Micro PIV  An optical diagnostic technique for microfluidics (e.g. MEMS, biological tissues, inkjet printer head) Requirements: Measure instantaneously vectors Spatial resolution of  m Wide velocity range: 50  m/s 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 d p = /NA, NA (Numerical Aperture)= n sin  For comparison, recall diffraction limit for camera: d diff = 2.44 /(D/f)=2.44  f#)  Microscopy + PIV Resolve particles of sub-microns Measurement of particle displacement Image field: 30~300  m n  dpdp

Micro PIVvs.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 ~ 1  m 30 ~ 300 mm 1 ~ 10 mm 2 ~ 20 mm Laser sheet thickness ~ 1 mm 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 D p = 0.3 ~ 0.7  m  Regular PIV Small enough to track flow, need to be detectable by the camera D p = 3 ~ 30  m

Challenges by Sub-micron Particles  1. Optical Resolution: need D p = 300 – 700 nm (Nd:YAG:  ~ 500 nm) Visible light 400 nm   750 nm If NA <1, cannot resolve d p less than sin  <1 n: index of refraction between specimen & objective  2. Low Light Signal

Solutions  Oil immersion lens (n  1.5) to get NA >1 NA =1.4 for 60x  100x objectives  Fluorescence (epi-illumination, reflection) d p < & 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 (~ 1  m) 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