Simulation of light scattering from a colloidal droplet using a polarized Monte Carlo method: application to the time-shift technique.

This study is devoted to the development and application of a Monte Carlo ray-tracing model to simulate light scattering when a colloid suspension droplet passes through a highly focused Gaussian laser sheet. Within this study, a colloidal suspension droplet refers to a spherical droplet containing multiple spherical inclusions. Such scattering scenarios arise when using the time-shift measurement technique for particle sizing. The incident laser sheet is treated as a large number of polarized light rays: the Stokes vector of each light ray is tracked, achieved by multiplication of the rotation matrix and the Mueller matrix after each scattering event. For the Monte Carlo simulation of light scattering, a very important issue is to generate the deflection angle and azimuthal angle after each scattering event. The scattering from embedded inclusions is computed using the Lorenz-Mie theory and by employing the rejection sampling technique to update the new propagation direction. Multi-reflection and refraction within the droplet is accounted for, as is total reflection at the drop interface. For this, the Mueller matrix formulation is invoked at the drop surface to update the Stokes vector. To validate this simulation code, the scattering diagram from a nanoparticle is computed with this Monte Carlo method and compared with the scattering diagram computed with the Lorenz-Mie theory, the agreement is excellent. This Monte Carlo code is then applied to simulate signals arising from a time-shift device, when a colloid suspension droplet passes through a focused polarized laser sheet, with the objective of measuring the concentration of colloidal particles in the droplet. Measurements verify the ability of the code to properly simulate this light scattering scenario.

Modeling photon transport in turbid media for measuring colloidal concentration in drops using the time-shift technique.

Colloidal drops-suspensions, dispersions, emulsions-are widespread in the process industry but are difficult to characterize by size, velocity, and concentration of particulate matter in the drop. The present study investigates the use of the time-shift (TS) technique for such measurements. Numerically, a model based on ray tracing is developed, incorporating interactions with randomly placed monodispersed scattering centers within the spherical drop. The model creates a random walk propagation trajectory, known from radiative transfer problems. The model approximates Mie scattering from each internal particle with a Gaussian distribution. Experiments are performed using a conventional TS instrument, first with water as a reference and for validation, and then with different concentrations of a milk/water emulsion. Comparison of the modeled and received signals exhibits very good agreement, confirming the possibility of measuring the colloidal concentration in drops using the TS technique.