Biomedical Engineering Reference
In-Depth Information
Figure 6.59  Schematic of the flow focusing.
the second focusing, it is confined to a diamond-shaped region surrounded by the
secondary liquid. By adjusting the secondary flow velocities, the liquid filament can
be centered in the exit channel. Again, upon integration of (2.47), an approximated
expression for the radius r of the “spot” is given by
2
Q
9
4
r
1
»
(6.109)
Q
wd
6.5.1.3 Discussion
In the two preceding sections, the focusing of a flow has been presented. The precision
of the focusing depends in the first place of the flow rate ratios, as indicated by (6.108)
and (6.109). Note, however, that relatively large objects such as cells will continue
along their streamlines, while smaller objects—DNA strands, for example—will have
a convective-diffusive behavior and progressively smear out from their streamlines.
6.5.2  Pinched Channel Microsystems
Separation of macromolecules, particles, and, above all, cells is fundamental in
biotechnology. For instance, extracting a well-defined population of cells in a wide
variety of cells is at the basis of blood analysis. An interesting device is the pinched
channel device (Figure 6.62). After focusing along a wall, the cells follow different
streamlines according to their size [25, 26]. Large particles follow the streamline
passing by their gravity center. After focusing, smaller particles are located closer to
the wall and, after the sudden enlargement of the channel, they follow a streamline
far apart from that of larger particles.
Figure 6.60  Different focusing obtained with the numerical software COMSOL for different flow
conditions.
 
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