Topics in Particle and Dispersion Science

Fibers and small particles contained in paper pulp have quite dissimilar optical and acoustical properties. Fibers, which have diameters on the order of several micrometers and the length of up to several millimeters, produce forward scattering of light, i.e. have a strongly asymmetric optical scattering function and exhibit significant acoustic attenuation in the MHz frequency range. In contrast, small particles, up to several micrometers in size, have a less asymmetric optical scattering function and also lower acoustic attenuation. The paper pulp absorbs light weakly at a wevelength of 1064 nm chosen by the authors. However, due to a high concentration of particles, it exhibits significant multiple scattering of light whose spatial distribution depends both on the concentration and optical properties of the dispersion particles.

These properties of the paper pulp have been exploited in an application of the optoacoustic method of light absorption measurement for the determination of relative contents of fibers and small particles by Zhao et al (2006). In an attempt to merge optical and acoustical measurements with just one detector they used two optical absorbers in both optical and acoustic contact with the dispersion as the optoacoustic converters and a single acoustic detector. The absorbers were placed symmetrically about the incident light beam axis, with one absober in acoustic contact with the acoustic detector.

Following illumination of the dispersion with a narrow pulsed laser beam, the acoustic detector receives three acoustic signals in succession. The first signal is generated by multiply scattered light absorbed by the light absorber in contact with the detector. The second signal is generated by the absorption of light at the beam location, and the third signal is generated by the light absorber at the other side of the light beam. The amplitude ratio of the 1st and 2nd signal is related, albeit in a complex way, to the absorption coefficient of light by the dispersion, the asymmetry of the optical scattering function, as well as to extent of multiple scattering of light. The amplitude ratio of the 1st and 3rd signals gives an estimate of the acoustic absorption of the dispersion. These two ratios depend ultimately on the relative contents of fibers and small particles in the pulp, which enabled Zhao et al (2006) to estimate the relative contents of fibres and small particles in the studied paper pulp samples.