Interfacial flow of a surfactant-laden interface under asymmetric shear flow

Milad Eftekhari; Karin Schwarzenberger; Sascha Heitkam; Kerstin Eckert

doi:10.1016/j.jcis.2021.04.126

Interfacial flow of a surfactant-laden interface under asymmetric shear flow

J Colloid Interface Sci. 2021 Oct:599:837-848. doi: 10.1016/j.jcis.2021.04.126. Epub 2021 Apr 28.

Authors

Milad Eftekhari¹, Karin Schwarzenberger², Sascha Heitkam², Kerstin Eckert²

Affiliations

¹ Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany; Institute of Process Engineering and Environmental Technology, Technische Universität Dresden, 01062 Dresden, Germany. Electronic address: m.eftekhari@hzdr.de.
² Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany; Institute of Process Engineering and Environmental Technology, Technische Universität Dresden, 01062 Dresden, Germany.

PMID: 33991800
DOI: 10.1016/j.jcis.2021.04.126

Abstract

Hypothesis: The shear stress of the axisymmetric flow field triggers a nonuniform distribution of the surfactants at the surface of a rising bubble, known as stagnant cap. The formation of the stagnant cap gives rise to Marangoni stresses that reduce the mobility of the interface, which in return reduces the rising velocity. However, the conditions in technological processes usually deviate from the linear rise of a single bubble in a quiescent unbounded liquid. Asymmetric shear can act on the bubble surface e.g. due to the vorticity in the surrounding flow, bubble-bubble interactions, or influence of the reactor wall. A different surfactant distribution at the interface is expected under asymmetric shear, which can change the hydrodynamic behavior of the interface drastically.

Experiments: Here we conduct model experiments with a bubble or a drop at the tip of a capillary placed in a defined flow field. Thereby we investigate the influence of asymmetric shear forces on the interface in the presence of surfactants. Microscopic particle tracking velocimetry is employed to measure the velocity of the surfactant-laden interface for different degrees of asymmetry in the surrounding liquid flow.

Findings: We show a direct experimental observation of the circulating flow at the interface under asymmetric shear, which prevents the formation of the typical stagnant cap. Additionally, we reveal that the interface remains mobile regardless of the surfactant concentration. Our results confirm that increasing the degree of asymmetry increases the shear forces and thus the interfacial velocity.

Keywords: Asymmetric shear flow; Interfacial flow; Internal circulations; Marangoni effect; Particle image velocimetry; Rising bubble; Stagnant cap; Surfactant distribution.