Transparent porous medium for optical fluid flow measurement using refractive index matching

We present a method for obtaining a transparent porous medium for optical fluid flow measurement using refractive-index matching. We report on the design of three transparent porous media, one of which was used in a study about the dynamic spreading of convection. The method relies on shadowgraphy, a well-known imaging technique that enables index matching between a porous material and a saturating fluid. The method was tested using borosilicate glass beads as the porous material, while the refractive index of the fluid mixture was determined using a calibrated refractometer operating at multiple wavelengths. The best matching condition is obtained by merging two criteria: the maximum intensity ratio and the minimum standard deviation. The previous knowledge of the fluid refractive index additionally allows us to recover the refractive index of the solid phase, which is ∼1.471 at λ =670nm and T=293.15K. The paper discusses the importance of understanding the fluid flow through porous media and optical measurement techniques in studying these processes.