Velocity Field within Droplets Falling in Liquid Media inside a Rectangular Channel

The concept of droplet microfluidics and droplet reactors is becoming popular in the chemical processing industry, especially to handle expensive specialty chemicals. The convective mixing within a single droplet or two coalescing droplets plays an important role during the reaction involving two-phase systems, which requires a good understanding of the internal fluid motion within the droplet. In this work, we study the flow field inside a liquid droplet falling through an immiscible liquid in a narrow-width rectangular vertical channel using particle image velocimetry (PIV). The droplet can be considered a representation of a midplane cutting through a spherical droplet. The effect of the size and release position of the droplet on the velocity field is examined. When the droplet is released at the center of the channel, a pair of symmetric counter-rotating vortices is observed inside the droplet. The vortices are positioned in the bottom half of the droplet and move toward its horizontal centerline as the diameter increases. However, when the droplet is released at an off-center position, the vortices are not symmetric. The terminal velocity of the droplet is observed to increase with an increase in droplet diameter, and therefore, coalescence occurs when a bigger droplet is released following a smaller one. The growth of the neck of the combined droplet with time and the velocity field at the neck are studied during coalescence.