Abstract
This paper elucidates parts of the mystery behind the interfacial and colloidal stability of the novel bubble system of bulk nanobubbles. Stable bulk nanobubble suspensions have been generated in pure water using hydrodynamic cavitation in a high-pressure microfluidic device. The effects of pH adjustment, addition of different types of surfactant molecules and salts on the nanobubble suspensions have been studied. Results show that nanobubble interfaces in pure water are negatively charged, suggesting the formation of an electric double layer around the nanobubbles. It is presumed that the external electrostatic pressure created by the charged nanobubble interface, balances the internal Laplace pressure; therefore, no net diffusion of gas occurs at equilibrium and the nanobubbles are stable. Such stability increases with increasing alkalinity of the suspending medium. The addition of mono- and multi-valent salts leads to the screening of the electric double layer, hence, destabilizing the nanobubbles. Different surfactant molecules (non-ionic, anionic, cationic) affect the stability of bulk nanobubbles in different ways. Calculations based on the DLVO theory predict a stable colloidal system for bulk nanobubbles in pure water and this could be a further reason for their observed longevity. All in all, in pure water, the long-term stability of bulk nanobubbles seems to be caused by a combination of ion-stabilisation of their interface against dissolution and colloidal stability of the suspension.
Highlights
Bulk nanobubbles are a novel type of nanoscale bubble system
It is presumed that the external electrostatic pressure created by the charged nanobubble interface, balances the internal Laplace pressure; no net diffusion of gas occurs at equilibrium and the nanobubbles are stable
Bulk nanobubble suspensions were generated in pure water using a microfluidic device
Summary
Bulk nanobubbles are a novel type of nanoscale bubble system. They have a typical mean spherical diameter of 100–200 nanometres and they exist in bulk liquid.[1]. Amongst other suggestions is that the nano-entities observed are not bubbles but could be supramolecular structures,[38] solid nanoparticles or nanodroplets. This is a key question which needs to be resolved. Ohgaki et al.[2] observed that the density of water reduces significantly when such nano-entities are present, suggesting they must be bubbles Their observation of hollow hemispherical structures via scanning electron microscopy based on a free-fracture replica technique indicated the existence of cavities. Apart from the theoretical challenges, there are a number of experimental challenges including developing efficient techniques for the generation of bulk nanobubble suspensions with meaningful gas fractions as well as their characterization, including the ability to distinguish between nanobubbles and solid nanoparticles. Using our experimental measurements, we provide a description of the colloidal stability of the nanobubble suspensions based on the DLVO theory
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