Causes and Conditions of Foam Formation
To define the terms "bubbles" and "foam," we note that in engineering, bubbles typically form when gas disperses through small openings into a liquid. Foam, on the other hand, is created when insoluble gas is introduced into a liquid with low surface tension under external forces, resulting in the gas being trapped and separated by the liquid. At the gas-liquid interface, the liquid film (the boundary between liquid and gas) plays a crucial role. A bubble is defined as having a single interface, while a collection of bubbles with multiple interfaces is referred to as foam.
According to the esteemed surface physicist Professor Zhao Guoxi, foam is defined as a dispersion system where gas is dispersed in liquid; the gas is the dispersed phase (discontinuous phase), and the liquid is the dispersion medium (continuous phase). Bubbles rise to the surface, forming a cluster of gas separated by liquid membranes.
Once we understand these definitions, we can analyze the principles behind foam formation. During the production or use of metalworking fluids or lubricants, air can enter the liquid through various means, such as mechanical stirring, circulation, sudden pressure changes, or vigorous splashing. This provides the necessary conditions for foam formation: continuous and sufficient contact between gas and liquid.
Additionally, the rate of foam generation must exceed the rate of foam destruction. This condition requires the formation of a resilient film, allowing bubbles to last longer. It's important to note that pure liquids typically do not produce foam. In pure liquids, any bubbles that form will quickly rupture upon contact or when escaping from the liquid. For instance, when air is injected into pure water, it produces only single bubbles, as the lifespan of bubbles in pure water is approximately 0.5 seconds. Once they reach the surface, they exist only momentarily, making stable foam impossible.
In summary, the three essential conditions for foam formation are: