Ultrasonic diffraction grating spectroscopy and the measurement of particle size

The results of measurements using ultrasonic diffraction grating spectroscopy (UDGS) are found to be dependent upon the particle size of the slurry. This illustrates the emergence of a new technique for measuring particle size. Theoretical calculations are underway to describe and predict them as well. The ultrasonic grating surface is in contact with the slurry. The diffraction grating was formed by machining triangular grooves with a periodicity of 483 microm on the flat surface of an aluminum unit with send and receive transducers fastened to it. For this experimental configuration, the ultrasonic beam strikes the back of the grating at an incident angle of 30 degrees and produces a transmitted beam of spectral order m=1 in the slurry. The angle of this transmitted beam changes with frequency and, at the critical frequency of 3.47 MHz, it is located at an angle of 90 degrees. The receive transducer measures the reflected m=0 beam at an angle of 30 degrees. At a frequency slightly less than the critical frequency, the m=1 transmitted beam no longer exists and its energy is shared with all other beams. Therefore, due to energy conservation, the signal in the receive transducer exhibits a peak at the critical frequency. During the transition, the wave interacts with the particles of the slurry and ultrasound amplitude is reduced. Therefore, the peak observed by the receive transducer is reduced, compared to that for water. Data have been obtained for slurries of polystyrene spheres, ranging in size from 215 microm to 463 microm, and for weight percentages of 1-12%. Polystyrene spheres of different diameter show differing amounts of energy loss and thus, particle size identification is possible.