Kinetics of liquid bridges and formation of satellite droplets: Difference between micellar and bi-layer forming solutions

- Effective surface tension can decrease as surfactant laden bridge goes to pinch-off.
- Marangoni stresses are the reason for a decrease in effective surface tension.
- A decrease in effective surface tension depends on surfactant equilibration rate.
- Size of satellite droplets increases with decrease of equilibrium surface tension.
- The concentration dependence of the size depends on surfactant equilibration rate.

The process of drop detachment from a capillary tip and formation of satellite droplets is studied for solutions of trisiloxane surfactants above the critical aggregation concentration. Two of the studied surfactants self-assemble in bilayer based phases, whereas the third forms micelles. The difference in the aggregates formed results in an essential difference in the rate of equilibration between the surface and the bulk solution and in a different behaviour near the pinch-off point. The difference in behaviour becomes pronounced when the viscosity of solutions increases 2-6 times (and therefore diffusion coefficients decrease correspondingly). In particular, when surfactant solutions are prepared in a water/glycerol mixture with a viscosity six times larger than water, the size of satellite droplets formed by the micellar solutions increases more than twice, whereas the size of droplets formed by the bilayer-forming solutions remains almost constant over a range of concentration covering two orders of magnitude. The bilayers forming solutions demonstrate a decrease in the effective surface tension near to pinch-off which can be related to the Marangoni stresses generated by surface flow during the thinning of the capillary bridge connecting the main drop with the liquid in the capillary.

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