(24e) Characterization of Particle Size and Shape with a Novel 3D Light Scattering Sensor (3D-LSS) for Aerosols

In the submicron regime particles gain a variety of interesting properties, not only by their size, but also by their structure. Therefore, these particles become nowadays a wide field of applications in pharmaceutical, optical and electronic industry [1,2].

In order to control a production process for this kind of particles properly, the particle size as a one-dimensional quantity is not sufficient. To ensure for example the performance of a catalyst, information about the structure is indispensable. The state of the art methods to get information about the structure of particles in the gas phase in the submicron regime are imaging methods like SEM or TEM. These methods can only be used offline. In order to reduce the analysis time and to control the production processes it is necessary to get information about the particle size and the structure online.

This information can be estimated through analysis of the distribution of the light scattered by an aerosol particle. Common used optical particle sizers for the submicron (i.e. the Mie-) regime measure the intensity of scattered light emitted by a single particle at one particular position. In this case, the effect of elastic light scattering (ELS) is utilized and the result is a one-dimensional equivalent diameter.

The novel developed 3D Light Scattering Sensor (3D-LSS) is also based on the optical technique of ELS, but the setup uses 16 photomultipliers at fixed scattering angles acquiring scattering intensities of single particles for size measurement.

The centerpiece of the system is a hollow, spherical aluminum shell with an inner diameter of 140 mm. It consists of two hemispherical shells which are bolted together. Perpendicularly, two bores are installed, through which the examined aerosol is introduced by aerodynamic focusing and sucked off again. A window for feeding the laser beam and a beam trap are installed horizontally. The intensities of the light scattered by particles can be measured at a variety of positions (up to 16 postions simultaneously in a wide range of scattering and azimuthal angles). For this purpose, bores are provided in the shell. Detectors, which can be introduced into these, pass the scattered light via glass fibers to photomultipliers. The output current of the photomultipliers is converted into a voltage signal by operational amplifiers and low-pass filters. These signals can then be saved on a PC for further investigation. Due to the flexible structure with regard to the positioning possibilities of the detectors and the possibilities to influence the laser beam, the system can be adapted to different measuring tasks.

By the use of multiple detectors in the 3D-LSS at different positions and suitable algorithms for data inversion, information about the 3D structure of the investigated particle can be gained online. E.g. for spherical particles the light scattered in the azimuthal domain at a scattering angle of 90° is homogenous at every position, while the azimuthal distribution of the scattered light emitted by a cylindrical particle is inhomogeneous if the incident light is circularly polarized. Thus, if the intensity of the scattered light is measured at different azimuthal angles and the variance of these values is then determined, conclusions can be drawn on the sphericity of the particles being examined [3].

The design and function of the sensor system in detail as well as measurements on different non-spherical particle systems will be presented.

We gratefully acknowledge for the financial support: Deutsche Forschungsgemeinschaft (DFG, project AN782/6-2) in the framework of the joint research program “multi-parameter characterization of particle-based functional materials by innovative online measurement technology” (PAK688).

[1] J. A. Champion, Y. K. Katare, S. Mitragotri; Journal of Controlled Release 121 (2007) 3-9

[2] Q. Xue; European Polymer Journal 40 (2004) 323-327

[3] H. Barthel; PhD Thesis (1999), TU Kaiserslautern, Kaiserslautern, Germany