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National Advisory Committee for Aeronautics, Technical Notes - A General Method of Selecting Foam Inhibitors

Most known foam inhibitors and antifoaming agents as
found in this investigation and described in the literature
are insoluble. A criterion is described here for selecting
foam inhibitors from insoluble liquids that form emulsions
with the foaming liquid. By determining the surface tensions
of the foaming liquid and of the additive saturated with that
liquid and the interfacial tension between them. spreading
and entering coefficients may be calculated. High positive
spreading and entering coefficients are characteristic of a
liquid foam inhibitor for the specified foaming liquid.
Negative spreading and entering coefficients are characteristicv
of liquids which will not reduce foaming, Fifty—four ex— ' -
perimental cases are cited, for three foaming syste_ms, to _ _""“
which the principle has been applied with high correlation. "

A mechanism of foam inhibition is described. Bubble
coalescence, rather than bubble rupture at the top of the
foam. is emphasized. It is shown that a spread film of a
foam inhibitor on the surface of the foaming liouid may
actually stabilize bubbles at the surface, although promoting
rapid coalescence.

The problem of foam inhibition is one of perennial interest
in many commercial processes and arises in numerous and diversi—
fied applications. While many substances are known which act
as more or less effective foam inhibitors for specific foam—
ing systems, no general principles for selecting such in—
hibitors have been recognized; This paper‘describes such a
general principle, which should be of practical value.

Generally speaking, foam is excessive and obnoxious only
when air or other gas is being continuously introduced'into
a liquid capable of foaming, by injection under the surface,
by agitation, by aspiration through a circulating pump, by
occlusion under a stream of liquid impinging on a liquid
surface in a tank,_and by other similar means. Presumably,"
the introduction of such gas cannot be avoided, and the best
way in which objectionable foaming may be eliminated is to so—
treat the liquid that gas will pass through it harmlessly,
each bubble breaking the moment it reaches the surface.