The Science of Defoaming

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4

Mode of Action

of Antifoams

4.1 INTRODUCTION

This chapter attempts a complete review of the various mechanisms proposed for the

action of antifoams over the past seven or so decades. It is a feature of thissubject

that some proposed mechanisms, although plausible, have been speculative. Thus,

unequivocal experimental evidence has sometimes been lacking. Indeed, the full

theoretical implications of proposed mechanisms have also often not been fully

developed. In the main, all of this derives from the extreme complexity of the rel-

evant phenomena. As we have seen, foam is itself extremely complex, consisting of

(usually) polydisperse gas bubbles separated by draining lms. These lms exhibit

complicated hydrodynamics involving the distinct rheology of air–liquid surfaces

and, for thin lms, colloidal interaction forces. The nature of the foam lm collapse

processes that are intrinsic to foam are still imperfectly understood.

Antifoams for aqueous systems are, for example, usually hydrophobic, nely

divided, insoluble materials. Their presence therefore further complicates the com-

plexities associated with foam. Indeed commercial antifoams for aqueous solutions

usually consist of hydrophobic particles dispersed in hydrophobic oils. The action of

such antifoams concerns the effect of a dispersion (of antifoam in foaming liquid) of

a dispersion (the antifoam) on yet a third dispersion (the foam).

However, despite this complexity, signicant progress has been made over the

past three decades concerning the mode of action of antifoams. Much speculation

has been replaced with rm theoretical and experimental evidence. In particular, the

role of oil lenses in causing foam lm collapse has been largely established together

with the realization of the importance of the stability of the so-called pseudoemul-

sion lms of foaming liquid, which separate antifoam oil drops from the relevant gas

phase. Much progress has also been made in establishing the role of solid particles

in antifoam action, particularly their role in breaking aqueous pseudoemulsion lms.

The latter is central to understanding the reason for the inclusion of hydrophobic par-

ticles in commercial oil-based antifoams intended for control of the foam of aque-

ous solutions. Particularly noteworthy has been the insights made by Denkov and

coworkers [1] arising from measurement of the critical applied capillary pressures

necessary for rupture of pseudoemulsion lms.

Theories of antifoam mechanism appear to fall into two broad categories: those

that concern modication of surface tension gradients and those that concern forma-

tion of capillary instabilities in foam lms. Theories that concern surface tension

gradients appear to be rather speculative. Some of these theories attribute antifoam

action to the generation of a surface tension gradient that supposedly drives uid