Light Scattering of Complex Particles: Application to the Time-Shift Technique

Abstract

Particles with complex shapes and compositions are omnipresent in industrial processes, such as encountered in the chemical, pharmaceutical, automobile and food industries. For instance colloidal drop, i.e. drop with solid particles dispersed in them, are inherent in the spray drying process used frequently in the food industry. Thus, in order to optimize such processes, there is a need to characterize these particles in terms of size, colloidal (solid particle) concentration, the size of the inclusions and possibly also velocity. The present study addresses this need for drop characterisation and concentrates on some particular examples of complex particles: drops with single embedded spheres; drops with single embedded platelets (e.g. aluminum flakes); and drops with multiple micro- or nanoparticle inclusions (colloidal suspension drops). The characterization of a homogeneous spheroidal particle has also been studied, serving as a validation and reference case. This thesis focuses on the light scattering of complex particles and characterization of such particles with the time-shift measurement technique. Within the scope of this study, spheroidal drops, drops with single embedded flakes or spheres, and drops with multiple spherical inclusions have been studied. According to the shape, composition and the size parameter of the particle, corresponding simulation methods have been chosen to simulate the light scattering. The ray-tracing method has been used to investigate the light scattering properties of spheroidal drop by varying its aspect ratio, and drops with single embedded flakes or spheres by varying the position of the sphere and the orientation of the flake within the drop. For drops with multiple inclusions, the polarized Monte Carlo ray-tracing method as well as the discrete dipole approximation method are used to study its light scattering properties. In addition, the time-shift signals, which are generated when the particle falls through a highly focused Gaussian beam of the time-shift instrument, have been simulated for the drops mentioned before. The goal is to investigate if the time-shift technique is able to unequivocally detect whether a drop contains a spherical particle or not, or detect if the drop contains flake. The possibility to use the time-shift technique to estimate the volume concentration of the inclusions within the drop has been studied as well. To validate the simulation results, corresponding experiments have been conducted to obtain the raw time-shift signals by using the time-shift instrument to measure pure water drops as well as the colloidal drops, which have multiple polystyrene latex nanoparticles embedded. During the measurement, the size and the volume concentration of the nanoparticles have been varied. Subsequently, comparison has been made between the measured and simulated time-shift signals to validate the simulation results. Through signal processing, the relative scattering strength from the inclusions has been evaluated from the measured time-shift signal to estimate the volume concentration of the inclusions. The size of the inclusions is estimated through evaluation the attenuation ratio of the time-shift signal.