ABSTRACT
Once the gas supply is stopped, the films at the bottom simply obey the Reynolds equation, with Eq. (133) as the initial condition. For the present calculations, xF0 was taken to be 500 nm.
The results of some simulations done to examine the effect of various parameters on foam collapse are discussed below. (a) Effect of Superficial Gas Velocity on the Steady-State Height. It is practical to use the steady-state height as a measure of foam stability only for relatively short-lived foams [37]. For more stable foams, it is possible that the height of the foam when collapse starts will be too large to be conveniently measured in a laboratory. It is also evident that the superficial gas velocity used to generate the foam will have a significant impact on the steady-state height. Fig. 21 shows the steady-state height (H0) as a function of the superficial gas velocity (G). It is clear that the steady-state height increases with G. The slope of the curve increases and seems to become unbounded for G= 0.00035 m/s, indicating that there is an upper limit on the superficial gas velocity beyond which a steady-state foam height will never be achieved. Some explanation for this can be provided as follows. It can be inferred from Eqs. (60) and (62) that the rate of foam
collapse is determined by Because the film thickness at the collapsing front is fixed, qPB is determined primarily by the value of the Plateau border radius at the top. However, because collapse can occur only if the capillary pressure at the top exceeds the maximum disjoining pressure, there is an upper limit to the Plateau border radius at the top given by:
(134)
