Backward-mode photoacoustic transducer for sensing optical scattering and ultrasonic attenuation: determining fraction consistencies in pulp suspensions

Abstract

An innovative backward-mode photoacoustic transducer was developed, consisting of an optical fibre, a composite absorber, piezoelectric film and high impedance preamplifier. By receiving scattering light from a turbid suspension, the transducer produces a photoacoustic source in it. This source emits two photoacoustic waves travelling in opposite directions. The waves' amplitudes relate to the optical scattering properties of the suspension, and the echo of a wave returning from the suspension carries information of acoustic attenuation. By assessing the optical scattering and acoustic attenuation, fraction consistencies in a two-fractional suspension can be determined if one fraction dominantly scatters light and the other mainly attenuates ultrasound. This technique is used in this paper to investigate paper pulp suspensions. Pulp consists of wood celluloses and wood fines (or extra-added fillers in some cases), where cellulose lengths range from a few sub-millimetres to millimetres and fines/filler sizes are a few tens of micrometres or smaller. Due to their different size and shape, celluloses and fines (or fillers) have different optical scattering and acoustic attenuation properties. Experimental results showed that the transducer can measure pulp consistency with good linearity at least in the range from 0.5% to 3%, and that it can distinguish pulp cellulose from fines or fillers (TiO2 particles). Needless to say, this technique is also suitable for determining other suspensions in the food, pharmaceutical and mineral industries.