Marangoni retardation of the terminal velocity of a settling droplet: The role of surfactant physico-chemistry

JOURNAL OF COLLOID AND INTERFACE SCIENCE

卷:178

Article

Chen, J ; Stebe, KJ

关键词: Marangoni stresses;    surfactants;    Frumkin isotherm;    physico-chemical hydrodynamics;    dynamic surface tension;   

DOI  :  10.1006/jcis.1996.0102

来源: Elsevier

【 摘 要 】

Nonlinear adsorption models accounting for monolayer saturation and nonideal surfactant interactions are used to find the terminal velocity U' of a droplet settling through a surfactant solution. Most prior research uses a linear adsorption model which cannot capture these effects. The solution concentration C'(eq) is assumed to be large enough for the surfactant mass transfer to be adsorption-controlled. The Langmuir model accounts for monolayer saturation by incorporating an upper bound for the surface concentration, Gamma'(infinity). Two competing effects result which alter U' from that predicted by the linear model. For slow adsorption-desorption kinetics, strong marangoni stresses develop when the surface concentration at the rear pole approaches Gamma'(infinity). These stresses favor strong retardation in U'. The adsorption flux is proportional to the unoccupied space on the interface, so depleted regions are supplied more rapidly. This diminishes Marangoni stresses and favors weak retardation. This effect dominates for rapid sorption kinetics. The Frumkin framework incorporates monolayer saturation and nonideal surfactant interactions which alter the amount of adsorbed surfactant, the sensitivity of the surface tension, and the dynamics of adsorptive-desorptive exchange. For a fixed mass of adsorbed surfactant, U' retarded for no interactions is increased by intersurfactant repulsion and decreased for cohesion. At elevated C'(eq). U' asymptotes to a value less than Hadamard-Rybczynski velocity U'(o) for the Langmuir case and for cohesive interactions. For repulsive interactions, U' approaches U'(o) in this limit. These asymptotes indicate the degree of surface remobilization attainable for finite adsorption-desorption kinetics and non-ideal interactions. (C) 1996 Academic Press, Inc.

【 授权许可】

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