Biomedical Engineering Reference
In-Depth Information
Figure 2.21
Flow lines in the microchamber. The delector does not induce any recirculation
(COMSOLcalculation).
been performed with the numerical software COMSOL using the complete Navier-
Stokes equations. The flow lines are shown in Figure 2.20, for an inlet velocity of
1 mm/s from left to right. If the flow is reversed, the pattern of the flow lines is ex-
actly the same (Figure 2.21). This is typically a case where the Stokes equations are
sufficient to describe the flow.
The property of reversibility is very important because it shows that, at very
low velocities, it is not possible to design a microfluidic “diode” where the pres-
sure drop would be small in one direction and large in the opposite direction. If
one wants to design such a diode, the flow velocity must be sufficiently large to be
outside the Stokes hypothesis or, as mentioned earlier, be non-Newtonian [17]. For
example, micropipes can give directionality to micropumping, based on a dissym-
metrical design, as shown in Figure 2.22, if the fluid flow is sufficiently important.
A simulation of such a design has been performed using the COMSOL software
(Figure 2.23). For a flow of 1 mm/s from left to right, the pressure drop is 296 Pa;
it is 324 Pa if the flow is reversed. For a flow of 100
m
m/s, the pressure drop is 29
Figure 2.22
Directionalityofpumpedlowobtainedbydissymmetricalpipes.Oscillationsofthe
piezo-electricallyactuatedmembranetriggeradirectionallowonlyinthecaseofasuficientlyhigh
Reynoldsnumber.
