Electrooptic effects in dilute suspensions of bacterial cells and fractal aggregates

The relative change in optical density (orientational turbidimetric effect) of dilute suspensions of bacterial cells is studied theoretically and expe rimentally. Optical properties of bacterial cells are modeled by homogeneou s spheroids using the T-matrix method. Calculated differential spectra of t he steady-state turbidimetric effect, as well as its electric field depende nce at fixed wavelengths, are shown to be in good agreement with experiment al measurements for dilute suspensions of Escherichia coli and Bacillus pol ymyxa cells. The theoretically predicted unusual non-exponential decay of o ptical density (after removal of the orienting field) has been illustrated by an experimental electrooptical example. By using the Rayleigh-Debye-Gans approximation, we have also computed orientational effects for dilute susp ensions of 3D-lattice ballistic and diffusion-limited fractal aggregates bu ilt from optically soft small spheres. We have found that the turbidimetric effect is determined solely by the gyration radius diffraction parameter a nd is essentially independent of the cluster mass. In terms of orientationa l effects, the effective structural anisotropy of the random fractal aggreg ates is close to the anisotropy of the homogeneous spheroidal model with th e axial ratio 2 for small clusters and reduces to 1.5 for larger clusters. (C) 1999 Elsevier Science Ltd. All rights reserved.