Ultrafine Bubbles, Nanobubbles, Dissolved Oxygen, Micro Bubbles, Milli Bubbles, the world of bubbles can be confusing at times.
Bubbles are gas-filled cavities in water, and they remain suspended in water. Dissolved oxygen are unbound oxygen molecules in water. Unbound oxygen behaves differently and it’s important to understand the difference between a dissolved gas and a gas cavity.
Milli-bubbles are bubbles smaller than one millimetre in diameter, but larger than one micrometre. Micro-bubbles are small bubbles with a diameter between 10 to 50 μm and decreasing in size and remain underwater. Ultrafine bubbles (nanobubbles) are miniature gas bubbles in liquids with a diameter smaller than 200 nm and have several unique physical properties other than ordinary milli-bubbles.
They remain stable in water for a long time because of their negatively charged surface which can be calculated by the zeta potential, whereas milli-bubbles increase in size, rise rapidly, and burst at the water surface.
Smaller bubbles have better reactivity by surface enlargement
In the same volume of water, the contact area between bubbles in water filled with tiny bubbles is much larger than water filled with bigger bubbles. The increase in the contact area is enhanced, be it the aerobic bacteria activities in the liquid by using oxygen gas, or anaerobic activities by creating nitrogen bubbles. Moreover, the efficiency of chemical reactions is increased between the supplied gas and liquid ingredients. In practical applications, CO2 is more easier available for algae and O2 is easier available for plant roots, or aerobic bacteria in soil and water remediation.
Looking at it mathematically: when generated, small bubbles can be created at higher concentrations than larger bubbles. The surface area of a volume of bubbles is in inverse proportion to the bubble diameter; thus, one mL of 100 nm diameter bubbles (2×10.15 bubbles) has 1000 times more surface (240 m2) than one mL of 0.1 mm bubbles (2×10.6 bubbles, 0.24 m2).
Bubbles have 3 components, gas phase, shell material and aqueous or liquid phase. The gas phase is the gas inside the bubble which is a single gas or a gas mixture. The shell material is the water or a liquid surrounding the gas phase. The bubbles formation and the mechanical properties of bubbles depend on the property of shell material. The last component is aqueous phases which are the liquid or combined solution surrounding the shell material.
Furthermore, ultrafine bubbles have an electrically charged surface which can generate free-radicals with the bubble collapse. In addition, some researchers reported that air micro/nano bubbles were pseudo-elastic and spherical in aqueous solutions. Regarding the fluid dynamic properties, bubbles have a low rising velocity in the liquid phase and low reducing frictional resistance.
Nanobubbles in pure water are negatively charged. The zeta potential measured in water with oxygen ultrafine bubbles was from -45 mV to -34 mV while air ultrafine bubbles are a little lower from -20 mV to -17 mV. The large specific surface area and charged surface enable tiny bubbles to effectively absorb opposite charged molecules and / or small particles.
Micro/Nanobubble Surface Tension and Gas Pressure
The gas pressure inside a small bubble is higher than in a large bubble, therefore the surface tension of a small bubble is higher as well. For this reason, the gas of a small bubble dissolves quicker than that of a large bubble.
Small bubbles rise slower than large bubbles to the top of the water surface, and because of this extra time, the gas transport from bubble to liquid is more efficient. Small bubble coalescence less (stick less together) than large bubbles, this is beneficial because when bubbles get bigger, they raise quicker to the surface giving them less time for gas transport.
In the table below, examples are given of the pressure inside the bubble depending on the size of the bubble. The calculations are based on the Young-Laplace equation.
| Bubble Diameter | Pressure inside Bubble in water |
|---|---|
| 1mm | 1.003atm |
| 100μm | 1.03atm |
| 10μm | 1.29atm |
| 1μm | 3.9atm |
| 500nm | 5.8atm |
| 300nm | 9.7atm |
| 200nm | 14.6atm |
| 100nm | 29.7atm |
The diameter of the bubbles in water is reflected in buoyancy and rising rate. The rising rate depends on the solution properties, and Reynolds number corresponds to approximately 1 at about 100 μm of diameter. In addition, in the case of Re < 1, Stokes Law adapts well because bubbles behave as balls, due to flux conditions on the interface of globular bubbles.
Based on the Stokes law, in the table are given 3 examples of different bubble sizes and the rising speed of a bubble in water. Since ultrafine bubbles are so small and move through the liquid randomly, Stokes law is not applicable to them.
| Bubble Diameter | Rising Rate Through Water |
|---|---|
| 100μm | 5440μm/sec |
| 10μm | 54,4μm/sec = 19.6cm/hour |
| 1μm | 0.544μm/sec = 2mm/hour |
Understanding the physicochemical properties of a compound such as solubility, stability, form definition, solid-state properties, partition coefficient and ionization constants is essential. Among the physicochemical characteristics of micro-nanobubbles, there is the large specific area and the high pressurization of gas inside the bubble, which confer to these bubbles’ high gas dissolution capability.
The smaller the bubble size, the higher the oxygen pressure pO2 values in water, suggesting that nano-bubbles increase the pO2 values in water to greater extent than that of micro bubbles (10-50 micrometre in diameter).
Why do ultrafine bubbles live so long?
In laboratory circumstances ultrafine bubbles can be kept for 3 to 6 months. In real-life applications it is much shorter.
The reason for the long-lived presence of ultrafine bubble is that the ultrafine bubble gas/liquid interface is charged, introducing an opposing force to the surface tension, so slowing or preventing their dissipation. In an electrolyte solution the positive ions become concentrated around the gas nucleus due to its negatively charged surface and act as shells that prevent the gas from dispersing (the salting-out phenomenon). Due to this characteristic of ion behaviour, ultrafine bubbles remain stable for more than 6 months in electrolyte solution.
In a real-life situation, there is demand on the oxygen caused by the contaminants and bacteria/microbial needs of the water, which is seeking oxygen. Nature knows the oxygen is there, so it gets used.