To plot the concentration profile, the μX-CT images were analysed in 2D slices with regularly-spaced vertical positions. A total of 16 positions were chosen (from slice 180 to slice 1680, with 100 slides between each position). At each position, the average grey-scale value was taken from 20 representative areas of the aqueous phase. The slice number was then converted to depth using the equation: 𝐷𝑒𝑝𝑡ℎ(𝑚) = × (the voxel size is 12.1043 μm for the first diffusion experiment, and 14.0537 μm for the second diffusion experiment). The greyscale values of the liquid phase were plotted against the depth (m).