As mentioned in Section 10.7,a fairy high value of the Gibbs elasticity EG is a needed to keep a film stable by means of the Gibbs mechanism,depicted in Figure 10.29c.Figure 10.35 shows examples of EG as a function of surfactant concentration c in the liquid.There is an optimum concentration copt for EG.If a film is stretched,its thickness will decrease,leading to a high value of EG.More important,however,is that the magnitude of c will decrease on stretching,because additional surfactant will become adsorbed onto the enlarged film surfaces(replenishing of surfactant from the bulk of the liquid is negligible in a thin film).As long as the local concentration is above copt ,EG will increase on stretching,but if c is or becomes < copt,EG will rapidly decrease,and the film will probably break .Hence there is a critical concentration for breaking the film,probably somewhat below copt.This has been comfirmed by stretching of isolated films.
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