Effects of surface active agents on fall velocities of drops

Skelland, A. H. P.; Huang, Yang H.

doi:10.1002/cjce.5450550302

Cited by 7 publications

(4 citation statements)

References 17 publications

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“…In both plots the data follow the shape of the Klee and Treybal correlation, but are all higher. In their work, Klee and Treybal (1956) did not take great care to eliminate impurities in their materials, yet the presence of surfactants has since been shown to reduce droplet terminal velocities (Skelland and Huang, 1977). The presence of parallel droplet streams appears to have no noticeable effect on the terminal velocity of the drops.…”

Section: Terminal Velocitymentioning

confidence: 99%

Effects of interaction between circulating or oscillating droplets on drop formation, free fall, and mass transfer

Skelland

Vasti

1985

Can J Chem Eng

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The effects of interaction between circulating or oscillating droplets on drop formation, terminal velocity, and the continuous phase mass transfer coefficient during free fall were studied. To examine the mass transfer coefficient during free fall both end effects were eliminated by subtracting the mass transferred in a short column from that transferred in a long column. A correlation for the prediction of drop volume based on the equations of Scheele and Meister (1968) for non‐jetting drops, and of Meister and Scheele (1969a, 1969b) for drops formed under jetting conditions, has been developed. The terminal velocity of the drops was enhanced by vertical interaction with adjacent drops at values of L/dp < 5. The continuous phase mass transfer coefficient for circulating drops was reduced due to vertical interaction at L/dp < 10; however, for slightly oscillating drops it was not affected by vertical interaction over the range of 1.5 < L/dp < 20. By conducting experiments with single and triple nozzle arrangements, horizontal interaction effects were found to be negligible for both systems.

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Section: Terminal Velocitymentioning

confidence: 99%

Effects of interaction between circulating or oscillating droplets on drop formation, free fall, and mass transfer

Skelland

Vasti

1985

Can J Chem Eng

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“…Many of the previously mentioned studies of the reduction in the internal circulation or the decrease in interphase mass transfer observed the reduction in terminal velocity. Other studies in which the measurement of the terminal velocity as a function of surfactant concentration was the primary focus of the study include: Griffith (1962), Edge & Grant (1972), Skelland & Huang (1977), Suzin & Ross (1985), Yamamoto & Ishil (1987), Skelland et al (1987), Bel Fdhila & Duineveld (1996), Sam, Gomez & Finch (1996), Jachimska, Warszynski & Malysa (2001), , Zhang, McLaughlin & Finch (2001), Krzan & Malysa (2002), Zhang, Sam & Finch (2003) and Liao et al (2004).…”

Section: Introductionmentioning

confidence: 99%

Theory and experiments on the stagnant cap regime in the motion of spherical surfactant-laden bubbles

2006

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The buoyant motion of a bubble rising through a continuous liquid phase can be retarded by the adsorption onto the bubble surface of surfactant dissolved in the liquid phase. The reason for this retardation is that adsorbed surfactant is swept to the trailing pole of the bubble where it accumulates and lowers the surface tension relative to the front end. The difference in tension creates a Marangoni force which opposes the surface flow, rigidifies the interface and increases the drag coefficient. Surfactant molecules adsorb onto the bubble surface by diffusing from the bulk to the sublayer of liquid adjoining the surface, and kinetically adsorbing from the sublayer onto the surface. The surface surfactant distribution which defines the Marangoni force is determined by the rate of kinetic adsorption and bulk diffusion relative to the rate of surface convection. In the limit in which the rate of either kinetic or diffusive transport of surfactant to the bubble surface is slow relative to surface convection and surface diffusion is also slow, surfactant collects in a stagnant cap at the back end of the bubble while the front end is stress free and mobile. The size of the cap and correspondingly the drag coefficient increases with the bulk concentration of surfactant until the cap covers the entire surface and the drag coefficient is that of a bubble with a completely tangentially immobile surface. Previous theoretical research on the stagnant cap regime has not studied in detail the competing roles of bulk diffusion and kinetic adsorption in determining the size of the stagnant cap angle, and there have been only a few studies which have attempted to quantitatively correlate simulations with measurements.This paper provides a more complete theoretical study of and a validating set of experiments on the stagnant cap regime. We solve numerically for the cap angle and drag coefficient as a function of the bulk concentration of surfactant for a spherical bubble rising steadily with inertia in a Newtonian fluid, including both bulk diffusion and kinetic adsorption. For the case of diffusion-limited transport (infinite adsorption kinetics), we show clearly that very small bulk concentrations can immobilize the entire surface, and we calculate the critical concentrations which immobilize the surface as a function of the surfactant parameters. We demonstrate that the effect of kinetics is to reduce the cap angle (hence reduce the drag coefficient) for a given bulk concentration of surfactant. We also present experimental results on the drag of a bubble rising in a glycerol–water mixture, as a function of the dissolved concentration of a polyethoxylated non-ionic surfactant whose bulk diffusion coefficient and a lower bound on the kinetic rate constants have been obtained separately by measuring the reduction in dynamic tension as surfactant adsorbs onto a clean interface. For low concentrations of surfactant, the experiments measure drag coefficients which are intermediate between the drag coefficient of a bubble whose surf...

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“…(ii) Kumar andHartland (1982, 1984) stated that "de Chazal and Ryan's (1971) model is complex because it requires the terminal velocity, ut to be calculated as a function of drop diameter using Hu and Kintner's (1955) correlation." But the precision required for ut would not be forthcoming from this correlation in our work with surfactants, because of the extreme sensitivity of terminal velocity to traces of surfactant contamination (Garner and Skelland, 1955;Skelland and Huang, 1977).…”

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confidence: 99%