Fiber characterization in a stationary ultrasonic field

An instrument has been developed to characterize the mean dimensions of softwood fiber samples. It is based on the phenomenon that 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 when subjected to a stationary ultrasonic field. 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 excitation is monitored with a collimated beam of light. Scattered light signals collected off-axis in the plane perpendicular to the acoustic nodal planes are shown to be a function of the weighted average fiber length. Results are presented for pulp samples in the average fiber length range of 0.2 to 3 mm. It was found that there is a region where the scattered light is linearly related to concentration. Acoustooptical measurements obtained at initial concentration in this linear region, for all fractions, have shown that the longer the average length from screen classifier is, the faster the layer formation is. Since the fiber length the radius are proportional for a wood species, this observation is in agreement with the theoretical prediction.