Interfacial mobility of surfactant and nanoparticle -laden interfaces under asymmetric shear flow

The shear stress of an axisymmetric flow field triggers a nonuniform distribution of the surfactants at the surface of a rising bubble, known as stagnant cap. This nonuniform surfactant distribution 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 curl-free. Thus, it cannot be compensated by the redistribution of the surfactants at the interface [1].
Here, we conduct model experiments with a bubble at the tip of a capillary placed in a defined asymmetric flow field. Thereby, we investigate the mobility of the interface in the presence of surfactants and nanoparticles using microscopic particle tracking velocimetry. Compared to surfactants, nanoparticles have substantially higher desorption energy, leading to irreversible adsorption. Thus, a different interaction between the bulk flow and the interface is expected for different types of adsorbed materials.
In this study, we show a direct experimental observation of the circulating flow at the interface under asymmetric shear stress. The results indicate that the interface remains mobile regardless of the surfactant concentration [2]. 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.