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