Accuracy of RGD approximation for computing light scattering properties of diffusing and motile bacteria

Sensitivity study on the contribution of scattering by randomly oriented nonspherical hydrosols to linear polarization in clear to semi-turbid shallow waters

The influence of hydrosol nonsphericity on the polarization characteristics of light under water is investigated by combining accurate single-scattering models for randomly oriented spheroidal scatterers with a radiative transfer model that employs Stokes formalism and considers refraction of direct unpolarized solar radiation and 100% linearly polarized radiation at the air-water interface followed by single scattering. Variations in what we call the "linear polarization phase function" (the degree of linear polarization as a function of scattering angle and the angle of linear polarization as a function of scattering angle) are examined for a wide range of spheroid aspect ratios and complex refractive indices of hydrosols. Implications for polarization-sensitive marine organisms and for remote sensing of the marine environment are discussed.

Dark field differential dynamic microscopy enables accurate characterization of the roto-translational dynamics of bacteria and colloidal clusters

Micro- and nanoscale objects with anisotropic shape are key components of a variety of biological systems and inert complex materials, and represent fundamental building blocks of novel self-assembly strategies. The time scale of their thermal motion is set by their translational and rotational diffusion coefficients, whose measurement may become difficult for relatively large particles with small optical contrast. Here we show that dark field differential dynamic microscopy is the ideal tool for probing the roto-translational Brownian motion of anisotropic shaped particles. We demonstrate our approach by successful application to aqueous dispersions of non-motile bacteria and of colloidal aggregates of spherical particles.

Approximation to extinction efficiency for randomly oriented spheroids

A semiempirical approximation to the extinction efficiency Q(ext) for randomly oriented spheroids, based on an extension of the anomalous diffraction formula, is given and compared to the extended boundary condition method or T-matrix method. Using this formula, Q(ext) can be evaluated over 10(4) times faster than by previous methods. This approximation has been verified for complex refractive indices m = n - ik, where 1.01 </= n >/= 2.00 and 0 </= k >/= 1 and aspect ratios from 0.5 to 4. We believe the approximation is uniformly valid over all size parameters and aspect ratios. It has the correct Rayleigh and large particle asymptotic behavior. The accuracy and limitations to this formula are extensively discussed.

Determination of motile behaviour of prokaryotic and eukaryotic cells by quasi-elastic light scattering

Information on the swimming motion of microscopic calls and the factors which affect it is important to a wide range of disciplines. The functions of the motile apparatus of simpler organisms such as bacteria or algae can provide useful clues on the operation of more complex contractile systems such as muscles. Often celluar motility is a response to conditions external to the cell (e.g. chemotaxis) and, hence, can lead to increased understanding of the cell's sensory capability. On the medical front there is a physical similarity between the flagellar beat of the motile spermatozoa and the ciliary activity of epithelial lining of respiratory and reproductive tracts. Impairment of the motile apparatus can lead to sterility and to a variety of pathological conditions. In animal husbandry, in the artificial insemination industry and in sperm banking estimates of the extent of cellular motility and the fraction of cells which are motile are key quantities which can determmine the fertilization capability of a semen sample.

Light scattering by randomly oriented spheroidal particles

Light scattering properties of an assembly of randomly oriented, identical spheroidal particles are studied. A computation scheme has been developed to integrate the solution of Asano and Yamamoto for scattering from a homogeneous spheroid over all the particle orientations. The extinction and scattering cross sections, asymmetry factor, and scattering matrix elements are calculated for randomly oriented prolate and oblate spheroids and compared with both calculations for spheres and laboratory measurements, The scattering cross section, single scattering albedo, and asymmetry factor of spheroids tend to be larger than those for spheres of the same volume. The normalized scattering matrix has a symmetrical form with six independent elements. The angular scattering behavior of spheroids is found to be greatly different from that of spheres for side scattering to backscattering directions. In general, prolate and oblate spheroids of the same shape parameter have similar angular scattering patterns. The angular distribution of scattered intensity is characterized by strong forward scattering and weak backscattering. The linear polarization tends to be positive at intermediate scattering angles. The linear polarization and depolarization are discussed in application to scattering in the earth and planetary atmospheres.