(355g) Direct Measurement of Length and Orientation of Microfibers in Solution

Size measurement of non-spherical objects represents a fundamental scientific challenge. Most particle sizers report volume averaged distributions which are converted to size distributions assuming that the particles are spherical. For non-spherical particles, this assumption is questionable and hence the conversions result in erroneous results. In the recent times, imaging based techniques are being deployed to measure the lengths of non-spherical particles, e.g. micro fibers. However, fibers pose an important challenge also for imaging based systems. Using conventional imaging tools, it is possible to only measure the projected lengths of fibers and not their real lengths, as the angle of tilt is seldom measurable.

In this work, we present the use of a novel measurement technique using digital holography. 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. The use of digital holography has been demonstrated for the measurement of particle size and shape measurements [1, 2]. The proposed method is first applied to experimentally recorded holograms of single fibers of known length and orientation placed on a mount, which can be fixed at a pre-determined off-axis tilt. The hologram of the fiber is captured and processed by a customized image process algorithm which provides the location, orientation and the length of the fiber. 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°). The proposed method is applied to the fiber suspension, where the length, position and orientation of the multiple fibers are determined without any a-priori information. Finally the results from holography are compared with those obtained by microscopy which show an excellent match confirming the potential of digital holography for measurement of micro fibers. The potential of this technique for use as an online monitoring tool for crystallization will be discussed.

1.E. Darakis, T. Khanam, A. Rajendran, V. Kariwala, T. Naughton, A.K. Asundi, Microparticle characterization using digital holography, Chem. Engg. Sci. 65:1037-1044, 2010.

2.M. Kempkes, E. Darakis, T. Khanam, A. Rajendran, V. Kariwala, M. Mazzotti, T. Naughton, A. Asundi, Three dimensional digital holographic profiling of micro-fibers, Opt. Express, 17:2938-2943,2009.