Micro nano bubbles are essentially a high-efficiency gas dissolution technology, which can not only increase the dissolution rate, but also effectively increase the apparent solubility of the gas. Its characteristics are as follows:
1. Slow Rising Speed
According to Stokes law, the rising speed of bubble in water is proportional to the square of bubble diameter. The smaller the bubble diameter, the slower the rising speed of bubble. A bubble with a bubble diameter of 1mm rises in water at a speed of 6m/min, while a bubble with a diameter of 10μm rises in water at a speed of 3mm/min, which is 1/2000 of the former. If the increase in specific surface area is taken into consideration, the dissolving capacity of micro nano bubbles is 200,000 times higher than that of ordinary air.
2. Self-pressurized Dissolution
There is a gas-liquid interface around the bubble in water, and the existence of the gas-liquid interface makes the bubble subject to the surface tension of water. For the bubble with spherical interface, the surface tension can compress the gas in the bubble, so that more gas in the bubble can dissolve into water.
The dissolution of micro nano bubbles in water is a process of bubble shrinking gradually. The rise of pressure will increase the dissolution rate of gas. With the increase of specific surface area, the speed of bubble shrinking will become faster and faster, so that it will eventually dissolve into water. Theoretically, the pressure on the bubble when it is about to disappear is infinite.
3. Surface Charging
The potential difference generated by the surface charge of microbubbles is often characterized by potential, which is an important factor in determining the adsorption performance of the bubble interface. When the micro nano bubbles shrink in water, the charged ions are rapidly concentrated and enriched at the very narrow bubble interface, which is manifested as a significant increase in the zeta potential, and a very high zeta potential value can be formed at the interface before the bubble bursts.
4. Produce a Lot of Free Radicals
At the moment of micro nano bubble breaking, due to the drastic change of gas-liquid interface disappearance, the accumulated chemical energy will be released by the high concentration ions gathered on the interface, which can stimulate the generation of a large number of hydroxyl radicals. Hydroxyl radical has ultra-high redox potential, which can degrade pollutants such as phenol which are difficult to be oxidized and decomposed under normal conditions, so as to realize the purification of water quality.
5. High Mass Transfer Efficiency
In the process of the volume shrinkage of micro nano bubbles, due to the increase of specific surface area and internal pressure, more and more gases are dissolved into water through the bubble interface. With the decrease of bubble diameter, the effect of surface tension becomes more and more obvious. Finally, the internal pressure reaches a certain limit value, which leads to the bubble interface rupture. Therefore, the self-pressurization characteristics of micro nano bubbles during the contraction process can continuously enhance the mass transfer efficiency at the gas-liquid interface. And this characteristic makes the micro nano bubbles continue to carry on the gas mass transfer process and maintain high efficiency even when the gas content in the water reaches the supersaturation condition.
6. High Gas Solubility
Micro nano bubbles have the characteristics of slow rising speed and self-pressurized dissolution, which makes the micro and nano bubbles gradually reduce to nano scale in the slow rising process, and finally disappear and dissolve into water, so as to greatly improve the solubility of gases (air, oxygen, ozone, carbon dioxide and so on) in water. For ordinary bubbles, the solubility of gas is often affected and limited by environmental pressure, and there is saturated solubility. In the standard environment, the solubility of gas is difficult to reach above the saturation solubility. Since the pressure inside the micro nano bubbles is higher than the ambient pressure, the supersaturated dissolution condition of gas calculated under the assumption of atmospheric pressure can be broken.