When making a bubble you put a gas into the liquid, a bubble is family of a droplet which is a liquid in a gas. Another member of the bubble and droplet family is a particle which is a solid in a gas or in liquid. It's important to distinguish these three terms from each other. There are a few methods to make bubbles, hydrodynamically, acoustic, optic and via particle cavitation. The most cost-effective method and most efficient method to make ultrafine bubbles is hydrodynamically. Hydrodynamics is a branch of physics that deals with motion on fluids and the forces acting on solid bodies immersed in fluids and in motion relative to them. To say it simple to create a bubble you need a moving liquid, add a gas and bring a force on the gas and liquid and the bubbles are created. In everyday life when you open a can of beer or a bottle of coke, by the change in pressure (the force) the bubbles are created, this is visible by the eye and by the sound.
1982 was the first time to publish an article about nano bubbles and until recently the existence of nano bubbles was heavily debated, the recent hype in ultrafine bubble technology or nano bubble technology is caused by two factors, first, there is now equipment on the market available that can measure ultrafine bubbles size and density and now most of the scientists agree that nano bubbles exist. Second the big advantage of the measuring equipment was that ultrafine bubble makers are now able to further develop and optimize their ultrafine bubble generators and they further develop applications.
Basically, there are two types of nano bubble generation technologies, first there are gas-water circulation types of nano bubble generators. Second there is a gas-water pressurization-decompression also often referred to as pressurized dissolution type of nano bubble generator. Most units are based on one principle or the other or a combination of both.
We can distinguish the following production methods of fine bubbles; the first 4 methods are described in more detail:
- Pressurized dissolution
- Rotational Flow
- Turbulent Static mixer
- Ejector Nozzle
- Ultrasonic (Supersonic vibration)
- Mixed vapor direct contact condensation
Pressurized dissolution method
This method of ultrafine bubble generation is based on the principles of Henry`s Law, which relates the concentration of a gas to the partial pressure. This means that more gas can be dissolved into a solution at a higher pressure. The principle of the ultrafine bubble generator is as follows: Via a venturi system the liquid and the gas is mixed together, in the next step in the mixing box the gas is melted into the water via pressurization. In the last step via a nozzle the water and gas is discharged. Due to drastic drop in pressure of the supersaturated liquid gas solution, the gas is expelled as fine bubbles and ultrafine bubbles in the liquid. The figure illustrates the process.
- Liquid is pumped into the unit under pressure.
- By narrowing the size of the pipe, the speed of the incoming liquid flow is increased, which converts most of the pump pressure into dynamic pressure, thus reducing static pressure and air being suctioned through negative pressure.
- After the liquid and suctioned gas become saturated with bubbles, the liquid/gas flow is sent through a wider pipe to reduce the speed of the flow, where dynamic pressure is converted back to static pressure and the process of pressurized dissolution of gas takes place.
- After the gas is completely dissolved into the liquid, the liquid/gas is ejected at once using atmospheric pressure, causing the liquid to become over-saturated, and massive ultra fine nano-bubbles are released.