Bubbles attraction between

The retention time in the flotation chamber is usually about 3 to 5 min, depending on the characteristics of the process water and the performance of the flotation unit. The process effectiveness depends upon the attachment of air bubbles to the particles to be removed from the process water.57 The attraction between the air bubbles and particles is primarily a result of the particle surface charges and bubble size distribution. The more uniform the distribution of water and microbubbles, the shallower the flotation unit can be. 

The coalescence of bubbles is driven by the two mechanisms. One is the attractive radiation force between bubbles called secondary Bjerknes force. The other is the other radiation force called the primary Bjerknes force which drives active bubbles to the pressure antinode of a standing wave field, ft should be noted, however, too strong acoustic wave repels bubbles from the pressure antinode as described in the next section [29, 30]. 

While the secondary Bjerknes force is always attractive if the ambient radius is the same between bubbles, it can be repulsive if the ambient radius is different [38]. The magnitude as well as the sign of the secondary Bjerknes force is a strong function of the ambient bubble radii of two bubbles, the acoustic pressure amplitude, and the acoustic frequency. It is calculated by (1.5). 

One of the central questions in the stability of foams is why are liquid films between two adjacent bubbles stable, at least for some time In fact, a film of a pure liquid is not stable at all and will rupture immediately. Formally this can be attributed to the van der Waals attraction between the two gas phases across the liquid. As for emulsions, surfactant has to be added to stabilize a liquid film. The surfactant adsorbs to the two surfaces and reduces the surface tension. The main effect, however, is that the surfactant has to cause a repulsive force between the two parallel gas-liquid interfaces. Different interactions can stabilize foam films [570], For example, if we take an ionic surfactant, the electrostatic double-layer repulsion will have a stabilizing effect. 

A comparison between a horizontal sparged contactor and a bubble column has been made, and it appears that for a variety of situations, horizontal sparged contactors may prove to be more attractive than bubble columns. 

Based on the Ps bubble model, [10, ll] experimental data on o-Ps lifetimes, and the widths of the narrow component of the ACAR spectrum, it was found that in practically all investigated liquids of various chemical nature, values of V0Ps = U are close to +3 eV. Thus, from Eq. (6) we obtain Eb —(0.5 to 1) eV. This means that qf-Ps is a loosely bound structure although the long-range Coulombic attraction between e+ and e always provides for the existence of the bound state. Because e+-e separation, rep, in qf-Ps is expected to be large, we may rewrite E, in the following form ... 

As two bubbles approach at any reasonable rate, there must arise a hydrodynamic repulsive force, due to the need to expel water molecules from the film between bubbles. In water, coalescence is observed. Thus, there must exist an attractive force that overcomes the hydrodynamic repulsion. The attractive van der Waals force calculated between two bubbles in water is found to be orders of magnitude smaller than the hydrodynamic repulsion present at reasonable approach rates. As bubbles are highly hydrophobic (/air-water = 72mjm 2) it is reasonable to assume that the "hydrophobic force" is present, and acts to produce coalescence. Available force measurements are found to give an attraction of sufficient magnitude to overcome the hydrodynamic repulsion. This implies that for salts and sugars to reduce bubble coalescence, the attractive hydrophobic force is reduced in their... 

Measurements of the hydrophobic attraction between solid surfaces using an atomic force microscope have also been performed in solutions of NaCl and NaClOa. The attraction was found to be equivalent to that in water in 0.2M NaClOs, whilst in 0.2M NaCl the attractive force was much reduced, supporting expectations. (NaClOa, has no effect on bubble coalescence, NaCl does, cf. Table 3.1.) It has also been observed previously that the range of the attractive force acting between bubbles is substantially reduced from about lOOnm to 40nm on addition of KCl above the transition concentration. 

Air bubbles and clathrate hydrates in polar ice cores have attracted considerable interest because they provide the most direct record of past atmospheric gas compositions (e.g., Raynaud et However, the processes of air clathration in polar ice sheets should be taken into account when considering gas analyses. It is known that extreme fractionation of gases in air inclusions occurs when air bubbles change into clathrate hydrates by the diffusive mass transfer of air molecules between bubbles and hydrates. To estimate the effects of hydrate formation on the distribution of atmospheric gases in deep ice, it is essential to understand the structure and physical properties of natural air hydrates. 

Secondary Bjerknes forces arise when two oscillating bubbles are present in a pressure field. Attractive forces between bubbles with inphase pulsation cause coalescence. Bubbles oscillating out of phase are repelled. The bubble oscillation is in phase when both bubbles are smaller or larger than the resonance size and attractive forces dominate. If one bubble is smaller and one larger than the resonance size, they oscillate in and out of phase and repel one another. 

Parker, J.L., Claesson, P.M., Attard, P. Bubbles, cavities, and the long-ranged attraction between hydrophobic surfaces, J. Phys. Chem. 98, 8468-8480 (1994)... 

Increasing ionic strength decreases adsorption, decreases thickness of double layer, decreases strength of attractive force between bubble (surfactant coated and floe particle), decreases effective size of floe particles, and reduces flotation efficiency. 

Ultrasound affects bubble interaction in two ways - vibration causes an attraction between two bubbles (Bjerkens force) and the Bernoulli force arises from the flow of the fluid parallel or normal to a bubble. A perpendicular flow also causes interbubble attraction. Additionally, if bubbles increase in size, due to the ultrasound, they rise much more quickly, proportional to the (radius). As mentioned above (Section 10.5.1.2), ultrasound enhances cavitation, which is also beneficial. 

Fig. 2. Portion of solid surface soon after contacting with a viscous liquid. Some wetting has taken place, but a pocket of gas has been trapped in a depression. Molecular attraction between liquid and solid at the peripheiy (locations A and B) of the micro-bubble tend to increase the true contact area.

For any adsubble method, if the material to be removed (termed the colligend) is not itself surface-active, a suitable surfactant (termed the collector) may be added to unite with it and attach or adsorb it to the bubble surface so that it may be removed (Sebba, Ion Flotation, Elsevier, New York, 1962). The union between colligend and collector may be by chelation or other complex formation. Alternatively, a charged colhgend may be removed through its attraction toward a collector of opposite charge. 

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