(83g) Two and Three Dimensional Microparticle Characterization Using Digital Holography

For particulate processes, such as crystallization, polymerization and granulation, particle imaging has been proven to be an effective technique for on-line particle size and shape measurement [1,2]. Imaging based methods extract size and shape information from acquired images using image analysis algorithms. The successful application of image analysis algorithms for conventional 2D imaging systems [1,2], however, is often hindered by the problems arising due to limited depth of focus imposed by required magnification as well as undetermined perspective position and random orientation of the particles [3]. Thus, conventional imaging systems provide 2D information about the 3D particles, which can result in inaccurate inference of particle's 3D size and shape [3,4].

In this paper, we propose a novel method to accurately characterize the properties of microparticles. The proposed method uses digital holography as the imaging tool. In digital holography, a hologram of the object is stored digitally. Subsequent numerical processing yields reconstructions focused at particular depths of the scene. Thus, 3D information can be obtained using a single hologram by reconstructing it at different depths [5]. This distinct advantage allows digital holography to overcome the aforesaid limitations of the 2D imaging systems. This paper highlights two key aspects of the application of digital holography. Firstly, we demonstrate that digital holography is a potential tool for microparticle analysis. Secondly, we develop an algorithm to accurately profile and to measure true lengths of microfibers.

Firstly we apply to measurement of particle size distributions of a population of microparticles. An automated image processing algorithm is proposed to extract particle size and shape from the reconstructed images [4,6]. The performance of the proposed method is studied using both spherical and needle shaped objects, which represent the morphological extremes encountered in practice. Furthermore, experiments are carried out under different conditions, such as static, suspended and flow-through system. In the static condition, dry particles are placed on a glass slide, whereas in the latter cases particles are contained in a volume (cuvette or flow-through cell) in order to study the out of focusing effect as well as effect of flow on the measurements. In all cases, the technique automatically determines the best focused depths and yields good measurement of the size and shape of the particles. These measurements have been validated using independent methods such as optical microscopy or scanning electron microscopy (SEM).

Secondly, we propose a novel method to accurately measure size, orientation and position of needle shaped microparticles. In this method, a tailor-made image analysis algorithm is applied to the reconstructed images to extract length, position and orientation of the microfibers simultaneously [7]. The proposed method is applied to experimentally recorded holograms of a single fiber of known length and orientation for benchmarking purpose. Here, a single microfiber is placed on a mount, which can be fixed at a pre-determined off-axis tilt. The measured length and tilt of the microfiber are found to be in excellent agreement with their true values. It is worth noting that the image analysis algorithm is automated, which provides the same accuracy for microfibers with different lengths (150 and 1320 microns) and different tilts along the optical axis (0-60 ^o). Finally, the proposed method is applied to a suspension, where the length, position and orientation of the multiple fibers are determined without any a-priori information. These results successfully demonstrate the potential of digital in-line holography for characterization of needle shaped particles and opens up avenues for particle analysis, where simultaneous measurement of both particle size and shape are of immense interest.

References

1.Eggers, J., Kempkes, M. and Mazzotti, M., Measurement of size and shape distributions of particles through image analysis. Chemical Engineering Science, vol. 63, pp. 5513-5521, 2008.

2. Larsen, P. A. and Rawlings, J. B., The potential of current high-resolution imaging-based particle size distribution measurements for crystallization monitoring, AIChE Journal, vol. 55, pp. 896-905, 2009.

3. Wang, X. Z., Roberts, K. J. and Ma, C., Crystal growth measurement using 2D and 3D imaging and the perspectives for shape control, Chemical Engineering Science, vol. 63, pp. 1173-1184, 2008.

4. Khanam, T., Darakis, E., Rajendran, A., Kariwala, V., Asundi, A. K. and Naughton, T. J., On-line digital holographic measurement of size and shape of microparticles for crystallization processes, in Ninth International Symposium on Laser Metrology, 7155, SPIE, 2008.

5.Schnars, U. and Juptner, W., Digital Holography: Digital Hologram Recording, Numerical Reconstruction, and Related Techniques, Springer, Berlin, 2005.

6. Darakis, E., Khanam, T., Rajendran, A., Kariwala, V., Asundi, A. K. and Naughton, T. J., "Processing of digital holograms for size measurements of microparticlese," in Ninth International Symposium on Laser Metrology, 7155, SPIE, 2008.

7 Kempkes, M., Darakis, E., Khanam, T., Rajendran, A., Kariwala, V., Asundi, A. K., Mazzotti M., and Naughton, T. J., Three dimensional digital holographic profiling of micro-fibers, Optics Express, vol. 17, pp. 2938-2943, 2009.