Micro PIV w An optical diagnostic technique for

Micro PIV w An optical diagnostic technique for microfluidics (e. g. MEMS, biological tissues, inkjet printer head) Requirements: n Measure instantaneously 103 - 104 vectors Spatial resolution of 1 - 10 mm Wide velocity range: 50 mm/s - 400 m/s n Accurate to within 3% full scale n n References n Meinhart, Wereley and Santiago (1999) n Santiago et al. (1998) n Private communication

Video Microscopy w 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: n ddiff = 2. 44 l/(D/f)=2. 44 l(f#) w Microscopy + Resolve particles of sub-microns Image field: 30~300 mm PIV Measurement of particle displacement q dp

Micro PIV w w w Field of View: 30 ~ 300 mm Vector Spacing: 1 ~ 10 mm Interrogation Cell: 2 ~ 20 mm vs. PIV 30 ~ 300 mm 1 ~ 10 mm 2 ~ 20 mm (50 % overlap) min. 10 pairs of particles for correlation w “Plane” Thickness dz: Depth of Field of microscope ~ 1 mm Laser sheet thickness ~ 1 mm Shrink 1000 times

Tracer Particles w Micro PIV Small-1. 2. 3. Follow flow Do not clog the device Do not alter fluid property But not too small-1. 2. w Regular PIV Small enough to track flow, need to be detectable by the camera Suppress Brownian motion Generate enough light signal Dp = 0. 3 ~ 0. 7 mm Dp = 3 ~ 30 mm

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

Solutions w. Oil immersion lens (n 1. 5) to get NA >1 NA =1. 4 for 60 x - 100 x objectives w. Fluorescence (epi-illumination, reflection) dp < l & stronger signal w. Differential Interference Contrast (DIC) microscopy Shearing interference to highlight refraction change

Light Source and Camera Mercury arc lamp n n Exposure Pulse delay t ~ 2 ms Dt ~ 100 ms Pulsed laser (Dual Nd: YAG laser) t ~ 5 ns Dt ~ 500 ns (Also depend on camera transfer) n Velocity up to 50 mm/s 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 w. Correlation w. Significant Noise: n n Out-of-plane motion Brownian motion w. Ensemble-averaging correlation technique (average 20 instantaneous correlations) w. 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 – 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 wallnormal direction near the wall

Inkjet Printer Head w Field of view 50 ~ 500 mm w Need objective lens working distance >1 mm (Cover Glass) w Smaller NA Larger particle size (~ 0. 6) (~ 0. 7 mm) Unsteady flow in the cycle of droplet ejection: need instantaneous or phase-averaged measurement

Basic Limitation of Micro PIV w. DOF (~ 1 mm) limits to strictly 2 D flow n n n Not only 2 D vector map, Out-of-plane motion cause measurement to fail Hence must select a plane with only 2 D motion PIV Plane