Micro PIV

1
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 mm
• Wide velocity range 50 mm/s - 400 m/s
• Accurate to within 3 full scale
• References
• Meinhart, Wereley and Santiago (1999)
• Santiago et al. (1998)
• Private communication

2
Video Microscopy

• Mature technology in bio-medical fields
• The smallest resolvable size
• dp l/NA , NA (Numerical Aperture) n sinq
• For comparison, recall diffraction limit for
  camera
• ddiff 2.44l/(D/f)2.44l(f)
• Microscopy PIV
• Resolve particles of sub-microns Measurement of
  particle displacement
• Image field 30300mm

n
q
dp
3
Micro PIV vs. PIV

• 30 300 mm
• 1 10 mm
• 2 20 mm
• Laser sheet thickness 1 mm

• Field of View 30 300 mm
• Vector Spacing 1 10 mm
• Interrogation Cell 2 20 mm
• (50 overlap)
• min. 10 pairs of particles for correlation
• Plane Thickness dz
• Depth of Field of microscope 1mm

Shrink 1000 times
4
Tracer Particles

• Regular PIV
• Small enough to track flow, need to be
  detectable by the camera
• Dp 3 30 mm

• Micro PIV
• Small--
• Follow flow
• Do not clog the device
• Do not alter fluid property
• But not too small--
• Suppress Brownian motion
• Generate enough light signal
• Dp 0.3 0.7 mm

5
Challenges by Sub-micron Particles

• 1. Optical Resolution need Dp 300 700 nm
• (NdYAG l 500 nm)
• Visible light 400 nm ----- 750 nm
• If NA lt1, cannot resolve dp less than l
• sin q lt1
• n index of refraction between specimen
  objective
• 2. Low Light Signal

6
Solutions

• Oil immersion lens (n ? 1.5) to get NA gt1
• NA 1.4 for 60x - 100x objectives
• Fluorescence (epi-illumination, reflection)
• dp lt l stronger signal
• Differential Interference Contrast (DIC)
  microscopy
• Shearing interference to highlight refraction
  change

7
Light Source and Camera

• Mercury arc lamp
• Exposure t 2 ms
• Pulse delay Dt 100 ms
• (Also depend on camera transfer)
• Velocity up to 50 mm/s

• Pulsed laser
• (Dual NdYAG laser)
• t 5 ns
• Dt 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)
8
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

9
Example 1

• Santiago et al. (1998)

10
Result

• Santiago et al. (1998)

11
Example 2

• Meinhart, Wereley and Santiago (1999)

12
Result

• Meinhart, Wereley and Santiago (1999)

Ensemble-averaged velocity-vector field measured
in a 30 mm deep, 300 mm wide, 25 mm channel. The
spatial resolution is 13.6 mm x 4.4 mm away from
the wall, and 13.6 mm x 0.9 mm 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
13
Inkjet Printer Head

• Field of view 50 500 mm
• Need objective lens working distance gt1mm (Cover
  Glass)
• Smaller NA Larger particle size
• ( 0.6) ( 0.7 mm)
• Unsteady flow in the cycle of droplet ejection
• need instantaneous or phase-averaged
  measurement

14
Basic Limitation of Micro PIV

• DOF ( 1mm) 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