Characterization of PIV Seed Particles for Flow in Porous Media

The ability to perform particle image velocimetry (PIV) in porous media presents many challenges in order to minimize the inherent errors in the process. Index of refraction matching is one method to visualize the entire region of interest. One important question is the available signal to noise ratio when using an index of refraction technique that may have small differences of index between the liquid and solid phases. Scattering that will occur at the phase interfaces are enhanced as the index mismatch increases. Consequently the highest intensity seed scattering is desired along with reduction of unwanted interface scattering. This paper examines four seed types: silver coated hollow glass spheres, polystyrene spheres, dyed polystyrene spheres, and fluorescent filled polystyrene spheres and the scattering power of each is measured in a porous media environment. It is found that the silver coated spheres coupled with the use of P polarized light provides the best scattering intensity coupled with low noise background. Also seed size required, based on Stokes number criterion, to follow inertial flows in porous media is presented.