Unraveling the behavior of highly-concentrated nanobubble solutions: The influence of colloidal nature on their generation and stability

Nanobubbles (NBs) exhibit properties such as unusual stability that make them valuable in a diverse suite of applications. Although NBs are of great interest, their formation, stability, and behavior in water, especially at high concentrations, are still not fully understood. Specifically, the duality of NBs (i.e., gas/liquid interface or colloidal suspension) presents significant challenges in terms of forming theoretical and practical frameworks. This study aims to expand the understanding of dual behavior of NBs at ultrahigh concentrations. NBs were generated by applying shear forces using different gases (i.e., O₂, N₂, air, Ar) and they were monitored during long-term storage, stirring, sonication, and dilution in terms of concentration, size, gas saturation, and zeta potential (ζ -potential). The results demonstrated that NBs (∼100 nm) with ultrahigh concentrations (∼3.0 × 10⁹ particles mL⁻¹) can be efficiently produced using a low-energy (∼125 Wh) generator system regardless of the gas type. The nanoparticle tracking analyzer (NTA) analysis demonstrated greater NBs characterization accuracy compared to dynamic light scattering (DLS). In general, Ar and N₂ NBs showed greater resistance to physical perturbations, while air and O₂ NBs were more susceptible, likely due to the reactivity and polarizability of O₂. On one hand, the highly negative surface potential indicated by ζ between −30 to −60 mV confirmed colloidal stability through electrostatic repulsions in agreement with the DLVO theory. On the other hand, the linear relationship between concentration and volume in diluted NBs solutions resembled an ideal solution, reinforcing the juxtaposition of NBs behavior as colloids and homogeneous solutions of gas cavities.