Tortuosity factor τ versus accessible pore volume fraction ε_t in hindered diffusion. The accessible pore volume fractions ε_t are numerically measured on microstructures after dilation of size r_g. The tortuosity factors are estimated with Equation (23) and corresponding effective diffusion coefficients shown in Figure 13. The red dotted curve is obtained by curve fitting using Equation (6), with the points in ε_t > 0.4.