92168.pdf

Emulsions and foams 275

required for surfactant molecules to diffuse to this surface region and

restore the original surface tension (Marangoni effect), this increased

surface tension may persist for long enough to cause the disturbed

film region to recover its original thickness^228.

As an extension of the Marangoni effect, Ewers and Sutherland^229

have suggested a surface transport mechanism in which the surface

tension gradient created by local film thinning causes foaming agent

to spread along the surface and drag with it a significant amount of

underlying solution, thus opposing the thinning process.

An absence of the Gibbs-Marangoni effect is the main reason why

pure liquids do not foam. It is also interesting, in this respect, to

observe that foams from moderately concentrated solutions of soaps,

detergents, etc,, tend to be less stable than those formed from more

dilute solutions. With the more concentrated solutions, the increase

in surface tension which results from local thinning is more rapidly

nullified by diffusion of surfactant from the bulk solution. The

opposition to fluctuations in film thickness by corresponding

fluctuations in surface tension is, therefore, less effective.

Surface rheology

The mechanical properties of the surface films (as in the case of
emulsions) often have a considerable influence on foam stability.
Several considerations may be involved.
A high bulk liquid viscosity simply retards the rate of foam
collapse. Hfgh surface viscosity, however, involves strong retardation
of bulk liquid flow close to the surfaces and, consequently, the
drainage of thick films is considerably more rapid than that of thin
films, which facilitates the attainment of a uniform film thickness.
Surface elasticity facilitates the maintenance of a uniform film
thickness, as discussed above; however, the existence of rigid,
condensed surface films is detrimental to foam stability, owing to the
very small changes in area over which such films show elasticity.

Equilibrium film thickness

If the balance of van der Waals attraction, electric double layer
repulsion, capillary pressure, structure propagation, etc., favours an
equilibrium film thickness, random fluctuations in film thickness will,
in any case, tend to be neutralised.