The particle size distribution of the disperse phase is a key parameter for estimating the process or further processing properties of liquid dispersions. In principle, a large number of different analysis methods are available to determine this parameter. Depending on the application and material, however, the methods require special sample preparation, as they can often only be used in diluted systems – e.g. in the case of optical methods. This influences the particle size distribution of the sample due to the change in electrochemical properties. Microscopic methods (SEM, TEM), on the other hand, only represent a very small fraction of the entire sample quantity and are therefore hardly representative!
In order to characterize such concentrated suspensions or emulsions, such as those used in ceramic, construction materials or battery industries regarding particle size distribution, a method is required that can macroscopically analyze the dispersions in their original state. This is achieved by acoustic spectrometry [1, 4]: the particle size is determined in proportion to mass by measuring the attenuation of ultrasonic waves in concentrated dispersions with particle sizes ranging from the nanometer to the upper micrometer range.
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Particle size 1nm – 1000 µm (ISO 20998-1) | Particle size 1nm – 1000 µm (ISO 20998-1) |
The principle of acoustic spectrometry or ultrasound attenuation spectrometry is shown in the following figure:
It is a transmission method, the ultrasonic transmitter and detector are positioned opposite each other (180°), with the sample in its original state to be characterized between them. Sedimenting or creaming dispersions can be pumped or stirred, stable systems can be measured without movement. Short wave pulses (bursts, intensity IIn) are coupled directly into the dispersion and attenuated as they pass through the sample; the attenuated waves are detected (intensity IOut). During the measurement, the frequency of the pulses is varied between 1 and 100 MHz and a complete attenuation spectrum is measured. In addition, the distance between the US transmitter and detector is varied (maximum 0.15 – 20 mm) in order to obtain significantly more stable spectra and to cover a maximum wide sample concentration range (0.1 – 60 vol.%).
The following first approximation applies to the acoustic attenuation:
F is the ultrasound wave frequency and L is the distance between the transmitter and detector. The measured, resulting attenuation spectrum is directly related to the particle size distribution of the disperse phase. The ultrasonic absorption mechanisms of the particles are:
Sound scattering only plays a role for particles > approx. 5-7 µm. Dissipative mechanisms are loss mechanisms of smaller particles, whereby the visco-inertial effect in hard materials, a type of heat loss due to friction at the particle-medium interface, is the dominant mechanism [2]. In emulsions, on the other hand, the energy loss is due to a thermal effect caused by the different thermal expansion of the continuous and emulsified phases [2]. In paste-like, often binder-containing dispersions, in addition to the mechanisms described above, there can also be structural effects that attenuate the transmitting wave [2]. Viscous losses play a particularly important role here, rather than the elastic components.
During the analysis by the DT device software, the measured sound attenuation spectrum is fitted on the basis of the implemented theories on the attenuation effects mentioned above [2,4] and the particle size distribution is calculated. The following example shows the measurement of an aqueous, 12 wt% aluminum oxide suspension at different pH values (left: acoustic attenuation spectra, right: calculated particle size distribution by fitting on the basis of the visco-inertial effect, [3]).
/1/ ISO 20998-1: Measurement and characterization of particles by acoustic methods.
/2/ A. Dukhin, P. Goetz: Characterization of Liquids, Nano- and Microparticulates, and Porous Bodies using Ultrasound. 2nd Edition. Oxford: Elsevier, 2010.
/3/3P APPNOTE 6-01: acoustic in battery research
/4/ Brochure DT Series: Particle size and zeta potential using acoustic and electroacoustic Spectrometry
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