Nano Particle Image Velocimetry (npiv); Data Reduction Challenges

Transcription

1 Nano Particle Image Velocimetry (npiv); Data Reduction Challenges Dr. Reza Sadr Micro Scale Thermo Fluids (MSTF) Laboratory Department of Mechanical Engineering P. O. Box 23874, Doha, Qatar 1

2 OUTLINE 2 Introduction NanoParticle Image Velocimetry (npiv) Experimental set up Numerical method Theory & Simulation Results Surface forces Particle distribution near the wall Effect of Brownian motion + non uniform illumination Effect out of plane velocity gradient Experimental results Conclusions

3 npiv New diagnostic technique to study near wall flows at thesub micron ( nano ) scale Measure two components of nearly instantaneous velocity parallel to wall [Li and Yoda, Exp Fluids, 2008] Extension of standard (macroscale) PIV Use evanescent wave illumination: TIR Seed flow with neutrally buoyant fluorescent particles Record tracer particle images over time Particle velocity flow velocity 3

4 npiv: IMPLEMENTATION 4 Illuminate flow with evanescent wave from TIR of light at solid fluid interface 1 Occurs at angle of incidence sin n c n Evanescent wave propagates parallel to wall y p{ p}, Intensity exp{ z / z p }, z p nm n 2 a z U=G (z+b) n 1 > n 2 b θ x z p n 2 sin 4n1 1 n / 2 Brownian motion causes particle drop out/in illumination region [Sadr et al., Exp. Fluids, 2005]

5 BROWNIAN MOTION Random motion of submicron particles in a fluid due to thermal energy Brownian diffusion hindered by wall D kt 6a Out of plane (z) diffusion [Bevan & Prieve 2000] (a=50nm, T=300 K) B Over t = 6.5 ms ( f =153 Hz) Langevin equation t 0 D 0.47 D for h/ a 1 B zb 160 nm for h/ a 1 t t x ( D) t r χ =Normally distributed random numbers ; mean=0, σ=1 B 5

6 Electrostatic force SURFACE FORCESS F el = f n (k,, T, a, ε 0 0, ε,, e, λ,, ζ p, ζ w ) = permibility, e = elementry charge =Debye lengths, = Zeta potential [Oberholzer et al., J Chem Phys 1997] van der Waals forces F vdw = f n (z, a) Buoyancy forces, F b = f n (V, ρ f, ρ p ) Langevin Equation Surface effects tt D x 0 D t r F t t kt 1

7 SIMULATION PARAMETERS Tracer: size and number density Fluid: velocity profile: Shear vs. uniform Hindered Brownian motion Surface and Buoyancy forces Illumination characteristics: z v Uc G a 2 Uniform, Z v = 280 nm Linear, Z v = 280 nm (non real!) (non real!) Exponentially decaying, Z p = 120 nm (real) Camera characteristics: shot and electronic noise 7

8 3D VIEW 8

9 npiv IMAGES Real image x Simulated image Pixel in ntensity a) Standard image processing FFT based cross correlation method to obtain average tracer displacement 2D Gaussian peak finding algorithm x Real image Simulated image [Sadr et al., Exp. Fluids, 2005] [Li et al., Exp. Fluids, 2006] 9

10 MEASURED MEAN VELOCITY Shear flow with no Brownian motion Light illumination profiles: 10 Uniform, I I for 0 0 zzv a I Linear, noise I 0 I ( z a ) I zv Exponential, exp z I I0 z p Decaying Light illumination affects measured velocity for shear flow Strongest effect is for exponentially decaying light tu c r search radius s 0 U U U c Each line contains all the shear rates study in this work, i.e. G=1000 to 3000 s 1

11 BIAS IN PIV DATA REDUCTION RESULTS No Surface effects 1 Shear flow without Brownian motion Significant effect of illumination Small effect oftime delay No affect of shear Shear flow with Brownian motion Significant effect for illumination Significant ifi effect for time interval Effect of shear depends on all shear, illumination, and time interval U/U c

12 12 EFFECT OF CORRECTION FACTORS Surface effects Correction factors for Brownian motion [Sadr et al., JFM, 2007] F U / U c exp * * * Surface effects are not considered Based on particle displacement No illumination affects are present F( ** ) [Huang et al., JFM, 2009] Based on particle displacement No illumination affects are present Estimated tracer velocity well predicted U/U c 1.6 The bias pattern is well predicted The models fail to correct the correlation based PIV data reduction results G = 1000 s s s -1 F( * ) F( ** ) v D * z 0.8a * t

13 EXPERIMENTAL RESULTS Inverted epi fluorescent Leica DMI6000 microscope Adapted for evanescent wave illumination from Ar + laser 40 objective w/adjustable /dj tbl working distance Princeton Pro EM 512 On chip gain (no intensifier): quantum effic. 90% at 500 nm Image size pixels Image pair time delay t=1.3ms Fused silica micro channel H=25 m, W=53 m Constant flow Syringe pump G (s 1 ) z v (nm) Dt (ms) Correction factor U cal (m/s) Theory U exp (m/s) Measured U corr (m/s) Corrected Velocity E E E 4 13

14 SUMMARY npiv: Near wall velocity measurement at Z <300 nm Surface forces are significant ifi at nano scale and cause non uniform tracer distribution near the wall Combination of shear and non uniform illumination also affects obtained velocity using correlation methods Existing correction for tracer displacement does not correct for PIV dt data reduction via correlation lti method Our results partially corrects the underestimation of the obtained velocity 14

15 ACKNOWLEDGEMENTS Thank you This work was supported by Qatar Foundation 15