Stability of dynamic foams

Dynamic foams are these in which a state of dynamic equilibrium between the rate of foam formation and the rate of foam collapse is reached. The behaviour of such foams as well as the reasons for their stability has been considered in many monographs [e.g. 13,25,39-41,113,114], An analysis of the stability of dynamic foams based on the properties of foam films is presented here. 

If a barbotage technique is employed in foam formation and the foams produced are of low stability, it is possible to reach a steady-state at which the rate of foam formation becomes equal to the rate of the decrease in foam column height, and during a long period of time the volume of the foam remains constant. It should be emphasised that a certain inaccuracy in the measurement of the foam column height can originate from an non-distinct (diffuse) liquid/foam boundary or roughness of the upper foam boundary (especially in structured foams, e.g. from proteins). 

The duration of establishing a steady-state regime depending on foam stability is usually of the order of seconds or minutes. The maximum foam volume Vr mM increases with the rise in the volumetric gas rate Q. The ratio VFmax/Q or H,mx/Q remains constant within a large range of gas rates [13]. Other authors suggest that constant should be the quantity (Hmax -ao)/Q or Vq IQ (where ao = const) [13,115]. At relatively high rates of gas supply all the expressions practically coincide. 

According to [116] Hmax IQ remains constant under a dynamic foaming regime when Reynolds number Re 100 (such a tube diameter is accepted as a characteristic size). When 250 Re 100, Hmax/Q first increases and then decreases. Similar dependences have been observed by Pattle [117], 

As a stability characteristic of dynamic foams Bikerman has proposed the average time of gas retention in the foam (see Eq. (7.19)). In order to determine the foam stability by Bikerman s method along with cylindrical vessels (A = const), conical shaped foam vessels (a 

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