The essence of foam is the entrapment of a large amount of gas within a small quantity of liquid, forming an aggregation of bubbles separated by liquid films. Pure liquids struggle to form stable foam because surface tension drives the liquid films to drain rapidly, thin out, and ultimately rupture. However, the addition of surface-active substances, primarily surfactants, alters this situation.

Foaming mechanism: When surfactants dissolve in water, their molecules rapidly migrate and adsorb at the gas--On the liquid interface, the hydrophobic tails face the air while the hydrophilic heads remain in the water, thereby reducing the surface tension of the water and making it easier for bubbles to form and expand. With the assistance of mechanical forces such as shaking or stirring, air is introduced into the liquid. Surfactant molecules immediately envelop these bubbles, forming the initial foam.

Stabilization Mechanism: Surfactants prolong the duration of foam through the following means: 1. Reducing surface tension: makes the liquid film more resistant to contraction and rupture. 2. Formation of a bimolecular adsorption film: Both sides of the liquid film between bubbles are adsorbed with surfactant molecules, forming a robust layer."Armor"Resist external disturbances. 3. MarangoniEffect: When the liquid film locally thins, the surfactant concentration in that area decreases, leading to an increase in surface tension. The surrounding liquid, with higher concentration and lower surface tension, automatically flows toward the thinned region, replenishing it."Repair"This self-healing process effectively prevents the bubble from bursting. 4. Imparting charge to foam bubbles: If an ionic surfactant is used, the hydrophilic heads adsorbed on the liquid film surface carry the same charge, repelling each other and preventing bubble coalescence.

In daily chemical products, such as shampoo, abundant foam not only provides a pleasant user experience but, more importantly, these surfactant-rich bubbles and liquid films can carry trapped oil and dirt particles, making them easier to rinse away with water. In food products, like whipped cream, natural surfactants such as proteins encapsulate air during the whipping process, forming stable foam that imparts a light and airy texture to desserts. Understanding the mechanisms of foaming and foam stabilization helps us better design and control products to meet the needs of various applications.