ABSTRACT
Thus, for values of G for which dz2/dt (Eq. (129)) is larger than the maximum value given by Eq. (136), dz2/dt will always be larger than dz1/dt and a steady-state height [for which (dz1/dt)=(dz2/dt)] will never be achieved. (b) Effect of Electrolyte Concentration on Foam Collapse. The concentration of electrolytes in the surfactant solution plays a crucial role in determining the stability of a foam due to its effect on the repulsive electrical double-layer forces in a film [37]. The primary effect of an increase in electrolyte concentration is to compress the electrical double layer (i.e., the Debye length decreases). The lower the value of Пmax is, the smaller is the capillary pressure required to cause collapse. On the other hand, the smaller the critical thickness is, the larger is the residence time required for the film to become critical. Figs. 22 and 23 show the variation of Пmax and the critical thickness (xFc) with electrolyte concentration. At small concentrations, the double layer is not too compressed. The maximum of the disjoining pressure therefore corresponds to large film thicknesses for which the van der Waals forces are not very large. An increase in the electrolyte concentration therefore raises ПDL without affecting ПVDW much and raises Пmax. At higher electrolyte concentrations, the double layer is compressed and Пmax corresponds to very small thicknesses at which the van der Waals force is significant. Пmax now decreases with an increase in electrolyte concentration because ПVDW rises sharply with a decrease in the film thickness. Πmax therefore increases, attains a maximum, and then decreases as the electrolyte concentration increases. The position of this maximum, however, moves to smaller film thicknesses. This means that the critical thickness decreases with an increase in electrolyte concentration. It is therefore reasonable to expect that as the electrolyte con-
