Nanobubble

Nanobubbles are nanoscopic and generally too small to be observed using the naked eye or a standard microscope, but can be observed using backscattering of light using tools such as green laser pointers. Nanobubbles can be generated using techniques such as solvent exchange, electrochemical reactions, and immersing a hydrophobic substrate into water while increasing or decreasing the water's temperature. but they differ in that nanoparticles have different properties such as density and resonance frequency. The study of nanobubbles faces challenges in understanding their stability and the mechanisms behind their formation and dissolution. == Theory ==

There is a theory created by Andrei Dukhin and Ren Xu that explains existence of the stable nanobubbles as a result of interaction between structured interfacial water layer and electric double layer. The hypothesis of water molecules structuring at hydrophobic interfaces exists for more than century with dozens of papers published on this subject, some of them reviewed in the book . It was confirmed with several measuring techniques: Atomic force microscopy, Sum frequency generation spectroscopy , Raman spectroscopy , Ultrasound . There are also many studies revealing electric charges on the nanobubbles interfaces leading to formation of electric double layer characterized with certain zeta potential (ζ). We reproduce here results of the paper by Meegoda et al (see Figure on the right) because it illustrates correlation between bubble diameter and zeta potential proving importance of this parameter. The red line represents result of theory . Individually, neither of these interfacial layers can explain nanobubbles longevity. However, their interaction can, as it is shown in the paper . This interaction leads to the two new surface forces. The normal force is exerted by inhomogeneous electric field of EDL on the oriented water molecules dipole moments. The name dielectrostatic was assigned to this force. It turns out that it compensates for the other normal force caused by Young-Laplace excessive pressure in the bubble.Balance of these two normal forces determines size of the stable nanobubble as according to the following equation: : \ a_s = \frac{2\gamma}{MLNd_w\zeta\kappa^2} where M=55500 is number of moles of water in 1 m3, N is Avogadro number, dw is dipole moment of the water molecule, ζ is zeta potential of the bubble, κ is reciprocal Debye length. Parameter L is a thickness of the structured water layer. It equals approximately 0.24 nm for monolayer. Assumption of two layers of the structured water molecules leads to the nanobubble stable size of 250 nm, as shown on the Figure above as a red line. This can be considered as an experimental support of this theory. There is one more experimental fact supporting validity of this theory. The nanobubble size is reciprocal proportional to zeta potential according to the Figure above. The same dependence is predicted by the theory according to the equation for the stable nanobubble size. The tangential force balance is achieved due to the additional tangential surface force - the repulsion of the of the oriented water molecules dipole moments. It competes with classical surface tension. It was shown that this new surface force is sufficient for compensating for the surface tension in the paper . == Properties ==

Nanobubbles possess several distinctive properties: • Stability: Nanobubbles are more stable than larger bubbles due to factors such as surface charge and contaminants that reduce interfacial tension, allowing them to remain in liquids for extended periods. • High Internal Pressure: The small size of nanobubbles leads to high internal pressure, which influences their behavior and interactions with the surrounding liquid. • Large Surface-to-Volume Ratio: This property is crucial for efficient gas transfer between the nanobubbles and the liquid, which is beneficial for various applications. == Usage ==

In aquaculture, nanobubbles have been used to improve fish health and growth rates and to enhance oxidation. Nanobubbles can improve health outcomes for fish by increasing the dissolved oxygen concentration of water, The use of nanobubbles to increase dissolved oxygen levels can also promote plant growth and reduce the need for chemicals. Nanobubbles have also been shown as effective in increasing the metabolism of living organisms including plants. In regards to oxidation, nanobubbles are known for generating reactive oxygen species, giving them oxidative properties exceeding hydrogen peroxide. Researchers have also proposed nanobubbles as a low-chemical alternative to chemical-based oxidants such as chlorine and ozone. == References ==