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.
We can distinguish the following production methods of fine bubbles; the first 3 methods are described in more detail:
- Pressurized dissolution
- Rotational Flow
- Static mixer
- Ultrasonic (Supersonic vibration)
- Nozzle (Ejector)
- 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.
Rotational flow is also often called Swirl Method or Spiral Flow. This fine bubble generator generates bubbles according to the Bernoulli's principle. In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli who published it in his book "Hydrodynamica" in 1738. Centuries later fine bubble generators are made based on this principle. The first product based on this technology is the Ranque-Hilsch Vortex Tube in 1933. Followed 50 years later by the Swirling jet flame. In the mid-nineties the first swirling type micro-bubbles was invented in Japan.
The principle of the fine bubble generator is as follows: water is put into a cylindrical tank from the top-side and made to flow in a spiral downwards. From the center bottom of the cylinder the gas is sucked in. The rotating water is sheared to the top of the cylinder producing fine bubbles.
The static mixer has its origin from mixing two liquids, the first patent for a static mixer was filed in 1965. Instead of mixing two liquids there is also the possibility of mixing a liquid and a gas. The benefits of the static mixers is that they can treat large volumes of water at once. They are not sensitive to clogging. The acniti technology is based on this principle.