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Journal of the Optical Society of America B

Journal of the Optical Society of America B


  • Vol. 7, Iss. 1 — Jan. 1, 1990
  • pp: 15–20

Diffusing-wave spectroscopy in a shear flow

X-L. Wu, D. J. Pine, P. M. Chaikin, J. S. Huang, and D. A. Weitz  »View Author Affiliations

JOSA B, Vol. 7, Issue 1, pp. 15-20 (1990)

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We present a new technique for measuring velocity gradients for laminar shear flow, using dynamic light scattering in the strongly multiple-scattering regime. We derive temporal autocorrelation functions for multiply scattered light, taking into account particle displacements arising from deterministic shear flow and random Brownian motion. The laminar shear flow and Brownian motion are characterized by the relaxation rates τS−1 = Γ¯k0l/√30 and τB−1 = Dk02, respectively, where Γ¯ is the mean shear rate of the scatterers, k0 = 2πn/λ is the wave number in the scattering medium, l is the transport mean free path of the photons, and D is the diffusion coefficient of the scatterers. We obtain excellent agreement between theory and experiment over a wide range of shear rates, 0.5 sec-1 < Γ¯ < 200 sec−1. In addition, the autocorrelation function for forward scattering is independent of the scattering properties of the medium and depends only on the mean shear rate and sample thickness when τs is much less than τB. Thus the mean shear rate can be simply determined by a single measurement.

© 1990 Optical Society of America

X-L. Wu, D. J. Pine, P. M. Chaikin, J. S. Huang, and D. A. Weitz, "Diffusing-wave spectroscopy in a shear flow," J. Opt. Soc. Am. B 7, 15-20 (1990)

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  10. The correction to Brownian motion that is due to the convective flow is called Taylor dispersion. Taylor dispersion modifies particle diffusion in the direction of velocity gradient. In the entire range of shear rate in this experiment the correction (Γτ)2/3 is much smaller than 1. This justifies our approximation that the particle diffusion and convective shear are decoupled.
  11. For some scattering geometries there will be an additional term in the sum corresponding to the difference between the input and output wave vectors. This term is proportional to the velocity (rather than to the velocity gradient) and does not contribute to the homodyne correlation function. We also note that for the common case that the flow direction is perpendicular to the input and output wave vectors, this term is identically zero.
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  20. We note that the average intensity of transmitted light, (I), through the sample did not vary with Γ¯. Since 〈I〉 ˜l/L, we conclude that l, and hence P(s), does not vary with Γ¯ for our samples (see Refs. 6 and 8). This also suggests that S(q) is essentially independent of Γ for the weakly interacting samples used in this study.
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