The most universal characteristic of any Defoamer is the fact that it is surface active, but highly insoluble in water. It has to be formulated so that it will be dispersed as tiny droplets, i.e. as an emulsion.
The surface-active nature of Defoamer causes it to spread very rapidly onto any air-water interface that it encounters. This is especially the case if that interface already is covered by the types of surface-active materials that tend to stabilize foams. Defoamer contains particles that pierce the surfaces of foam bubbles, causing them to collapse when it spreads at the interface.