Aggregation of particles is associated quite often with decay of the particles zeta potential. Electrostatic repulsion is an important component preventing particles aggregation. This repulsion gets weaker when particles zeta potential and, consequently, surface electric charge decreases. That is why simultaneous characterization of the particle size and particle zeta potential is so important.

Acid/base titration is the normal way to vary surface charge because it is sensitive usually to pH value. Figure 10 shows titration zeta potential curve measured for concentrated rutile dispersion ( 7%vl). It is seen that isoelecric point of this rutile is about pH=4. According to the general principles we should expect appearance of the large aggregates in the vicinity of pH=4.

Evolution of the simultaneously measured particle size distribution is shown on Figure 11.

Initial median particle size of the stable rutile dispersion at pH=8.9 is about 0.3 microns. This size correlate very well with independent Sedigraph data. Particle size distribution becomes bimodal indeed when pH because large aggregates with size of several microns appears.

It is interesting that the both acoustic and electroacoustic measurements indicate second instability zone at high pH around 12. We believe that instability zone is related to the higher ionic strength. Electroacoustics alone would not be sufficient because zeta potential drops only about 10 mV. It looks like this small change of the zeta potential reflects much larger changes in the DL structure at the high pH.

This titration experiment with rutile dispersion shows that combined acoustic and electroacoustic spectroscopy is capable to characterize the both types of aggregation instability associated with electric surface charge. The first instability zone near pH=4 is caused by surface charge neutralization, whereas the second instability zone near pH=12 is related to the double layer suppression by increasing ionic strength and probably some changes in DL structure.  

System containing two or more chemically different dispersed phases is referred to 
as "mixed dispersed system". Acoustic spectroscopy is capable to characterize particle size distribution in such a system. This capability is related to the new acoustic theory which 
does not use superposition assumption.

In order to verify this capability of acoustic spectroscopy we measured 5%vl alumina and 
zirconia dispersions separately and mixed together with 1:1 ratio. Corresponding experimental
and best fit theoretical attenuation spectra are shown on Figure 12. It is seen that all three attenuation spectra are very different. Theory provides a good fit to the experimental data.

Particle sizes corresponding to theoretical curves on the Figure 12 are shown on Figure 13. It is seen that calculated particle size distributions for each component of mixed system almost coincides with particle size calculated for individual system. The heights of the zirconia histograms are different because these particles present only 50% of total volume in the mixed system. The same reason explains the difference of the alumina PSD heights.

There are also some differences in the distribution width, especially for AA-2. We attribute 
these discrepancy to the restriction on the mode width of the bimodal distribution. This 
instrument software assumes the same width of the bimodal distribution modes. Particle size distribution of zirconia is wider than that of the alumina AA-2 as it is shown on Figure 13 for individual systems. Artificially chosen equality of these distributions width for bimodal alumina-zirconia mixture might cause some interpretation problems.

Ceramics is one of the most important and effective applications of acoustic spectroscopy. The density of ceramic powders are usually much higher than density of dispersion medium. As a result the mechanisms of viscous and scattering losses are dominant. Theories of these two mechanisms are completed and successfully tested experimentally. Reliability of characterization is consequently quite high.

Acoustic spectroscopy is capable to characterize particle size distribution of individual materials as well as of their mixtures. Results of mixed dispersed system characterization are presented in the section Characterization of mixed dispersed systems. This section is devoted to the individual materials.

Table 2

Corresponding particle size distributions are presented on Figure 17. There is an opportunity to compare these results with independent data provided by the manufacturers of these materials. For instance, zirconia on Figure 18 and Figure 17 is TZ-3YS produced by Toso Corp.. Expected particle size is 0.3 micron whereas acoustics gives 0.26 micron. Silicon nitrate on Figure 18 and Figure 17 is E-03 produced by Ube Industries with expected particle size 1 micron. Acoustics measures 1.1 micron.

There is no particle size distribution for silica geltech on Figure 17. This material is widely used as a test monodisperse system with particle size 1 micron. Acoustics gives also monodisperse particle size distribution which can not be plotted as a result. Particle size is a little bit different than claimed by Geltech. It is 1.1 micron.

One of the positive features of acoustic spectroscopy is its independence on electric properties. That’s why, for instance, acoustics is capable to characterize conducting silicon particles as it is shown on Figure 18 and Figure 17. Electroacoustics is not suitable for these purposes because there is no electroacoustic theory for conducting particles.

This dispersed system is certainly unstable. Particles aggregate and particle size distribution becomes bimodal. There is no lognormal PSD which produces attenuation spectra matching experimental data as it is shown on Figure 20. Attenuation spectra calculated for bimodal PSD shown on Figure 19 fits experimental data much better than attenuation spectra calculated for the best lognormal distribution. Error of theoretical fit is 16.1% for lognormal distribution and only 5.5% for bimodal distribution.

It is interesting to mention that reported size for alumina AKP-30 is 0.3 micron. The median size of the smaller mode on Figure 19 is very close to the reported value. It is 0.28 micron.

It looks like about 70% of the alumina particles coagulate in the nickel-plating bath and build aggregates with about 2 microns median size. 

Particle size characterization of small nanosize particles is very challenging because sound attenuation caused by these particles is comparable with attenuation of water. Nevertheless we have measured successfully many systems with particle size below 100 nanometers. Three examples are listed below:

1.The smallest particle size we have measured so far was 12 nanometers anataze in the 10%vl dispersion.

2. Silica Ludox with 30 nanometers particles is our calibration colloid for electroacoustic spectrometer. Particle size measurement of this silica using acoustic spectrometer gives sizes 30 ± 3 nanometers even at 40%vl.

3. We have done many measurements with chemical-polishing materials (see Chemical-polishing materials, alumina with additions, silica with additions).

Characterization of zeta potential of nanosize particles is simpler than for particles with micron size. There is no need in particle size correction. Electroacoustic spectrometer measures basically Smoluchowski electrophoretic mobility. 

The size of the microemulsion droplets should be measured for intact concentrated system. Dilution may destroy original droplet size. As a result traditional techniques based on light scattering are not suitable for microemulsions characterization. Electroacoustics is also not applicable for these systems because of the small size of droplets and low density contrast.

There is one new method which gives an opportunity to get some information about at least mean droplet size. This method explores neutron scattering. It is very expansive and complex.

Acoustic spectroscopy is much cheaper and much easier in use. In addition, acoustic spectroscopy is capable to characterize not only mean size but PSD width as well.

Characterization of the latex particle size distribution requires information of thermodynamic properties of latex. It makes this application more complicated than the other ones with high density contrast and rigid particles. Nevertheless, Acoustics is a very informative tool for latecies. Thermodynamic properties are known already in many cases. One of the examples is given above in the section Acoustic Spectroscopy, Experimental Test where neoprene latex is used as a test for theory of thermal losses.

We have done many experiments with silica in ethanol and methanol. Particle size agreed perfectly with expected data provided by either manufacturer or other techniques.

We don’t have enough experience with other liquids. We have required properties of more than 90 liquids in our database. We are looking for potential customers for developing further this application.