Abstract
Bulk nanobubbles are a novel type of nanoscale bubble system. Because of their extraordinary behavior, 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 nanoentities 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, that is, 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 toward 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 stabilization.
Highlights
In recent years, three new types of nanoscale bubble systems have been reported: (i) surface nanobubbles which form at solid−liquid interfaces and are either (i) spherical cap bubbles or (ii) micropancakes which are quasi-two-dimensional gaseous domains, with lateral dimensions of several microns and a uniform height of a few nanometers; and much more recently (iii) bulk nanobubbles which exist in bulk liquid and are spherical with a typical diameter of 100−200 nm
The technique of acoustic cavitation was shown to be effective at producing bulk nanobubbles in pure water in relatively large numbers approaching 109 bubble·mL−1 with a typical diameter of 100−120 nm
The nanoparticle tracking analysis (NTA) technique was shown to be more reliable for nanobubble size measurements than dynamic light scattering (DLS) which tends to give positively skewed distributions which are biased toward large bubble sizes
Summary
Three new types of nanoscale bubble systems have been reported: (i) surface nanobubbles which form at solid−liquid interfaces and are either (i) spherical cap bubbles or (ii) micropancakes which are quasi-two-dimensional gaseous domains, with lateral dimensions of several microns and a uniform height of a few nanometers; and much more recently (iii) bulk nanobubbles which exist in bulk liquid and are spherical with a typical diameter of 100−200 nm. The most peculiar characteristic of these three types of nanobubbles is their extraordinary longevity.[1] Because of established bubble theories, early reports on stable nanobubbles at surfaces were met with scepticism and an accepted description of their stability is still lacking. Surface nanobubbles were until recently a controversial topic despite mounting evidence for their existence which is firmly established, following investigations by a number of research groups.[1] despite a number of recent studies,[2] bulk nanobubbles are still an emerging field and speculation remains rife about their existence and their stability.[3,4].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
