Automated extraction of in situ contact angles from micro-computed tomography images of porous media

We present a simple and robust algorithm for the automated measurement of in situ contact angles from segmented micro-computed tomography images of immiscible fluid pairs in porous media and 2D slices of them. The algorithm comprises three steps: identification of contact points, an initial coarse estimate of contact angle, and its refinement. To obtain the coarse estimate we identify the vectors which point into the wetting phase and are also normal to the fluid/fluid and fluid/solid surfaces within the vicinity of each contact point. The coarse estimate is subsequently refined by fitting planes across fluid/fluid and fluid/solid surfaces to obtain the final estimate of contact angle. The algorithm was applied to a packed bed of glass spheres using air/water as the fluid pairs. A wide distribution of contact angles spanning the full range of possible values was measured for both the 2D and 3D cases. The distributions are skewed towards their respective means and are longer-tailed compared to a Gaussian distribution. The mean contact angles were found to be approximately 65 +/- 21 degrees, which is significantly larger than bulk contact angles of 35 +/- 3 degrees measured on a flat glass substrate using identical test fluids. The disagreement suggests that bulk contact angles can dramatically underestimate in situ contact angles even in the simplest porous media.