Vinyl chloride polymerization in microdroplet reactor - Les thèses en ...

Chapter I : Bibliographic review
Figure I- 10 : Droplet generation in a T-junction
II.B.1.b
Flow-focusing devices
In the case of these droplet generators, the discrete phase is injected into a co-flowing continuous
phase. At the flow-focusing junction, there can be a narrow constriction to focus the flow.
Figure I- 11: Different regimes observed in flow-focusing devices from Sullivan et al. (2008). (a)
Geometrically controlled break up, (b) dripping, (c) ‘narrowing jetting’, and (d) ‘widening jetting’.
Several flow regimes were observed in flow-focusing devices and they are presented in Figure I- 11.
Garstecki et al. (2006) explained that the dynamics of geometrically controlled break-up in a
microfluidic flow-focusing device is directed by the rate of supply of the continuous phase to the
region where break-up takes place. Droplets are produced in a uniform manner by the uniform
collapse of the liquid thread formed at the nozzle as a result of the slow progression of collapse and
the equilibrium between interfacial tension and hydrostatic pressure fields (a). Dripping (b) is very
similar to geometrically controlled break-up, except that pinch-off is not controlled by blockage of the
constriction, but instead by viscous drag on the gas thread. In both cases (a,b), the inner phase is
observed to grow from a minimum size, form a thinning neck behind a leading drop, pinch-off, and
finally retract to the original minimum. Stable and unstable jetting occurs when a long, thin thread
forms and subsequently breaks up due to capillary instabilities (c,d). These regimes have been
described more in depth by Anna et al. (2006) or Utada et al. (2007).
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