Bubble coalescence and dynamic adsorption layer

An extremely high bubbling velocity results in capture of bubbles in a foam which is subjected to coalescence. However, investigations by Rulyov (1985) show that enlargement of bubbles by coalescence takes place also at small bubbling velocities. Integral distribution functions of bubble diameters in an electroflotation cell, obtained by microphotography, are shown in Fig. 10.11. 

The curve ( ) corresponds to the region close to the electrodes (less than 2 cm), and curve ( ) to the region 2 to 8 cm away from the electrodes. As one can see, bubbles located near the source are approximately 25% smaller than in the remaining volume of the flotation cell. Calculations by Rulyov (1985) show that in the leading part of the cell 60% of bubbles are the coalescence product of two, and 40% of three initial bubbles. This results in a decrease of extraction intensity by a factor 1.5-2 when the height of flotated liquid layer increases from 2 to 8 cm. It was also shown by Rulyov (1985) that in gradient coalescence the rate of disappearance of initial bubbles can be presented by 

Unfortunately, generation of small bubbles ( 20 m) and maintaining their high concentration is one of the most difficult problems of microflotation which still remains to be solved. Since the surface of small bubbles is strongly retarded, their transport to the surface of large bubbles can be described, as a first approximation, by the equation derived for the sedimentation of solid spheres on a bubble surface, i.e. Eqs. (10.31) and (10.27). It has to be taken into account that small bubbles do not sediment on the surface of a large bubble. On the contrary, they rise from it so that the coefficient before the second term in Eq. (10.27) becomes negative, which yields 

The formation of bubble-bubble aggregates under microflotation conditions is complicated by the presence of dispersed particles on their surface. This question will be considered in the next paragraph. 

Gradient coagulation (cf. Van de Ven 1989) of small bubbles with the use of rather high amounts of cationic surfactant under microflotation conditions can be slowed down substantially due to electrostatic repulsion forces. High stresses develop in the zone of bubble 

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