micro piv an optical diagnostic technique for microfluidics (e.g. mems, biological tissues, inkjet...
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Micro PIV
An optical diagnostic technique for microfluidics(e.g. MEMS, biological tissues, inkjet printer head)Requirements:
Measure instantaneously 103 - 104 vectors Spatial resolution of 1 - 10 m Wide velocity range: 50 m/s - 400 m/s Accurate to within 3% full scale
References Meinhart, Wereley and Santiago (1999) Santiago et al. (1998) Private communication
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Video Microscopy
Mature technology in bio-medical fields
The smallest resolvable size
dp = /NA , NA (Numerical Aperture)= n sinFor comparison, recall diffraction limit for camera:
ddiff = 2.44/(D/f)=2.44f#)
Microscopy + PIV
Resolve particles of sub-microns Measurement of particle displacementImage field: 30~300m
n
dp
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Micro PIV vs. PIV
Field of View: 30 ~ 300 m Vector Spacing: 1 ~ 10 m Interrogation Cell: 2 ~ 20 m
(50 % overlap)
min. 10 pairs of particles for correlation
“Plane” Thickness z:
Depth of Field of microscope ~ 1m
30 ~ 300 mm
1 ~ 10 mm
2 ~ 20 mm
Laser sheet thickness ~ 1 mm
DOFn
NA
ne
M NA
( ) ( )2
Shrink 1000 times
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Tracer Particles
Micro PIV
Small--1. Follow flow
2. Do not clog the device
3. Do not alter fluid property
But not too small--1. Suppress Brownian motion
2. Generate enough light signal
Dp = 0.3 ~ 0.7 m
Regular PIV
Small enough to track flow, need to be detectable by the camera
Dp = 3 ~ 30 m
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Challenges by Sub-micron Particles
1. Optical Resolution: need Dp = 300 – 700 nm(Nd:YAG: ~ 500 nm)
Visible light 400 nm 750 nm
If NA <1, cannot resolve dp less than sin <1n: index of refraction between specimen & objective
2. Low Light Signal
dNA np
sin
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Solutions
Oil immersion lens (n 1.5) to get NA >1
NA =1.4 for 60x 100x objectives
Fluorescence (epi-illumination, reflection)dp < & stronger signal
Differential Interference Contrast (DIC) microscopy
Shearing interference to highlight refraction change
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Light Source and Camera
Mercury arc lamp
Exposure ~ 2 ms Pulse delay t ~ 100 ms
(Also depend on camera transfer) Velocity up to 50 m/s
Pulsed laser
(Dual Nd:YAG laser)
~ 5 ns
t ~ 500 ns
up to 1 m/s
Digital CCD Camera(1030 x 1300 x 12 bit cooled interlined transfer can record
back-to-back images within 500 ns)
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Data Processing
Correlation
Significant Noise: Out-of-plane motion Brownian motion
Ensemble-averaging correlation technique(average 20 instantaneous correlations)
Limited to steady or periodic flows
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Example 1– Santiago et al. (1998)
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Result– Santiago et al. (1998)
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Example 2– Meinhart, Wereley and
Santiago (1999)
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Result
Ensemble-averaged velocity-vector field measured in a 30 m deep, 300 m wide, 25 m channel.The spatial resolution is 13.6 m x 4.4 m away from the wall, and 13.6 m x 0.9 m near the wall. A 50% overlap between interrogation spots yields a velocity vector spacing of 450 nm in the wall-normal direction near the wall
– Meinhart, Wereley and Santiago (1999)
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Inkjet Printer Head
Field of view 50 ~ 500 m Need objective lens working distance >1mm (Cover Glass)
Smaller NA Larger particle size
(~ 0.6) (~ 0.7 m) Unsteady flow in the cycle of droplet ejection:
need instantaneous or phase-averaged measurement
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Basic Limitation of Micro PIV
DOF (~ 1m) limits to strictly 2D flow Not only 2D vector map, Out-of-plane motion can cause measurement to fail Hence must select a plane with only 2D motion
PIV Plane