In Taylor-Couette flow, characterized by the formation of axisymmetric vortex cells between concentric rotating cylinders, bubbles, droplets, and particles exhibit complex and intriguing motions that are strongly coupled with the underlying flow structure. In this study, the axial movement of a single droplet in a liquid-liquid two-phase Taylor-Couette flow was experimentally investigated under various Reynolds numbers, surfactant concentrations, and continuous phase compositions. Droplet movement was quantified by normalizing the axial position with the mean wavelength of Taylor cell pairs and by constructing histograms to estimate the number of crossings across cell boundaries. The results demonstrated that increasing the Stokes number significantly reduced the frequency of droplet movement crossing cell boundaries, indicating that droplet inertia strongly governs axial transport. Notably, when a glycerol aqueous solution was used as the continuous phase, droplets remained confined within a single pair of Taylor cells despite similar hydrodynamic conditions and Stokes numbers. This behavior is attributed to reduced droplet deformation associated with lower Capillary numbers and increased viscosity of the continuous phase. The findings suggest that axial movement of droplet in Taylor-Couette flow cannot be explained solely by inertial effects and requires consideration of interfacial tension and deformation dynamics. Further studies employing numerical simulations and systematic experiments will be essential to elucidate the mechanisms governing droplet migration and retention in multiphase Taylor-Couette flow.
Masuda et al. (Wed,) studied this question.