Bulk nanobubbles: Production and investigation of their formation/stability mechanism.

Nanobubbles (ΝΒs) have attracted concentrated scientific attention due to their unique physicochemical properties and large number of potential applications. In this study, a novel nanobubble generator with low energy demand, operating continuously, is presented. Air and oxygen bulk nanobubbles (NBs@air and NBs@O2) with narrow size distribution and outstanding stability were prepared in water solution. The bulk NBs' behavior was evaluated taking into consideration the hydrodynamic diameter and ζ-potential as a function of processing time, gas type, pH value and NaCl concentration. According to the results the optimum processing time was 30 min, whereas the effect of water salinity was stronger in NBs@O2 than NBs@air. In order to investigate further the NBs properties, Electron Paramagnetic Resonance (EPR) spectroscopy was applied for quantitative analysis of free radicals following the spin trapping methodology. The mechanism of bulk NBs' generation and their extremely long-time stability can be attributed mainly to the hydrogen bonding interactions. The formation of a diffusion layer, by absorption of OH- due to electrostatic interaction, contributing to negative surface charge, whereas the interaction of ions with the surface hydroxylic groups provide the equilibrium between the protonation and deprotonation of water and finally the formation of a stable interface layer. A remarkable highlight of this work is the long-time stability of generated bulk NBs which is up to three months.

A rotating sample cell for in situ measurements of adsorption with x-rays.

A sample cell which facilitates adsorption in conjunction with small angle x-ray scattering under a rotational field is presented. The device allows dynamic phenomena that take place within a pore system to be investigated in situ by x-rays. As an example, a sample of Vycor porous glass was measured at relative pressures p/po = 0 and p/po = 0.5. For the static measurements, the results were as expected. Under rotation, an increase in the scattered intensity of the loaded sample, over the corresponding static one, is observed. Fractal analysis has shown an increase in the fractal dimension even higher than that of the dry sample. It was suggested that the increase in the scattered intensity was due to the rotation, while the abnormality in the fractal dimension was due to asymmetric ripples of the adsorbed layers. The limits of the technique are given too.