?PIV study of droplet fission in a bifurcating microchannel

?PIV study of droplet fission in a bifurcating microchannel

Sun, C.
Liu, S.L.

2013-07-01

In this study, we exploit the ?PIV (microscale particle image velocimetry) analysis to investigate the internal flow field of a dividing droplet in a bifurcating microchannel. Herein, droplets are generated by a flow focusing scheme and sent downstream toward an asymmetric bifurcation. In order to control the proportion of split droplets, we design the lengths and widths of the two arms such that the ratio of the flow resistances conforms to the designate values and pressure drops are equalized across the two daughter channels. During the experiments, three bifurcation angles (? = 30°, 60°, 180°) and four flow resistance ratios (R1/R2 = 0.01, 0.13, 0.38, 0.62) are considered. Similar to the results reported by previous literatures, a pair of counter-rotating recirculation is formed inside the droplet that travels along the mother channel in order to roll against the walls. When the droplet approaches the bifurcation of ? = 30° or 60°, however, the presence of the pinnacle break up the two vortices and a saddle point emerges. Once the interface of the droplet touches the vertex of the bifurcation, fluid tends to be pulled into the wider branch because of smaller capillary pressure needed. As a result, the saddle point shifts toward the wider branch and eventually the sharp edge of the bifurcation snap off the end cap of the droplet. Once the fission is complete, flows inside the two daughter droplets gradually retrieve back to the normal rolling configuration as they travel downstream. For the bifurcation of ? = 180°, however, the droplet monotonically slows down when it approaches the T branch. The opposite two arms pull the droplet apart more evenly without the aid of a sharp edge and the saddle point remains close to the stagnation point during the fission process. We find that the size ratio of the two daughter droplets is less extreme for ? = 180° and they nearly move in the same speed after the fission.

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Conference proceedings

conference paper

(c) 2013 Sun, C.; Liu, S.L.