Refractive Index Matching With Distortion Measurements in a Bed of Irregularly Packed Spheres

Abstract

Experimental flow visualization in porous media is often conducted using optical techniques such as PIV and PTV for velocity field estimation and LIF for concentration field measurements. The porous bed is made optically accessible to laser light and imaging by matching refractive indices of the liquid phase to that of the solid matrix, including the channel walls. The methods most commonly employed to match the refractive indices have been to maximize the transmitted intensity through the bed or to rely on refractometers for measurement of the liquid and solid phases. Refractometers with sensitivity of 0.001 could still cause refraction problems in a porous bed, while accuracy and sensitivity of transmission based methods are limited by the camera resolution and noise scattered by impurities and stray light caused by reflections at interfaces. Both these methods fail to provide uncertainty estimates for particle position determination due to slight refractive index mismatching. This work presents a method for assessing the matching of refractive indices that relies on measuring distortion of a target when imaged through a porous bed. The target used is a grid of 250 μm dots irradiated with light at the necessary wavelength at which refractive indices are to be matched. Two principle types of distortion are quantified, distortion of the image centroid due to interface refraction and intensity distortion within the image for index mismatching as low as 0.0005.