Biomedical Engineering Reference
In-Depth Information
In this geometry, the polymeric chains stretch in the entrance of the tube and relax
at the outlet of the constriction (Figure 2.15).
The normalized velocity profile in a cross section differs considerably from the
Poiseuille-Hagen parabolic profile, as shown in Figure 2.16.
Non-Newtonian Microflow in a Microfluidic Network
Microfluidic networks
are commonly used in biotechnology. Later we will present some examples of mi-
crofluidic networks, such as a microneedle with multiple outlets and a network for
extracting plasma from blood. Such networks are difficult to balance (e.g., to design
in order to have the expected flow rate in each branch). Usually such networks are
designed considering Newtonian fluids. It is emphasized here that such networks
will not work properly for non-Newtonian fluids. Figure 2.17 shows the change of
flow rates in a network depending on the visco-elastic character of the fluid. Using
driving pressure conditions at the inlet, the computation shows that the flow rate in
the small “branches” is reduced in the non-Newtonian case.
2.2.3 Laminarity of Microflows
Regardless of the size—macroscopic or microscopic—a fluid flow is said to be lami-
nar when viscous forces dominate inertia. When this is the case, turbulences cannot
develop and the fluid flow lines are, at least locally, parallel. One can picture it by
considering that the flow is locally laminated. On the other side, a turbulent flow
Figure 2.16
Thenon-NewtonianvelocityproilediffersfromthePoiseuille-Hagenparabolicproile
(COMSOLcalculation).
