Resonant multiple scattering of light

The detection of ballistic and diffuse light in confocal and heterodyne imaging systems in transillumination is studied experimentally and theoretically. We find an optimum pinhole size for ballistic light detection and diffuse light rejection for confocal imaging. The ratio of ballistic and diffuse light is found to be determined primarily by sample parameters and aberrations introduced by the sample. For sample and illumination characteristics that are typical for biomedical imaging, the limits of ballistic light detection in confocal imaging are close to the noise limits of standard detectors. Heterodyne detection with narrow-bandwidth light can extend these limits, depending on the spatial and the temporal coherence properties of the transmitted scattered light.

Ballistic and diffuse light detection in confocal and heterodyne imaging systems

We report an experimental study of the surface flow and the creeping flow of glass spheres on a heap. We characterize the dynamics of the flow with particle tracking velocimetry and dynamic light scattering measurements. We observe a creeping flow with a dynamics which slows down exponentially with the depth. The characteristic distance for the decay is one bead diameter. A striking observation is that the exponential decay for the mean velocity holds over more than six orders of magnitude.

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Experimental study of a creeping granular flow at very low velocity

We are interested in the propagation of light in a random packing of dielectric spheres within the geometrical optics approximation. Numerical simulations are performed using a ray tracing algorithm. The effective refractive indexes and the transport mean free path are computed for different refractive indexes of spheres and intersticial media. The variations of the optical path length under small deformations of the spheres assembly are also computed and compared to the results of Diffusive Wave Spectroscopy experiments. Finally, we propose a measure of the transport mean free path and a Diffusive Wave Spectroscopy experiment on a packing of glass spheres. The results of those experiments agree with the predictions of this ray tracing approach.

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Diffusive wave spectroscopy of a random close packing of spheres.

The mean square of particle velocity fluctuations, \(\delta v^{2}\) , which is directly related to the so-called granular temperature, plays an important role in the flow, mixing, segregation and attrition phenomena of particulate systems and associated theories. It is, therefore, important to be able to measure this quantity. We report here in detail our use of diffusing wave spectroscopy (DWS) to measure the mean square particle velocity fluctuations for a 2D non-circulating gas fluidized bed of hollow glass particles whose mean diameter and effective density are 60 \(\upmu\) m and 200 kg/m3, respectively. Mean square particle velocity fluctuations were observed to increase with superficial velocity, Us, beyond the minimum fluidization velocity. Following the uniform fluidization theory of Batchelor (1988), the function \(f{\left(\phi \right)}\) in the expression \(\delta v^{2} = f{\left(\phi \right)}U^{2}_{\rm s}\) was also determined and shown to increase from zero at a solids loading of \(\phi \approx 0.33\) to a maximum at \(\phi \approx 0.4\) before decreasing again to zero at \(\phi \approx 0.53\) . The spatial variation of the mean square particle velocity fluctuations was also determined and shown to be approximately symmetrical about the centreline where it is also maximal, and to increase with height above the distributor.

Granular temperature in a gas fluidized bed

We have performed a series of measurements of transmission and surface intensity profiles vs. thickness in which the coherent depth of penetration and the surface reflectivities are varied independently. The excellent agreement of diffusion theory which includes interfacial coupling in a natural way allows us to determine the domain of validity of the diffusion model and to obtain the first accurate values for the coherent penetration depth and the optical transport mean free path.

T. D. Cheung

Papers

Transmission and Surface Intensity Profiles in Random Media