Particle characterisation in highly concentrated dispersions using ultrasonic backscattering method
Highlights
► A measurement setup for gathering ultrasonic reflection signals from dispersions is presented. ► Analysing the reflection or backscattering signals indicates a high sensitivity against particle size and concentration. ► We propose a semi-empirical approach to describe the influence of particle properties on backscattering behaviour. ► The absence of a measuring gap increases the potential inline/online capability of this measurement method.
Introduction
Nowadays there is a large variety of measurement techniques for the determination of particle size and concentration in suspensions and emulsions. Despite this, only few techniques facilitate the characterisation of highly concentrated dispersions, e.g. during process monitoring. Optical detection principles can usually not be employed in this case, because of the dispersions’s opacity. Using ultrasound can overcome this limitation. Moreover, ultrasound methods in general are capable to deal with field applications where sampling and preparation (i.e. dilution) should be avoided.
A well known technique, the ultrasonic attenuation spectroscopy, uses a transmission arrangement for measuring the portion of sound which is extinguished by the dispersion. Two acoustic transducers are arranged face to face with a gap between them (Fig. 1a). This gap, which contains the medium to measure, mostly can be adjusted to ensure a minimum signal to noise ratio for the transmission signal. The frequency dependent attenuation furthermore is used to determine particle size distribution, adapting the measured attenuation to the values predicted by physical model [1]. In case of pulse shaped excitation with properly adjusted time between pulses, transmission method allows a complete elimination of multiple scattering. Sound waves that originate from multiple scattering (in sense of redirection of sound waves) can be separated from the pulse signal that directly penetrates the medium, because multiply scattered waves cover a longer travelling distance and are thus received with a time delay [2].
For strongly attenuating media, like highly concentrated dispersions, the measuring gap has to be concentrated to a few millimetres. Under these circumstances, single coarse particles (e.g. foreign objects) or highly viscous media can plug the gap and reduce the inline capability of this method.
Beside the transmitted sound waves, which are evaluated within the attenuation spectroscopy, the ultrasonic waves scattered by the particles also contain information about the dispersion [3]. Taking advantage of this ‘particle-born’ sound signal, it is possible to arrange an ‘open’ measurement setup in reflection mode without any gap (Fig. 1b). In medical application this approach is used since many years to non-invasively characterise biological tissue [4]. Also, in non-destructive material testing (NDT) ultrasound reflection measurements are used to detect defects or cracks within a material matrix [5]. Although the detection and analysis of scattered or reflected ultrasound exhibit the advantages mentioned above, this method is not established within particle analysis yet. On the one hand, measuring the acoustic amplitude or power scattered by small particles (in micrometre range and below) is quite difficult because of the weak scattering at particles that are small compared to the wavelength of ultrasound in the lower megahertz range, which applies to all particles smaller 50 μm (especially submicron) at frequencies below 10 MHz. In addition, the interpretation of the measured scattering signals, based on models for sound propagation in dispersive systems, is challenging because of the physical complexity due to effects of multiple scattering [6]. In contrast to a transmission measurement, there is no possibility to eliminate the multiple scattering contribution from backscattering signals, as the travelling distance of the wave is not well defined. Hence, multiple scattering needs to be accounted for in signal analysis/interpretation. For several material mixtures like blood (red blood cells in plasma) [7], ultrasonic contrast agents [8] or marine sediments [9] results of ultrasonic backscattering measurements as well as algorithms for determining particle size and/or particle concentration can be found. Most of these applications are either limited to very small particles (compared to the wavelength) or low particle concentrations where weak scattering is assumed and multiple scattering effects can be neglected [3], [4].
This paper introduces a novel acoustic technique for the characterisation of concentrated dispersions, which uses an ultrasonic reflection arrangement to gather the sound waves which are reflected, respectively backscattered from the particles dependent on the time of travel (i.e. dependent on the penetration depth). The corresponding backscattering signal contains information about the scattering strength and even about the sound attenuation. Experimental data, presented within the paper, show a high sensitivity of both acoustic parameters to particle size and concentration. Sound attenuation, extracted from reflection measurements, is compared to values gathered with an ultrasonic attenuation spectrometer.
The potential benefit of ultrasonic backscattering measurements for particle characterisation is related to the existence of a transfer function relating acoustic (scattering amplitude, attenuation) to dispersive (particle size, concentration) parameters. As already mentioned, complete physically modelling of sound propagation in high concentrated dispersion is very complex. Alternatively, a semi-empirical approach is presented, which separately describes the effects of particle size and particle concentration on the scattering behaviour of the dispersion. The spectral decomposition of the backscattering signal furthermore gives information about the particle size, especially about the width of size distribution.
Section snippets
Scattering of ultrasound by single particles
Unlike the propagation of ultrasound in continuous media, sound propagation in dispersions coincides with sound scattering due to the presence of particles. In general, scattering describes a redirection of the sound wave incident on an obstacle (e.g. particle). The scattering behaviour of a particle is essentially determined by the size-to-wavelength ratio (between particle size x and sound wavelength , respectively sound frequency f), expressed in form of the dimensionless wave number:
Description of experiments
An ultrasonic reflection setup was build up for gathering ultrasound waves backscattered from particles dispersed in liquid media (Fig. 5). Using pulse shaped excitation, the transducer emits an ultrasound wave with a duration according the transducer’s bandwidth. Pulser and receiver are integrated into one device (UT340, UTEX Scientific Instruments, Inc.) additional containing a receiving amplifier. Furthermore, a digital oscilloscope (ZT410, ZTEC Instruments, Inc.) is used for capturing
Results
The main aim of the experiments is a validation of the ultrasonic backscattering probe, focussing on the sensitivity of backscattering parameters (cf. Eq. (5)) to changes in particle size and concentration. First of all, it can be noticed, that the backscattering signals, captured for the different suspension samples, are evaluable the way described before (Section 3). Especially for small particles (low scattering amplitude) and high concentrations (high sound attenuation) the signal-to-noise
Conclusion and perspectives
A novel method for characterising highly concentrated dispersions by means of ultrasonic backscattering is proposed. Analysing the reflected, respectively backscattered acoustic signal yields two measurement variables, that are sensitive against varying particle size and concentration. Experimental proof of concept, regarding signal capturing and interpretation, is given. Good agreement is found between time dependent decay of backscattering signal and attenuation, gathered with a conventional
Acknowledgements
The authors would like to thank for the financial support on the part of AiF (Grant Number 16681 BR/2). The research project 16681 BR/2 of the research association Forschungsgesellschaft für Messtechnik, Sensorik und Medizintechnik e.V. Dresden was sponsored by the AiF in context of the program for supporting the industrial research and development founded by the Federal Ministry of Economics and Technology, based on a resolution of the German Parliament.
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