(13c) Influence of detector geometries of common laser diffractometers on the particle size distribution of elongated particles

Laser diffraction is a fast means to measure particle size distributions, but in most commercially available devices data analysis is based upon the assumption that particles are statistically orientated in the measurement volume, which results in centrosymmetric and circular diffraction patterns. Provided, these highly symmetric patterns really exist, analysis of only a small section of the diffraction patterns is sufficient to calculate equivalent diameters of particle systems. Producers of common laser diffractometers use this fact, when they incorporate detectors consisting of only a limited number of azimuthal segments in different geometries instead of analysing the whole diffraction pattern.

On the other hand, depending on the flow velocity choosen (more precisely, the corresponding Reynolds number) either turbulent or laminar flow can prevail in the measurement volume. In the latter case (laminar flow conditions) particles with strongly elongated (prolate) shapes align with their longest axis parallel to the flow direction. Due to this preferred orientation the resulting diffraction patterns, although still centrosymmetric, are no longer circular (ring-shaped), as can be seen from Fig. 1. Thus, strictly speaking, the use of segmented detectors in combination with laminar flow conditions confines the application of laser diffraction to (sufficiently) isometric particles. Using segmented detectors, which analyse only parts of the diffraction pattern, the results of particle size calculations will strongly depend on the geometry and on the orientation of the detector segment with respect to the flow direction, i.e. the alignment of the particles [1]. On the other hand, laminar flow conditions in combination with suitable detector geometries, could provide valuable information about particle systems with strongly anisometric shapes. Without doubt the best choice for this purpose is a two-dimensional area detector, e.g. a CCD-chip, which offers the possibility to display and analyze the whole diffraction pattern simultaneously. Another ? cheaper ? possibility is the use of a detector, which covers a segment of a circle and which could be rotated to different azimuthal angles. Alternatively, a rotation of the measurement volume would allow the recording of the entire diffraction pattern [2]. Additionally a change of flow conditions in the measurement volume between laminar and turbulent could give information about an anisometry index of particles or particle systems even if common laser diffractometers are used. In the talk, we will show in more detail the influence of flow conditions and detector geometry of different laser diffractometers on the results of grain size distributions analyzing fibres and fibre collectives. Additionally, we will demonstrate the modification of an existing device, which allows to obtain more information about the length and diameter of fibres and fibre collectives.

Literature

[1] Berthold, C., Klein, R., Lühmann, J., and Nickel, K.G. (2000), First results to characterize fibres and fibre-collectives with common laser diffractometers. Part. Part. Syst. Charact. 17(3): p.113 - 116.

[2] Berthold, C.; Nickel, K.G. (1999), Method for determining grain shape using granulometric laser diffraction instruments, International PCT application, AZ PCT/DE 99/0260, WO 00/12998.