Читать книгу Additives for Water-borne Coatings - Wernfried Heilen - Страница 63

The spreading wave mechanism has been demonstrated only for slow-acting defoamers. The mechanism only works in very thin lamellas (approx. 1 micrometre). Spreading of a defoamer causes disruption and thinning of the surfactant film which stabilises the lamella. Resultant stresses in the lamella must be compensated by its deformation. The lamellas assume a wave like structure of varying thickness. This lamella is less resistant to mechanical stress than a defoamer free lamella. The spreading wave mechanism does not act locally at the point of entry of the defoamer droplet; rather the disruption created by the defoamer covers a larger area of the foam lamella. The mechanism has been proven only for slow defoamers in sufficiently large lamellas. Lamellas in the micrometre wavelength region and amplitudes in the nanometre range have been measured experimentally. The disruption caused by the defoamer extended over the whole foam lamella [8].