An imaging approach for in-situ measurement of refractive index of a porous medium

Abstract

Refractive index matching (RIM) is often used for flow measurement in porous media studies that use optical techniques. However, the refractive index of transparent porous media, often constructed of beads, is not always readily available or measurable with commonly used measuring devices such as refractometers. Therefore, an alternative technique is needed for determining the refractive index of the beads forming the porous media. The available methods in the literature involve a rigorous procedure and use a laser to illuminate the medium. Their setups are often complex and are sensitive to small changes in the temperature or wavelength and sometimes needs continuous stirring of the medium to have a homogeneous porous media. A new approach for measuring and matching the refractive index of beads that minimizes the limits of the available techniques is presented here. Unlike common RIM measurement approaches, the proposed technique involves no laser light and therefore is relatively simpler than similar approaches found in the literature. This approach uses shadow imaging to capture images of the beads forming the porous medium while the RIM fluid flows through it. A comparison approach using a reference image and a structural similarity metric to quantify the level of the refraction index matching is used to identify the refractive index of the dispersed phase. The approach is verified experimentally using borosilicate glass beads as the dispersed phase. Two different RIM fluids, Potassium thiocyanate (KSCN) solutions and a mixture of Drakeol 7, as silicone oil and soybean oil are used as the test fluids to compare the independency performance of the approach. The approach not only identifies the general refractive index of the dispersed phase but can also be used to identify local spatial variations in the porous media. In addition, using the imaging approach minimizes the effort for determining the refractive index of the solid for flow experiments and reduces the need for equipment or changes to the setup.