Abstract
HypothesisSurfactant redistribution in a liquid bridge close to drop detachment depends on competition between the bridge deformation rate and surfactant equilibration rate. Surfactant effect can be different in situations when diffusion coefficient changes independently of thinning kinetics or in line with it. Using moderately viscous liquids should allow both situations to be explored experimentally. ExperimentsFormation of liquid drops at the tip of capillary is studied experimentally for silicone oils and for surfactant-laden and surfactant-free water/glycerol mixtures of moderate viscosity with particular attention to the kinetics of liquid bridge close to pinch-off and formation of satellite droplets. FindingsEffect of surfactant depends on the dynamic regime of the bridge thinning. In the presence of surfactant, inertial kinetics slows down close to pinch-off demonstrating effective surface tension smaller than dynamic surface tension. An acceleration of thinning kinetics caused by depletion of surfactant from the liquid bridge was observed in viscous and visco-inertial regimes. The size of satellite droplets has a maximum versus viscosity; increasing with surfactant concentration at smaller viscosities and decreasing with an increase of surfactant concentration at largest studied viscosity, where inversion of the pinch-off point was observed for surfactant solutions.
Highlights
The results of the present study show that the slowing down kinetics of thinning of surfactant-laden liquid bridges is a general phenomenon not restricted to the certain type of surfactant or to concentrations above critical micellar concentration (CMC)
Particular attention was paid to the kinetics of thinning of the bridge connecting the detaching drop to the capillary close to the pinch-off time and the size of satellite droplets accompanying the formation of the main drop
Surfactant dynamic effects on drop formation were probed for two different situations: (i) when surfactant diffusion coefficient changes independently of thinning kinetics or (ii) when it changes in line with thinning kinetics
Summary
N.M. Kovalchuk et al / Journal of Colloid and Interface Science 516 (2018) 182–191 Nomenclature. AI proportionality constant in scaling law for inertial kinet- t0 ics of bridge thinning, Eq (2) l. AV proportionality constant in scaling law for viscous q kinetics of bridge thinning, Eq (4) r. AVI proportionality constant in scaling law for visco-inertial r0 kinetics of bridge thinning, Eq (1) rd. CMC critical micelle concentration g acceleration due to gravity re hm neck radius
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