Manipulable Formation of Ferrofluid Droplets in Y-Shaped Flow-Focusing Microchannels

The formation process of ferrofluid droplets in the Y-shaped flow-focusing microchannels with a cross section of 400 mu m x 400 mu m and different angles was investigated experimentally by a high-speed camera. Three flow patterns were observed: slug flow, dripping flow, and jetting flow. The effects of two-phase flow rates, the capillary number of the continuous phase, the magnetic flux density and the angle of a microchannel on the formation process, and the size of droplets were studied systematically. The formation process of a droplet includes four stages: expansion stage, extending stage, squeezing stage, and pinch-off stage. The relationship between the dimensionless minimum width w(m)/w(c) of the droplet neck and the dimensionless remaining time (T-t)/T-c could be described in a power law relationship. On the basis of the two-phase flow rates, the capillary number of the continuous phase, the magnetic bond number and the angle of a microchannel, a correlation for predicting the droplet size was proposed.