What we can do?

Our laboratory can quantitatively analyze "particle size distribution," "elasticity/viscosity," and "surface properties" of "microparticles dispersed in liquid" using ultrasonic methods. Since ultrasonic waves are used, it can be applied to "materials through which light does not transmit," and in addition, it is characterized by a very wide range of particle diameters from "several tens of nanometers" to several hundred "micrometers. Although there are commercially available ultrasonic devices, they differ in the following respects.

- In commercial devices, the structure and morphology of particles are limited to rigid spheres. This limitation is due to the analytical theory model incorporated in the device.

- Although commercial instruments can provide particle size distributions, they cannot perform elastic modulus analysis.

- In addition, our laboratory can analyze "hollow particles" and "core-shell particles. We also analyze the properties of "Pickering emulsions" and "Particles coated with polymers”.

We often hear the complaint that "the results obtained with commercially available equipment are completely different from the expected particle size distribution. Our laboratory clarifies the structure, characteristics, and dispersion state of samples that were thought to be impossible with commercially available equipment.

The system is used in various fields, including pharmaceutical particles, electrode slurries for fuel cells, metal pastes, paints, cosmetics, semiconductor particles, and functional polymer particles.

Difference from existing technology: Ultrasonic spectroscopy (US) method

In the so-called attenuation and transmission methods, the size distribution of particles is evaluated by measuring the attenuation (viscosity and scattering) due to the particles. Since the ultrasonic contrast (the factor that generates the signal) is the difference in elasticity (or more precisely, the difference in compressibility and density) between the microparticles and the surrounding liquid, ultrasonic experiments can determine "particle rigidity" and the "viscoelasticity of the suspension". Commercial devices are based on the assumption that the rigidity of the particles is known, and only the particle size distribution can be calculated. Pickering emulsions can be analyzed using the US method.

Difference from existing technology: Dynamic Ultrasound Scattering (DSS) method

This is an ultrasonic version of the Dynamic Light Scattering (DLS) method. When ultrasonic waves are impinged on microparticles in a liquid, a scattered signal is detected. Since smaller particles diffuse faster and larger particles diffuse more slowly, the rate of relaxation gives the size of the particles. It is similar to DLS in that it makes full use of the correlation function method. It was invented by Prof. John Page in Canada. At that time, the ultrasonic frequency used was low, and the particles studied were about "millimeters" or slightly smaller.

At our university, we have created a new technique that can detect particles "a few micrometers" in diameter (now it is possible to detect particles down to 10 nm, three orders of magnitude smaller). The advantages and disadvantages are as follows

Disadvantages of the DSS method

1. For particles as small as a few nanometers, the signal intensity is very small. If the sample is transparent, we recommend the commercial DLS method over our DSS method.

2. For nanoparticles, DLS is more sensitive for very low concentration samples (0.01% or less).

Advantages of the DSS method

1. In principle, nanoparticle concentrated systems can be measured without dilution (detailed studies of physical properties are still in progress).

2. It is possible to measure particle sizes larger than submicron (which is difficult with light scattering). If the sample is optically transparent, the DLS can measure sizes larger than 100 nm, but it requires two complex correction terms (structure factor and hydrodynamic factor), and these values are quite different under different spatial (scattering vector) conditions, making a correct evaluation very difficult. For electrostatically neutral spherical particles, these effects more or less cancel each other out and may be relatively unproblematic in the low concentration range. In contrast, the wavelength of ultrasound is about 100 times longer than that of visible light, making it relatively easy to analyze.

3. Multiple scattering of light is not a problem for us. Even if multiple scattering of ultrasound occurs in a concentrated system, single scattering can be analyzed by analyzing only the area in front of the sample. Since ultrasonic waves analyze the phase, it is easy to analyze only the area in front of the sample without using a large apparatus such as an interferometer.

4. Analysis is possible even if the wavelength of the beam is not known (FD-DSS method). In addition, since a broadband sensor is used, various wavelength information can be analyzed in a single acquisition. In the case of DLS, data for different scattering vectors must be obtained by varying the scattering angle.

5. Discriminate between nanoparticles and submicron particles is possible. This is because single nanoparticles exhibit Brownian motion and aggregates exhibit ballistic motion (hydrodynamic velocity fluctuations) accompanying long-range hydrodynamic interactions. Diffusive motion and velocity fluctuations have different mechanisms as well as different relaxation times. This can be distinguished by the shape of the spectrum, so that in addition to quantitative relaxation time evaluation, the analysis of the mode of motion can reveal the properties of the sample.

6. The beam can be set up in various orientations (top, bottom, horizontal, etc.) to suit sedimenting or diffusing particles, making it flexible in application.

7. The ability to view the size range from tens of nanometers to hundreds of micrometers over four orders of magnitude allows us to understand the hierarchical dynamics of primary nanoparticles, secondary aggregates, tertiary soft agglomerates, and so on.

8. We are developing a software system using multi-cores instead of relying on hardware correlator, so we can prepare each application according to our needs and update it at will. This is only possible in the age of digital society.

Polymer Molecular Engineering Laboratory
Department of Macromolecular Science and Engineering, Graduate School of Science & Technology,
Kyoto Institute of Technology,
Matsugasaki, Sakyo-ku, Kyoto 606-8585, JAPAN