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Here we present the study on the behavior of individual droplets in a Taylor vortex of immiscible working fluid. The droplets (water or ethanol with the volume of 15–30 μL) were introduced into a Taylor–Couette device with an aspect ratio of 6 and a radius ratio of 0.67, in which a mineral oil (density of 0.86 g/cm3 and viscosity of 0.066 Pa s) was used as the working fluid. This configuration ensures a laminar Taylor vortex flow with no occurrence of wavy vortex in the entire operating range. The behavior of the droplets was investigated with a high speed camera and a phase Doppler particle analysis (PDPA) system. The water droplet can be trapped in the vortex center at low Reynolds numbers, corresponding to a circular trajectory, which gradually develops into a three-dimensional toroidal motion with the increasing Reynolds number. Differently, the ethanol droplet exhibits the toroidal motion at low Reynolds numbers and follows the circular trajectory at high Reynolds numbers. Due to their deformability, both water and ethanol droplets are elongated as an ellipsoid shape when moving in the gap, although its length-to-diameter ratio varies along the annulus. Furthermore, the ethanol droplet subject to a sudden-start of the fluid can break into lots of small droplets with the size of microns, which later coalesce into a single droplet again after the Taylor vortex flow becomes stable. The study can help to better understand the droplet behaviors in a Taylor vortex and therefore benefit possible applications conducted in Taylor vortex devices. |
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