Experimental evidence of diffusion-induced bias in near-wall velocimetry using quantum dot measurements

Pouya, Shahram; Koochesfahani, Manoochehr; Greytak, Andrew B.; Bawendi, Moungi G.; Nocera, Daniel G.

doi:10.1007/s00348-008-0491-7

Cited by 13 publications

(14 citation statements)

References 10 publications

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“…The estimated diffusion coefficient in this case was found to be a function of interrogation time interval t, where the diffusion coefficient was increasingly overestimated as t increased. This diffusion-induced effect is similar to the mean velocity overestimation of particles in near-wall measurements, reported analytically by Sadr et al (2007) and experimentally by Pouya et al (2008) and Huang et al (2009).…”

Section: Discussionsupporting

confidence: 80%

“…The effect also becomes more pronounced as the domain size (H) shrinks. The diffusion-induced bias just described for the near-wall diffusion coefficient has also been observed in near-wall velocimetry studies, where the asymmetric diffusion leads to a larger apparent mean velocity that increases with delay time (Sadr et al 2007;Pouya et al 2008;Huang et al 2009). …”

Section: Near-wall Hindered Diffusionmentioning

confidence: 81%

“…In the simulations d p /a is set to 13, consistent with previous near-wall QD imaging experiments (d p ≈ 100 nm, a ≈ 8 nm) (Pouya et al 2008). Other values of d p /a can be considered easily, if needed, by interrogating the results for different values of intensity threshold, as described next.…”

Section: Brownian Simulationmentioning

confidence: 90%

“…only particles whose average intensity is less than this value are used in the data analysis. This value of intensity threshold is chosen based on the typical values used in previous experiments (Pouya et al 2005(Pouya et al , 2008. Results for other values of intensity threshold will follow.…”

Section: Near-wall Hindered Diffusionmentioning

confidence: 99%

“…in near-surface measurements with evanescent wave illumination using fluorescent coated particles (Zettner & Yoda 2003;Jin et al 2004;Sadr et al 2004) and quantum-dot (QD) nano-particles (Pouya et al 2005(Pouya et al , 2008. Such measurements often require longer detection exposure/integration periods to compensate for the much reduced optical signal of nano-particles.…”

Section: Introductionmentioning

confidence: 98%

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Effect of finite sampling time on estimation of Brownian fluctuation

2015

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We present a study of the effect of finite detector integration/exposure time E, in relation to interrogation time interval t, on analysis of Brownian motion of small particles using numerical simulation of the Langevin equation for both free diffusion and hindered diffusion near a solid wall. The simulation result for free diffusion recovers the known scaling law for the dependence of estimated diffusion coefficient on E/ t, i.e. for 0 E/ t 1 the estimated diffusion coefficient scales linearly as 1 − (E/ t)/3. Extending the analysis to the parameter range E/ t 1, we find a new nonlinear scaling behaviour given by (E/ t) −1 [1 − ((E/ t) −1 )/3], for which we also provide an exact analytical solution. The simulation of near-wall diffusion shows that hindered diffusion of particles parallel to a solid wall, when normalized appropriately, follows with a high degree of accuracy the same form of scaling laws given above for free diffusion. Specifically, the scaling laws in this case are well represented by 1 − ((1 + )(E/ t))/3, for E/ t 1, and (E/ t) −1 [1 − ((1 + )(E/ t) −1 )/3], for E/ t 1, where the small parameter depends on the size of the near-wall domain used in the estimation of the diffusion coefficient and value of E. For the range of parameters reported in the literature, we estimate < 0.03. The near-wall simulations also show a bias in the estimated diffusion coefficient parallel to the wall even in the limit E = 0, indicating an overestimation which increases with increasing time delay t. This diffusion-induced overestimation is caused by the same underlying mechanism responsible for the previously reported overestimation of mean velocity in near-wall velocimetry.

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Section: Discussionsupporting

confidence: 80%

Section: Near-wall Hindered Diffusionmentioning

confidence: 81%

Section: Brownian Simulationmentioning

confidence: 90%

Section: Near-wall Hindered Diffusionmentioning

confidence: 99%