Reaction mechanism of defoamers
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The local surface tension of foam decreases, causing the foam to burst
Some people studied the defoaming process of polysiloxane in the oil system. They took continuous photos of the foam system at a speed of 1/1000 seconds, and the photos were magnified 100 times. The schematic diagram of four consecutive photos shows that the silicone oil defoamer drops arrived on the foam film, causing the surface of the foam film to break and merge into large bubbles. From 1 to 3, the foam was merged from four to one large, and the foam film became thinner. Finally, the gas-liquid quickly separates, the large bubbles are eliminated, and the bubble is broken at 4. This objectively explains why a drop of defoamer can cause a series of bubbles to burst.
When the defoamer adheres to the foam film and immerses in the foam liquid, it significantly reduces the surface tension at that location. Because the active ingredients of defoamers have low solubility in water in aquatic systems, the decrease in surface tension is limited to the local area of the foam membrane, while the surface tension around the foam membrane remains almost unchanged. The part with reduced surface tension is strongly pulled around and extended, finally leading to the bursting of foam.