Phenomena occurring during bubble collisions with a water/air interface were studied. The bubble impact velocity was tuned by the following: (i) changing the bubble diameter and (ii) adjusting the distance between the bubble formation point and the water free surface (at the bubble acceleration stage). It was found that the bubble bouncing and the coalescence time, i.e., the time from the moment of the bubble's first collision to its rupture, increased with the impact velocity. The coalescence time varied from a few to ca. 120 ms when the bubble impact velocity was changed from 8.0 to 36.7 cm/s. It was found that a prolongation of the coalescence time was related to size of the liquid film formed during the bubble collision. Higher impact velocity means larger deformation of the bubble shape and larger radius of the liquid film formed. It was shown that the bubble bounces when the thinning water film between the bubble and the air/water interface does not reach its rupture thickness during the collision time.
A new, simple foam test, in which a well-controlled volume of gas is introduced into a definite volume of solution, is presented along with the method of analysis. Aqueous solutions of sodium dodecyl sulfate (SDS), n-octyl-β-D-glucopyranoside, hexadecyltrimethylammonium bromide (CTAB), and n-hexanol, i.e., four systems forming metastable and transient foams, were studied. The parameter R5, defined as the ratio of the height of the foam at 5 min after formation to the initial height, is proposed for the evaluation of foam stability. Foams having R5 values higher than 50% can be considered as metastable. Lower R5 values indicate low-stability foams. Changes of R5 values with concentration are similar to those of foam half-life with concentration. Thus, instead of measurements lasting hours for the foam half-life, one can obtain similar information from tests lasting only a few minutes. With this test also one can obtain information about the solution contents in foams. This parameter can be used as an additional criterion for the evaluation of foam stability. In the case of metastable foams formed by SDS, CTAB, and n-octyl-β-D-glucopyranoside, the initial foam volume was almost equal to the volumes of the dispersed gas and the solution carried into the foam by the bubbles. This shows that there was practically no rupture of foam films at the stage of the foam formation.Paper no. S1293 in JSD 6, 69-74 (January 2003).Foams of very different stabilities are commonly met in many industrial processes and everyday life; however, there exists neither a general theory explaining the mechanism of their stability nor a commonly accepted test enabling a reliable determination and evaluation of foams formed by different surfactants. This is caused by the fact that foam is a complicated gas/liquid dispersed system whose properties are determined, in our opinion, by nonequilibrium adsorption coverages (1) at the very enlarged gas-solution interfaces. The Bartsch (shaking) and the Ross-Miles (pouring test) methods are the most commonly applied simple tests for the comparison of the foamability of solutions (2). In the shaking test a certain amount of solution is vigorously shaken in a locked cylinder. In the Ross-Miles method, a definite amount of solution is poured from the upper vessel through an orifice of definite diameter onto a bed of the same solution located in a cylinder at a standard distance from the orifice. The volume of the foam formed and the lifetime of either the entire or half the height of the foam formed are measured in both methods (2,3). The main advantage of these methods and the reason for the wide application lie in their simplicity. To improve the reproducibility of these various methods, modifications and standardization were applied (4-6). Pinazo et al. (5) recently proposed an interesting modification. Unlike the classical Ross-Miles test, they kept the volume of liquid in the vessel constant by continuously pumping back the dropped solution. The initial foam height was measured after the solution had...
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