Biomedical Engineering Reference
In-Depth Information
if
P
e
<<
P
i
. Similar regimes occur with flow rate actuation, except that flow reversal
is not observed due to the conditions imposed by the syringe pumps.
It is practical to plot the flow domains in a [
Q
i
,
Q
e
] or [
P
i
,
P
e
] coordinates sys-
tem. The flow chart usually has the aspect shown in Figure 4.70.
Volume, Frequency, and Spacing of the Droplets
Volume
V
d
, frequency
f
, and spacing D of droplets produced in the MFFD are the
three main parameters that one would like to control. There are two relations link-
ing these numbers. The first relation links the flow rate of the dispersed phase to the
frequency and the droplet volume
Q f V
(4.101)
The second relation links the velocity in the outlet channel
U
s
to the frequency
and spacing
=
i
d
U
= D
f
(4.102)
s
The volume of the droplets is an important topic as encapsulation or emul-
sification applications require a precise, given volume of droplets. This raises the
question of the dependency of the droplet size on the different parameters of the
FFD. It has been experimentally observed that the size of the droplets depends on
the flow conditions, the geometry of the FFD, the viscosities of the fluids, and to
some extend to the surface tension between the fluids. Quantitative theory does not
exist for two-phase flows in FFDs, so we have to rely on the different experimental
results and observed trends.
Figure 4.70
Typical flow chart for FFDs. The droplet regime is the regime of choice for MFFDs
applications.
