Page 226 - MEMS Mechanical Sensors

9.1 Introduction to Microfluidics and Applications for Micro Flow Sensors 215

Outlet Mixer

Flow
sensor 2
Flow
sensor 1

Pump 1 Pump 2

Inlet 1 Inlet 2
Figure 9.2 Microchemical reaction system realized on a microfluidic circuit board. Dimensions of
3
the system are 3 × 3.5 × 0.3 cm .

a micromachined differential pressure flow sensor and took measurements aboard a
Spacelab [20].
Flow sensors are often used in connection with, or built inside microchannels,
which affects the performance of the sensor. The pressure drop within the channel is
an important criterion that influences the measurement range and the usability of the
flow-sensing device with other devices (e.g., a micropump, which can only pump
against a certain backpressure). The pressure drop in a microchannel is given by
Gravesen et al. [10]. Koo et al. [32] compare experimental observations with compu-
tational analyses of liquid flow. They argue that the entrance effect becomes more
important for short channels with high aspect ratios and high Reynolds number con-
ditions. For polymeric liquids and particle suspension flows, the non-Newtonian
fluid effects become important. Wall slip effects are negligible for liquid flows in
microconduits, and the surface roughness effects are a function of the Darcy number,
the Reynolds number, and cross-sectional configurations. For Reynolds numbers
above 1,000, turbulence effects become an important part. And finally, viscous dissi-
pation effects on the friction factor are nonnegligible in a microconduit, especially
for hydraulic diameters D < 100 µm{D =(4 ⋅ area)/circumference)}. The Reynolds
h h
number is an important parameter in microfluidics and is a measure for the transi-
tion from the laminar to the turbulent flow regime. A laminar flow means that the
different fluid layers glide over one another smoothly and do not mix. Smooth and
connected streamlines are formed around an obstacle [Figure 9.3(a)]. Turbulent flow

(a) (b)
Figure 9.3 Flow past an obstacle: (a) laminar flow; (b) turbulent flow.