Circulation, drop

Although the adsorption of surfactants tends to reduce mass-transfer coefficients by suppressing drop circulation, a sharp increase in mass transfer... 

The effect of slow accumulation of surface-active materials is indicated in Fig. 18, which is a series of photographs of drops suspended in a tapered tube (H9). Tiny amounts of fine solids of colloidal dimensions, as described by Elzinga and Banchero (El), gradually collected at the interface and were swept around to the rear of the drop. Circulation was progressively hindered until it was nearly stopped. Yet no measurable change could be detected in any physical property, including interfacial tension of the separated phases. 

The flow and shape transitions for small and intermediate size bubbles and drops are summarized in Fig. 7.13. In pure systems, bubbles and drops circulate freely, with internal velocity decreasing with increasing k. With increasing size they deform to ellipsoids, finally oscillating in shape when Re exceeds a value of order 10. In contaminated systems spherical and nonoscillating ellipsoidal... 

As a drop becomes larger, it begins to distort in shapa, oscillate, and finally break into smaller drops. Circulation occurs wjthin it. Skelland 1 provides a number of coreslalions for dispeized- and coatinuons-phase coefficients for various situations in the two phases, for example, a circulating drop with significant resistance in both phases, end en useillating drop with resistance mainly within the drop. It is clear thai... 

Drop circulation Hadamard stream function, approximation from (B12)... 

Large Drop circulation Simplified boundary-layer theory... 

Drop circulation Empirical, mass transfer gas-liquid liquid-liquid... 

Drop circulation or oscillation E < 0.5 Table A-1 Drop circulation or oscillation E > 0.5... 

Drop circulation is by far the most important and stimulating model and numerous workers have treated its momentum-, heat-, and mass-transfer aspects both theoretically and experimentally. ... 

If the rate of desorption of surfactants is very much greater than the rate at which the drop traverses through a distance equivalent to its diameter (about 0.02 sec), one may expect (D6) no accumulation of surfactants at the rear end. A quantitative semiempirical expression for the degree of drop circulation as a function of the viscous forces, drop diameter, densities and the compressional modulus of the surface film (surface-tension gradient), as well as the empirical fraction of liquid circulating, has been suggested by Davies (D6). 

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