines an electro-optic modulator with a rotary quarter-wave plate. The results of experiments involving distilled water and NaCl aqueous solutions are reported. The measured angular dependencies of the elements of the scattering matrix are compared with numerical computations based on the T-matrix method. The experimental scattering matrix data can be interpreted as a scattering on an ensemble of stochastic micron-scale clusters composed of polydisperse air bubbles having effective radii of 70-100 nm. The fractal dimension of such clusters was evaluated as 2.5-2.8. Their concentration increases with salt addition from 103 cm-3 in distilled water to 106 cm-3 in 0.8 M aqueous solution of NaCl.

Self-oscillating Water Chemiluminescence Modes and Reactive Oxygen Species Generation Induced by Laser Irradiation; Effect of the Exclusion Zone Created by Nafion

Samples of water inside and outside an exclusion zone (EZ), created by Nafion swollen in water, were irradiated at the wavelength l = 1264 nm, which stimulates the electronic transition of dissolved oxygen from the triplet state to the excited singlet state. This irradiation induces, after a long latent period, chemiluminescence self-oscillations in the visible and near UV spectral range, which last many hours. It occurs that this effect is EZ-specific: the chemiluminescence intensity is twice lower than that from the bulk water, while the latent period is longer for the EZ. Laser irradiation causes accumulation of H2O2, which is also EZ-specific: its concentration inside the EZ is less than that in the bulk water. These phenomena can be interpreted in terms of a model of decreasing O2 content in the EZ due to increased chemical activity of bisulfite anions (HSO3-), arisen as the result of dissociation of terminal sulfonate groups of the Nafion. The wavelet transform analysis of the chemiluminescence intensity from the EZ and the bulk water gives, that self-oscillations regimes occurring in the liquid after the latent period are the determinate processes. It occurred that the chemiluminescence dynamics in case of EZ is characterized by a single-frequency self-oscillating regime, whereas in case of the bulk water, the self-oscillation spectrum consists of three spectral bands.

Light-induced phase transitions in stratifying liquid mixtures

It is observed experimentally for the first time that light-induced barodiffusion is an important mechanism for local phase separation in a liquid stratifying mixture in the field of short laser pulses. This effect is applied for determination of the binodal location of the mixture, whereas inducing the laser cavitation in the mixture can be used to determine the spinodal location.