The essence of foam is the entrapment of a large amount of gas within a small amount of liquid, forming an aggregation of bubbles separated by liquid films. Pure liquids struggle to form stable foam because surface tension drives the rapid drainage and thinning of the liquid films, ultimately leading to rupture. However, the addition of surface-active substances (primarily surfactants) changes this situation.

Foaming mechanism: When surfactants dissolve in water, their molecules rapidly migrate and adsorb at the gas-liquid interface. 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, and surfactant molecules immediately encapsulate these bubbles, forming the initial foam.

Stabilization Mechanism: Surfactants extend foam longevity through the following approaches: 1. Reduce surface tension: makes the liquid film more resistant to contraction and rupture. 2. Formation of a bimolecular adsorption film: Surfactant molecules are adsorbed on both sides of the liquid film between bubbles, forming a robust layer of "armor" to resist external disturbances. 3. Marangoni Effect: 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. This "repair" self-healing process effectively prevents the bubble from bursting. 4. Imparting charge to foam: If an ionic surfactant is used, the hydrophilic heads adsorbed on the surface of the liquid film carry the same charge, causing mutual repulsion and preventing bubble coalescence.

In daily chemical products, such as shampoo, abundant foam not only provides a pleasant user experience but, more importantly, the bubbles and liquid films rich in surfactants can carry encapsulated 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 gives desserts a light and airy texture. Understanding the mechanisms of foaming and foam stabilization helps us better design and control products to meet the needs of different scenarios.