Abstract
When subjected to a stationary ultrasonic field, particles of cylindrical shape diluted in water and shorter than one-fourth of the acoustic wavelength migrate to nodal planes of acoustic radiation pressure and reorient parallel to these planes. Based on this phenomenon, a method was developed to characterize the mean dimensions of softwood fiber samples. The aim is to develop an acoustooptical method suitable for online measurements of pulp fiber dimensions in a paper mill. As the resonator operating frequency is 72 kHz, fibers up to 5 mm in length can be measured. The time evolution of the fiber suspension during ultrasonic excitations is monitored with a collimated beam of light. Scattered light signals collected off-axis are functions of the mean fiber dimensions. Results are presented for fiber samples in the mean length range of 0.2 to 3 mm. >
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