Collapse, foams

The rate of foam drainage is determined by the hydrodynamic characteristics of the foam, as well as the rate of internal foam collapse and breakdown of the foam column. Foam drainage is determined by measuring the quantity of liquid that drains from the foam per unit time. Various types of vessel and graduated tubes can be used to measure the quantity of liquid draining from a foam alternatively, changes in the electrical conductivity of the layer at the vessel mouth can be measured and compared to the electrical conductivity of the foaming solution [4]. 

This can be followed by measuring the bubble size distribution as a function of time, by using microphotography or by counting the number of bubbles. Alternatively, the specific surface area or average bubble size can be measured as a function of time. Other techniques such as light scattering or ultrasound can also be applied. 

Dickinson, E. (1992) Introduction to Food Colloids, Oxford University Press. 

Exerowa, D. and Kruglyakov, P.M. (1997) Foam and Foam Films, Elsevier, Amsterdam. 

Lucassen, J. (1981) Anionic Surfactants, (ed. E.H. Lucassen-Reynders), Marcel Dekker, New York, p. 217. 

as well as other liophobic disperse systems are thermodynamically unstable due to their high interfacial free energy. This high free energy provokes processes that lead to foam coarsening and eventual destruction, i.e. to separation of the liquid from the gas phase. 

Foam instability is expressed mainly by the processes of excess (referring to equilibrium quantity) liquid outflow, diffusion gas transfer from smaller to larger bubbles and coalescence. If the vapour pressure of the solvent in the surrounding medium of foam is lower than its saturated vapour pressure, then the process of evaporation influences significantly foam collapse. Finally, if a foam is produced from a gas phase, the main component of which is the solvent vapour, condensation of these vapours appears to be the determining process of bubble expansion. 

All these processes occur spontaneously in a foam without any external intervention . 

Depending on the type of external impact the processes in a foam could be accelerated (for example, creating pressure drop in the foam liquid phase leads to a drainage rate increase) or new processes can be initiated (for example, rupture of films that are stable if no pressure is applied). 

In the process of internal collapse of an isolated foam the total gas content remains constant (in a foam that is in contact with the atmosphere there is a certain decrease in gas quantity resulting from both gas diffusion from the surface bubble layer into the medium and coalescence of surface bubbles). From the equation of the state of a foam [1-3] 

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Most slabstock foams are open-celled, that is, the walls around each cell are incomplete. Towards the end of the foaming process, the polymer migrates from the membranes between cells to the cell struts, which results in a porous structure. In some cases, cells near the surface of the foam collapse to form a continuous skin, which may be trimmed off later. 

Treatment of albumen with trypsin, bromelain and fungal protease produced significantly greater volumes of foam compared to stored control albumen (49). However, all enzyme treatments had inferior foam stability when compared to controls. Both the rate and amount of foam collapse was greater in enzyme-treated samples. The fresh control produced the most stable foam. 

Foam characteristics in several commercial beers were evaluated using one-dimensional NMR image. Foam texture differences and regions of cling, head, and liquid beer can be clearly distinguished. Significant differences in rates of foam collapse were measured [22]. 

A high bulk liquid viscosity simply retards the rate of foam collapse. High 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. 

Considerable frothing occurs with loss of some isocyanic acid (faint smell resembling that of sulfur dioxide). The foam collapses readily on stirring. 

The foam prior to collapse normally sits on the surface of the fabric however, once the foam collapses there is a precipitous drop in viscosity and the resultant liquid rapidly flows along the fibers uniformly sheathing them with the dye or finish liquor. This is illustrated in Figures 3 and 4. 

Figure 3. Foam coated carpet before foam collapse (13)...

If measurements are carried out in the same sample at consecutive time intervals the rate of drainage, the rate of foam collapse, the changes in foam volume and the gas fraction in the foam can be determined in addition to the evolution of the bubble-size distribution. 

In order to estimate the influence of the rate of internal foam collapse on foam drainage, studies were conducted in which the change of border profile and border number were simultaneously measured [12], The volumetric rate of liquid flow at border mouth is a sum of the rate of change in border volumes as a result of their narrowing and the rate of liquid influx as a result of the reduction of border number because of internal foam collapse... 

A typical dependence of drainage onset on foam column height at foam expansion ratio n = 70 is given in Fig. 5.14. [6,22], For high foam columns (H > 16 cm) zb is small and does not practically depend on H. It is mainly determined by the hydrodynamic properties of the system (borders size and viscosity), i.e. of the microsyneresis rate. For small foam column heights t0 strongly depends on H and is determined by the rate of internal foam collapse. These dependences indicate that for a quantitative description of drainage detailed... 

The decrease in foam dispersity results from both bubble coalescence and diffusion bubble expansion. So, depending on the surfactant kind and the time elapsed after foam formation, one of these processes can have a prevailing effect on the rate of foam collapse. 

Techniques Involved in the Study of the Kinetics of Internal Foam Collapse... 

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