A pure liquid cannot produce a stable foam. The bubble rises from the inside of the liquid to the surface at a rate proportional to the square of the radius of the bubble and inversely proportional to the viscosity of the liquid, according to Stoks' law. This changes when the system contains multiple components, especially surfactants. The surfactant will Orient itself to the liquid/gas phase interface, reducing surface tension. At the same time, the membrane wall of the foam reaches a certain thickness, which has a stable effect on the foam.

The surfactant avoids the edge thinning of the film layer and stabilizes the foam through the Malangani effect and electrostatic effect.

 Ma Lange, effect (surfactant liquid backflow phenomenon, caused by offset gravity)

As the bubble rises to the surface, the liquid phase flows downward under the action of gravity, which causes the concentration of surfactant distributed on the bubble surface to change. The surface tension at the top of the membrane is higher than that at other parts due to less surfactant distribution. The liquid from other parts migrates back toward the top of the surface tension. If gravity and reflux forces are balanced between the critical bubble bursting thickness (10nm) of the bubble layer, the bubble will be stabilized.

Electrostatic effect

Surfactants have both oil-philic and water-philic properties. When the liquid in the bubble film is gradually discharged due to the action of gravity, the two surfaces of the film are close to each other, the polar or charge end of the surfactant will be close to each other, at this time the same polarity or charge end will repel each other, thus preventing the liquid in the film layer to continue to discharge. When the film layer in equilibrium is larger than the critical film thickness, the bubble will be stable.