SLOW RISING VELOCITY

Microbubbles' rising velocity depends on the physical properties of liquids where microbubbles are created and present.

This velocity follows Stoke's equation:

where V is the rising velocity, ρ is the liquid density, g is the gravitational acceleration, d is the bubble diameter and μ is the liquid viscosity.

Microbubbles of 10 to 40 micrometers are more useful for biological activities of living organism while the ones of 80 to 100 micrometers are normally used for chemico-physical separating operation

BUBBLE RISE VELOCITY - BUBBLE DIAMETER

EXCELLENT SOLUBILITY

Microbubbles have very high solubility thanks to their large surface areas in contact with the surrouding fluid and their internal high pressure.

Mass transfer rate from gas to liquid, or dissolving rate N (mol/s), is written by the below equation when the gas phase mass transfer resistance is neglected.
N = kLA(p - p*)/H
where kL is the liquid phase mass transfer coefficient (m/s), A is the bubble surface area (m2), p is the partial pressure of dissolved component in bubble (Pa), p* is the partial pressure of gas phase equilibrium with dissolved component in liquid (Pa) and H is the Henry constant (p = HC)

 D_L is the gas diffusion coefficient in liquid phase.

d is the bubble diameter and U is the bubble rising velocity

When one 1-mm bubble can be sheared off and dispersed into 100 micrometer (a large microbubble), the surface area of all the microbubbles is 10 times increased.

When such 1-mm bubble is split into 10 micrometer bubbles, such total surface area of all 10-micrometered bubbles is 600,000 m2 or 100 times increased.

Coupled with higher internal pressure inside the bubble, the mass transfer rate or dissolving rate of gas into liquid is greatly increased.

HIGH INTERNAL PRESSURE OF MICROBUBBLES 

Young–Laplace equation 

Internal bubble pressure (ΔP) increases when bubble diameter (d) decreases to a  level higher than the surrounding pressure due to the surface tension σ.

When bubble diameter decreases, bubble internal pressure - a dissolved gas component or the driving force of dissolution increases, which explains why the gas dissollution increases substaintially.

Zeta Potentials 

Fine bubbles (micro/nano bubbles) are electrostatically charged.

Zeta potential of fine bubbles ranges roughly from -20mV to -40mV under normal conditions regardless of microbubble diameter.

This value depends on the pH of the liquid solution. In alkali liquid, fine bubbles show positive charge which can be upto -100mV, while in acidic one Zeta potential can be Zero. 

 Charged bubbles show either repulsion or attractive force like Coulomb's force when two bubbles or foreign particles approach to each other. Charging mechanism has not been made clear yet.

When the surface of microbubbles is charged negatively, there is no possibility of coalescence of microbubbles in the case of dense microbubbled water. Separation operation is applied by charging the surface of suspended materials positively and adsorbing on the negatively charged surface of microbubbles.