Page 257 - MEMS Mechanical Sensors

246 Flow Sensors

Optical Wave Flow Wave
fiber guide guide
Drag
element
Cantilever
Silicon Rubber
(a) Optical beam
fiber

Strain gauge
IR receiver
Groove
Slugs
Flow
From
Towards laser
power meter
(b)
IR diode Electrode
(c)
Figure 9.36 Schematic diagrams of optical flow sensor: (a) The sensor employs a cantilever beam
++
−
and wave guides made of N /N . V-grooves in the silicon are used to place the optical fibers.
(After: [109].) (b) Optical drag force flow sensor. (After: [110].) (c) Flow sensor for detection of
multiphase flow (light gray: water, dark gray: decane, white: air). (After: [110].)

9.9.2 Particle Detection and Counting
Other optical setups are used to detect particles or cells out of the fluid flow. A
micromachined version in soda-lime glass using two embedded optical fibers was
presented by Lin et al. [111] and using integrated optical waveguides by Lee et al.
[112]. The particles/cells (5- to 20-µm polystyrene beads and diluted whole blood)
are squeezed hydrodynamically into a narrow stream by two neighboring sheath
flows so that they flow individually through the detection region. The resulting scat-
tered light is then detected optically. The flow rates for the sheath flows and the sam-
ple flow were 0.2 and 0.05 µl/min, respectively. Various micromachined devices for
particle counting, separation, deflection, sorting, or transporting within fluids are
described in the book by Koch et al. [4].

9.9.3 Multiphase Flow Detection

The flow of fluids in pipes, where more than one immiscible phase is present, are of
major importance in several industries, like the power-generation industry (steam
generators and some types of nuclear reactors) and the petroleum extraction indus-
try [113]. Multiphase flow could be, for example, steam/water, water/kerosene, and
crude oil/water/natural gas. Fordham et al. [113–115] use standard silica fibers and
internal reflection to distinguish drops, bubbles, or other regions of fluid in multi-
phase flows on the basis of refractive-index contrast. They tested various geometries
for the tip of the optical fiber and used a self-assembled monolayer of alkyl