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To study the hydrodynamics of rising droplets (especially less than 1 mm) in quiescent water, a microfluidic device with co-flowing configuration was integrated to prepare micron-sized droplets. Soybean oil and toluene droplets of size from 100 to 600 μm were obtained by five co-flowing devices with different capillary sizes. It is found that the capillary device with smaller tip size produces smaller droplets and, for a given device, the droplet size decreases with the increase of continuous phase flow rate. Alternatively, the increase of dispersed phase flow rate has little influence on the droplet size. To predict the droplet size, a correlation for dimensionless droplet diameter as a function of Capillary number (Ca), Reynolds number (Re) and viscosity ratio was proposed. Then, the hydrodynamics of a single droplet and of droplet swarms rising in quiescent water were extensively investigated. The experimental results show that the terminal velocity of a single micro-droplet is consistent with that of rigid spheres with the same size and density, while the terminal velocity of a droplet swarm is obviously higher than that of a single droplet. Experimental observation shows that the motion of a droplet swarm in static water is such a manner that each droplet alternately accelerates by chasing others, causing a higher terminal velocity. Furthermore, the terminal velocities of millimeter-sized toluene droplets rising in water were measured. Compared with rigid spheres, the droplets of dp=1~4.6 mm have higher terminal velocities, while the droplets of dp>4.6 mm have lower terminal velocities. Finally, by taking into account the interfacial tension, internal circulation and shape deformation, a comprehensive explanation is proposed to describe the variation of the drag coefficient with the droplet size ranging from microns to millimeters.

PREPARATION OF MICRON-SIZED DROPLETS AND THEIR HYDRODYNAMIC BEHAVIOR IN QUIESCENT WATER