Interface pinch-off in the presence of a soluble surfactant

TL;DR

This study combines numerical and experimental methods to analyze how soluble surfactants influence droplet breakup, revealing that certain surfactants maintain constant surface tension and affect filament formation.

Contribution

It provides a detailed comparison of diffusion-limited and insoluble surfactant effects on droplet pinch-off, highlighting the role of surfactant properties in breakup dynamics.

Findings

Surfactant transfer is enhanced by convection, keeping surface tension constant during most of the breakup.

Diffusion limits surfactant sorption only near pinch-off, affecting droplet shape.

Surfynol 465 and SDS maintain constant surface tension, unlike slow-kinetics surfactants like Triton X-100.

Abstract

We study numerically and experimentally the breakup of a pendant droplet loaded with a soluble surfactant. We consider the limit in which surfactant sorption is limited only by diffusion. Surfactant transfer toward the interface is enhanced by convection. As a consequence, diffusion does not constitute a significant barrier over most of the breakup, and surfactant sorption maintains the surface tension practically constant across the interface. Diffusion hinders the surfactant sorption only very close to the interface pinch-off. The droplet shape in the diffusion-limited model deviates significantly from that in the insoluble case over most of the breakup. In the insoluble case, the droplet shape is affected by surfactant depletion, which leads to a local increase in surface tension and Marangoni stress. The dynamics of a millimeter-sized droplet loaded with Surfynol 465 agree remarkably well with predictions from the diffusion-limited model, without any parameter fitting, down to pinching times of the order of $10-20$ $\mu$s. Sodium dodecyl sulfate (SDS) produces essentially the same effects as those for Surfynol 465. Therefore, both Surfynol 465 and SDS maintain a practically constant surface tension throughout most of the droplet breakup. Slow-kinetics surfactants, such as Triton X-100, differ significantly from Surfynol 465 and SDS. The most evident effect of the surfactant adsorption energy barrier is the shortening of the filament that bridges the upper meniscus and the detached lower drop. Comparing the filament length to that of a clean interface with the same surface tension allows one to evaluate the rate of surfactant adsorption.