Foam is an unstable system, which will cause structural changes with time. The stability of the foam depends on the ease of defoaming. Once the foam is broken, the surface energy of the system will be reduced. The security is relative. When the foam is cut, the membrane that separates the foam is mainly caused by the combined effect of gravity and the additional pressure of the curved liquid surface to create the foam to reach out and become thin until it bursts. For the foam made by a general blowing agent, the influencing factors are about the same.
The surface tension of pure water is very high, making foam molding difficult. After adding a surfactant in water, the surface tension of the aqueous solution is reduced, and foam molding is generally relatively secure and stable. Because of the surface energy of the foam increases as the surface area of the liquid increases. When the bubble bursts, the strength of the system also decreases. Therefore, if the surface tension of the liquid is low, the surface energy of the foam is relatively small. There is also less reduction in surface energy when the foam bursts. But the surface tension of the liquid is not the determinant of foam stability. The main factor that determines the stability of foam is the strength of the liquid film. Only when a specific strength film is formed on the liquid surface, low surface tension can promote the stability of the foam.
The strength of the liquid film determines the security of the foam, and the critical property of the power of the liquid film is the surface viscosity. The surface viscosity depends on the tightness of the molecular arrangement in the movie. Measuring the surface viscosity can identify the different states of the single molecular cinema, which is an essential factor for the stability of the foam.
The higher the surface viscosity, the greater the thickness of the liquid film, the higher the strength of the surface adsorption, the lower the fluid film rate, and the longer the life of the formed foam.
However, if the surface viscosity is too high or too low, it is impossible to form a stable foam. Only under a proper thickness can a more stable foam be formed.
The width inside the liquid is also one of the reasons affecting the stability of the foam. The liquid has a high viscosity, resulting in a thick liquid film, which increases the strength of the liquid film. The foam liquid film is often not easy to break, and the liquid on the surface adsorption layer is not natural to flow.
4. Liquid film elasticity
Liquid film elasticity refers to the repairing effect of surface tension. When the foam is impacted or disturbed by external forces, the liquid film will locally thin, which will increase the area of the liquid film. Increasing the surface area will require more work. When the liquid film shrinks, although the surface tension decreases, the surface adsorption concentration increases, which is not conducive to shrinkage. To make the membrane have better elasticity, the adsorption capacity of the foam stabilizer is often higher.
Besides, the foaming agent concentration, temperature, and parameters of the foam generator are all factors that affect the stability of the foam.