Optics and Lasers in Engineering 134 (2020) 106175 Contents lists available at ScienceDirect Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng An imaging approach for in-situ measurement of refractive index of a porous medium Reza Sabbagh, Shadi Ansari, David S. Nobes ∗ Optical Diagnostics Lab., Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada a r t i c l e i n f o Keywords: Refractive index matching Dispersed phase Flow measurement Structural similarity Shadowgraphy a b s t r a c t 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. 1. Introduction Fluid flow in a porous medium has many applications in industries including combustion, bio-industries, clinical, oil and gas extraction, fil- tration, waste water treatment, etc. [1]. Optimization of these systems and their operation related to the flow inside a porous media needs a deeper understanding of the details of the fluid mechanics. Such under- standing is connected to the ability to undertake flow measurement and visualization to develop a qualitative and quantitative understanding of aspects of the flow domain such as local flow velocities, flow paths and plugging of the flow as the continuous phase, that fluid, passes through the porous media. The flow field is dependent on the properties of the medium, such as pore size, pore geometry and surface properties of the material [2–4]. Any kind of invasive technique may change the nature of the flow characteristics in the porous media domain. Additionally, some experiments require the use of toxic or chemicals that hazardous to the operators and may change flow conditions is they are exposed. ∗ Corresponding author: Mechanical Engineering Department, University of Alberta, Edmonton AB, CANADA. E-mail address: david.nobes@ualberta.ca (D.S. Nobes). Therefore, noninvasive techniques are commonly a better option for un- dertaking flow measurements in porous media [5]. Noninvasive methods that have been applied for 3D flow measure- ments through porous media are typically imaging bases techniques. These include optical imaging [6], gamma radiation [5], magnetic reso- nance imaging (MRI) [7], nuclear magnetic resonance (NMR) [8], par- ticle (positron) emission tomography (PET) [9], X-ray tomography (CT scan) [10,11] and ultrasound or acoustic Doppler velocimetry (ADV) [8] or ultrasonic velocity profiling [12,13]. Each technique has its ad- vantages and disadvantages related to the cost (equipment, computa- tion, maintenance and operation), accuracy (spatial and temporal res- olution) and sensitivity to test conditions (such as humidity and tem- perature) [5,6]. Other factors such as operating procedure and the need for highly trained operators, manufacturing quality and optical proper- ties of beads constructing the porous medium, interference effects and safety may also be limiting factors. Among the imaging techniques that are used for investigations in a porous medium, optical imaging and https://doi.org/10.1016/j.optlaseng.2020.106175 Received 3 January 2020; Received in revised form 27 March 2020; Accepted 5 May 2020 0143-8166/© 2020 Elsevier Ltd. All rights reserved.