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 and liquid quickly separate, and the large bubble is eliminated. At 4, the bubble is broken. This also objectively explains why a drop of defoamer causes a series of bubbles to burst.
When the defoamer adheres to the foam film and is immersed in the foam solution, it will significantly reduce the surface tension at that location. Because in water systems, the active ingredients of defoamers have a lower solubility in water, and the decrease in surface tension is limited to the local area of the foam film, while the surface tension around the foam film remains almost unchanged. The part with reduced surface tension is strongly pulled around and extended, finally leading to the bursting of foam.
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