Response of a surfactant- and particle-laden bubble surface to asymmetric shear flow

The shear stress of an axisymmetric flow field triggers a nonuniform distribution of adsorbed
surfactants at the surface of a rising bubble. This creates a surface tension gradient that
counteracts the viscous shear stress of the flow and thus reduces the mobility of the interface.
However, in technological processes the flow field often is asymmetric, e.g. due to the
vorticity in the flow. Under such conditions, the interface experiences an unbalanced shear
stress that is not free of curl, i.e. it cannot be compensated by the redistribution of the surfactants
at the interface (Vlahovska et al., 2009). Here, we conduct model experiments with
a bubble at the tip of a capillary placed in a defined asymmetric flow field, in the presence of
surfactants and nanoparticles. Unlike classical surfactants, nanoparticles adsorb irreversibly
at the bubble surface. Thus, a different interaction between the bulk flow and the interface
is expected. In this study, we show a direct experimental observation of the circulating flow
at the interface under asymmetric shear stress (Eftekhari et al., 2021a,b). The results indicate
that the interface remains mobile regardless of the surfactant concentration. Additionally, we
show that the nanoparticle-laden interface adopts a solid-like state and resists the interfacial
flow upon surface compression. Our results imply that the immobilization of the interface
can be described by the ratio of the interfacial elasticity to the bulk viscous forces.
Vlahovska, P. M., Bławzdziewicz, J., & Loewenberg, M. (2009). Small-deformation theory for a
surfactant-covered drop in linear flows. J.Fluid Mech., 624, 293.
Eftekhari, M., Schwarzenberger, K., Heitkam, S., & Eckert, K. (2021). Interfacial flow of a surfactant-
laden interface under asymmetric shear flow. J. Colloid Interface Sci., 599, 837.
Eftekhari, M., Schwarzenberger, K., Heitkam, S., Javadi, A., Bashkatov, A., Ata, S., & Eckert, K.
(2021). Interfacial behavior of particle-laden bubbles under asymmetric shear flow. Langmuir,
37, 13244.