Biomedical Engineering Reference
In-Depth Information
droplets with an increased monodispersity; microflow focusing device (MFFD) is
actually the best tool to achieve such a goal [69, 70]. In the following, we present
the principles of a MFFD and the different geometries that are currently used.
4.4.3.2 Mechanism
The principle of a flow focusing device is shown in Figure 4.67. In the most com-
mon flow regime (i.e., the droplet regime—we shall see later that different flow
regimes may appear) the formation of a droplet can be described by the following
four stages. First a “tongue” of dispersed phase liquid enters the orifice. It forms
an obstacle to the flows coming from each of the sides and pinching of the tongue
occurs in a second step. A liquid blob is formed, which is linked to the incoming
dispersed phase by an elongating thread (stage 3). Finally, when it thins, the thread
becomes instable and breaks, liberating a droplet.
On a theoretical point of view, the physics of a MFFD is complex. Currently
there are no general quantitative theories for two-phase flow in a geometry such as
a flow-focusing configuration. There are many parameters to the problem:
The actuation parameters: Q i and Q e or P i and P e , respectively, the dispersed
phase and continuous phase flow rates or pressures.
The fluids characteristics: h i , h e , g , r i , and r e , denoting respectively the viscosity
of the dispersed and continuous phases, the surface tension between the two liquids,
and the density of the two liquids.
The geometry parameters, like the dimensions of the channels wi, i , w e , w n , w s , d
and the flow resistances of the channels Ri, Re and Rs, which are respectively the
widths of the dispersed phase channel, continuous phase channel, nozzle, outlet
channel, depth of the channels, and hydraulic resistances of the dispersed phase,
continuous, and outlet channels.
The droplet parameters, fr , L , V d (or f d ), and U d , corresponding to the droplet
release frequency, the distance between two droplets, the volume of a droplet (or its
diameter), and the velocity of the droplet in the outlet channel.
Figure 4.67  (a-d) Different geometries of MFFD. Type 1: systems for encapsulation; type II, systems
for producing emulsions.
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