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Defoaming via spreading is only possible with oils of low surface tension. Once a defoamer droplet enters the lamella it spreads on its surface.

Figure 3.6: a) Defoamer droplet on the surface of a lamella, b) Spreading of a defoamer droplet on the surface of a lamella

The ability of defoamer oil to spread is described by the spreading coefficient..

S= σ AW - σ OW - σ OA

Spreading of oil on the surface of a lamella causes a change in the surfactant distribution on the lamella’s surface [4] [5]. The newly created oil/water interface reduces the entry barrier for further defoamer droplets to penetrate into the lamella surface through the spread-out oil. Consequently, the area of the spread-out oil increases. Oil lenses caused by spreading may result in surface defects, such as craters or fish eyes, in coatings.

A positive spreading coefficient is a prerequisite for defoaming by spread out liquids or by the spreading-wave-mechanism [8].