Bikerman test with a cylindrical column Sparge tube technique
The Bikerman test is a relatively old method, which was first proposed in 1938. Samples of liquid are foamed in a thermally insulated glass column with a glass frit in the base to create the foam bubbles. The column is 70 cm high and 3 cm in diameter. The gas is thermally equilibrated, saturated with vapour from the liquid and bubbled through the liquid sample (which usually has a volume of about 50 ml). The equipment used is shown in Figure 2.9(a). The steady-state volume, V (cm3), is measured at a series of gas flow rates, U (cm3/s), and the ratio V/U = gives a time in which should be independent of the gas flow rate. Usually, a height/flow rate profile is determined Figure 2.9(b). Transitions in the slope of the profile can indicate transitions in the foam state near critical points in two-component systems. The Bikerman test assumes the steady-state foam volume to be independent of the container shape. The weakness of this cylindrical test is that wall effects
Figure 2.9. The Bikerman method for measuring both the dynamic and equilibrium foam stabilities by using a cylindrical container: (a) Schematic of the apparatus; (b) dynamic testing from foam volume as a function of gas flow; (c) equilibrium testing from foam volume as a function of decay time (note that wall effects may cause scatter in the results obtained)
occur and the height of the foam near the walls is higher than in the centre. This results in a non-distinct or diffused liquid foam boundary. The Bikerman test also assumes that the steady-state volume is independent of the container shape, although this is invalid and is a fundamental weakness of cylindrical containers. In addition, the surfactant is consumed during testing and in order to obtain a steady-state foam, then the duration of foaming or the volume of the foaming solution should be restricted.
It is also important to stress that the Bikermann test determines foam stability under dynamic conditions, and as the bubbles move up the column to the interface under the buoyancy force (Archimedes), they are also subjected to (Stokes) frictional forces. Therefore, the time to reach the surface can be be defined by the following:
where rj and p are the viscosity and density of water, respectively r is the bubble radius and / is the column length (to the water/air interface). In this test, the time period would normally correspond to about 10~2 to 10-1s, and this period may be insufficient for the surfactant to diffuse and adsorb on the bubbles, so that the surface coverage may not be complete. In this case, the thin film separating two bubbles at the surface may not be in equilibrium and the stability cannot be related to the disjoining pressure where relatively large liquid film separation distances are usually involved (2000 A). Under these circumstances, it is more feasible that the foam stability will be related to the dynamic rheological properties of the monolayer.
The foam equilibrium decay time can also be determined by switching off the gas flow and measuring the decay of the foam as a function of time (Figure 2.9(c)).
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