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
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
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
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
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
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
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)
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
Example 1– Santiago et al. (1998)
Result– Santiago et al. (1998)
Example 2– Meinhart, Wereley and
Santiago (1999)
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)
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
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