On the existence and stability of bulk nanobubbles

Bulk nanobubbles are a novel type of nanoscale bubble system. Due to their extraordinary behaviour, however, their existence is not widely accepted. In this paper, we shed light on the hypothesis that bulk nanobubbles do exist, they are filled with gas and they survive for long periods of time, challenging present theories. An acoustic cavitation technique has been used to produce bulk nanobubbles in pure water in relatively large numbers approaching 109 bubble.mL-1 with a typical diameter of 100-120 nm. We provide multiple evidence that the nano-entities observed in suspension are nanobubbles given that they disappear after freezing and thawing of the suspensions, their nucleation rate depends strongly on the amount of air dissolved in water, and they gradually disappear over time. The bulk nanobubble suspensions were stable over periods of many months during which time the mean diameter remained unchanged, suggesting the absence of significant bubble coalescence, bubble breakage or Ostwald ripening effects. Measurements suggest that these nanobubbles are negatively charged and their zeta potential does not vary over time. The presence of such a constant charge on the nanobubble surfaces is probably responsible for their stability. The effects of pH, salt and surfactant addition on their colloidal stability are similar to those reported in the literature for solid nanoparticle suspensions, i.e. nanobubbles are more stable in an alkaline medium than in an acidic one; the addition of salt to a nanobubble suspension drives the negative zeta potential towards zero, thus, reducing, the repulsive electrostatic forces between nanobubbles; and the addition of an anionic surfactant increases the magnitude of the negative zeta potential, thus, improving nanobubble electrostatic stabilisation.