When the solid is in contact with the liquid, the original solid/gas, liquid/gas interface disappears and a new solid/liquid interface is formed. This process is called wetting. For example, textile fiber is a porous material with a huge surface. When the solution spreads along the fiber, it enters the gap between the fibers and drives the air out, turning the original air/fiber interface into a liquid/fiber interface. It is a typical wetting process. At the same time, the solution will enter the inside of the fiber, and this process is called infiltration. Surfactants that aid in the wetting and osmotic action are known as wetting agents and penetrants.
Due to the emulsification of the surfactant, the grease stain particles detached from the solid surface can be stably emulsified and dispersed in the aqueous solution, and are no longer deposited on the surface to be re-contaminated.
The effect of the surfactant is explained below by the process of removing the liquid oil from the surface.
The liquid oil was originally spread on the solid surface. When the surfactant was added, because of its low surface tension, the aqueous solution of the surfactant quickly spread on the solid surface to wet the solid, and gradually replaced the oil. The oil stained on the solid surface gradually curls into oil droplets (the contact angle gradually increases from wet to non-wetting).
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