The Science of Defoaming

1

1

Some General

Properties of Foams

1.1 INTRODUCTION

Before considering the behavior of antifoams, we review the relevant properties of

foams. Only a brief summary is given here. It is for the most part only concerned

with those aspects that may have relevance for the understanding of antifoam action.

For more complete accounts, the reader is referred to the many books [1–9] and

reviews on the subject [10–27].

This brief review includes denition of the structural features of foams. A sum-

mary of the processes occurring in foam lms follows with particular emphasis on

the factors that determine the stability of those lms. Finally, we include an outline

of the processes of drainage and diffusion-driven coarsening, which concern the

entire body of a foam and not just the constituent parts.

1.2 STRUCTURE OF FOAMS

We rst consider the structure of a polydisperse foam. That structure is exemplied

by the photograph reproduced in Figure 1.1. This image depicts a foam that has been

aged, where both drainage and diffusion of gas from small bubbles to large bubbles,

as a result of differences in capillary pressure, have occurred. In the lower part of

the foam, bubbles are spherical (so-called kugelschaum) and of small size with a

relatively low overall gas volume fraction. Collections of spherical bubbles, without

the distortions associated with lm formation, form at gas volume fractions,

Φ

G

foam

,

of ≤~0.74 in the case of monodisperse bubbles and ≤~0.72 in the case of polydisperse

bubbles [28]. As the liquid drains out of, for example, a polydisperse foam so that

Φ

G

fo

am

becomes > 0.72, the bubbles distort to form polyhedra. This polyhedral foam

(polyederschaum), with a relatively high gas volume fraction, consists of thin foam

lms joined by Plateau border channels.

In the case of the foam depicted in Figure 1.1, there is clear segregation of bubble

sizes, with larger bubbles being present at the top of the foam column. The extent

of such vertical segregation in polydisperse foams varies according to the method

of generation. It probably depends on the extent of mixing during foam generation,

gravity segregation [5], and even the so-called brazil nut effect [29]. Segregation of

such a polydisperse foam can in fact apparently be facilitated by rapid continuous

wetting of a foam column from above so that high liquid volume fractions prevail.

Bubble movement can then occur without requiring the distortion of bubbles so that

gravitational segregation is in turn specically facilitated [5].

2 The Science of Defoaming: Theory, Experiment and Applications

It is worth noting that the Plateau borders between bubbles of different sizes in

Figure 1.1 are in fact curved, being concave with respect to the larger bubbles. This

implies that the adjacent foam lms are also curved so that the capillary pressure is

larger inside the smaller bubbles. Such differences drive the process of gas diffusion

leading to coarsening of the foam where average bubble sizes increase.

We illustrate the salient structural features of a foam in Figure 1.2, by relating

computer-generated simulations reported by Weaire and Hutzler [5] to two actual

images of polydisperse foams, each of different gas volume fraction, made by Hartland

and Barber [30]. The comparison is, however, intended to be only qualitative, particu-

larly with respect to gas phase volume fractions in the respective experimental images

and simulations. The latter concern assemblies of the so-called Kelvin cells, tetrakaid-

ecahedra, with six at quadrilateral faces and eight curved hexagonal faces. However,

polydispersity means that such polyhedra are not at all present in the foams imaged by

Hartland and Barber [30].

As shown in Figure 1.2b, Plateau borders join together to form a network of chan-

nels, containing almost all the liquid in the foam and through which drainage occurs

in the gravity eld. The junctions or nodes of the Plateau borders in the interior of

a dry foam (where

Φ

G

foam

1

→ ) invariably involve four borders meeting at a regular

tetrahedral angle of 109.5°. The Plateau border cross-sections are seen to be con-

cave triangular in shape, with each of the vertices terminating in a foam lm where

0.5 cm

FIGURE 1.1 Aged polydisperse foam with dry polyhedral foam (polyederschaum) at top of

column and spherical bubbles (kugelschaum) at bottom of column.