Nanobubbles of the dissolved gas in deep purified water

N.F.Bunkin, A.V.Shkirin, V.A.Kozlov

A.M.Prohorov General Physics Institute of RAS

The Bauman Moscow State Technical University, Moscow, Russia

As was obtained in experiments on measurement of the elements of the scattering matrix, macroscopic scatterers of light waves are present in doubly distilled water free of 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 10⁶ cm^-3.

Measurements of the indicatrix for laser beam scattering radiation as functions of the scattering angle in goniometric setups are well known to be a powerful method for determining the shape and sizes distribution of scattering objects. In the past 10 - 15 years, the techniques based on measuring the polarization characteristics of light scattering have increased in popularity, since they yield complete information about the interaction of optical waves with arbitrary objects. Here we refer to the angular dependence of the elements of the scattering matrix. These techniques are widely applied in the study of the structure of space dust, volcanic ashes, structures of the aerosols formed by microcrystals of an ice and drops of water, etc. As follows from the analysis of the quoted works, measurements of angular dependences of factors of a matrix of Muller allow us to retrieve the characteristics of the size distribution of the scattering particles. In the case where the size of these particles is greater than the wavelength of the laser radiation (the so-called Mie particles), it is possible to determine whether the given particles are "monolithic" ones or consist of separate monomers of sizes less than the wavelength (i.e., Rayleigh particles). Moreover, this technique allows us to define the type of aggregation of monomers resulting in the formation of a clustered Mie particle.

The last circumstance is very important in the context problem of the problem formulated below. As was shown in previous studies, any liquid saturated with a dissolved gas (for example, atmospheric air) and containing an ionic component are unstable with respect to the spontaneous formation of spherical gas cavities on the nanometer scale - so-called bubstons. The bubstons (an abbreviation of "bubbles stabilized by ions") are stable gas nanobubbles; their stability is conditioned by the adsorption of ions of the same sign on the bubston surface. These results in the appearance of repulsive Coulomb forces that are directed normal to the bubston surface and can compensate for surface tension forces. Consequently, the bubston is mechanically stabilized. In aqueous solutions of electrolytes with an ionic density of 10¹⁵ - 10¹⁶ cm^-3 (i.e., very dilute solutions), the radii of bubstons are between 10 - 100 nm. According to data from previous studies under the conditions of mechanical equilibrium, the pressure of the gas inside a separate bubston is precisely equal to the gas pressure over the surface of the liquid.

This explains the diffusion stability of the bubstons (as related to the gas inside the bubston and the gas dissolved in the liquid). Recently the problem of the Coulomb screening of an electrically charged bubston in an aqueous salt solution was solved; in the same work, the effect of viscous friction forces on the charged bubston surrounded by a cloud of counterions was analyzed. It appears that the viscous forces effectively "wash off" the peripheral layers of the counterion cloud. Thus two types of compound particles are generated in equilibrium. These compound particles represent the charged gas nuclei surrounded by counterion clouds of different (but fixed) thickness. The compound particles are not electrically neutral but have opposite electric charges. Therefore, they are capable of coagulating with one another due to the Coulomb attraction, resulting in the formation of bubston clusters. According to the data presented before a bubston cluster includes ~10² љseparate bubstons, and its effective radius (the gyration radius, see below) is about 0.5 mm. Note that the mechanism of bubble stabilization due to the charging of its surface is an alternative to the mechanism mechanical and diffusion the stabilization, based on the penetration of gas molecules into the pores and cracks of solid particles.

This work is devoted to experimental verification of the existence of bubston clusters in water free of external solid impurities. Here we studied the angular dependences of the elements of the scattering matrix for the samples of water and for aqueous suspensions of colloidal silica and polystyrene latex. In addition, the experiments on laser beam scattering indicated that the micron-sized particles consist, in turn, of nanometer-scale monomers.

The article is divided into the theoretical and the experimental parts. In the theoretical part, a detailed theoretical framework for the experiment is given. In the experimental part, the setup description and the analysis of the results of the experiments on laser beam scattering are summarized. In the final part, the basic conclusions are formulated. This study develops at a qualitatively new level earlier works devoted to this topic.