Light-scattering of tracer particles for liquid flow measurements

A variety of modern flow measurement techniques use tracer particles that should accurately follow fluid motions and should scatter sufficient light to be detectable by imagining systems. These two requirements are at odds if they are to be full-filled by varying the tracer size. For this reason, other particle properties such as material, structure, and coating are also considered. While the effect of these properties on the particle response time can be estimated, it is challenging to quantify their effect on the scattered light using the Mie scattering theory. To address this issue, we investigated the light scattering properties of several commercially available tracer particles and provided simple guidelines for selecting appropriate particles. The investigations were carried out using particle images recorded in forward, side, and backward-scatter angles that are typically used in 3D-particle tracking velocimetry. The selected particles represent a wide spectrum of particle sizes and included glass, polymer, and fluorescent particles used in liquid flows. Other properties such as hollow structures and metallic coatings were also investigated. The results showed that glass particles had greater light scattering in the forward-scatter direction, while the polystyrene particles scattered more light in the back-scatter direction. The fluorescent particles had a relatively narrow intensity distribution with a strong side-scatter. We found that silver-coated glass particles had two to four times higher image intensity in the side and back-scatter cameras when compared with uncoated glass particles. The hollow glass particles had a higher forward-scatter compared with the solid glass particles. The recorded images were also used to obtain 3D particle tracks. A large intensity variation was observed along the 3D tracks that was mainly associated with the discretization of particle images on the camera sensor.