Topological and geometrical disorders correlate robustly in two-dimensional foams

A two-dimensional (2D) foam can be characterised by its distributions of bubble area and number of sides. Both distributions have an average and a width (standard deviation). There are therefore at least two very different ways to characterise the disorder. The former is a geometrical measurement, while the latter is purely topological. We discuss the common points and differences between both quantities. We measure them in a foam which is sheared, so that bubbles move past each other and the foam is 'shuffled' (a notion we discuss). Both quantities are strongly correlated; in this case (only) it thus becomes sufficient to use either one or the other to characterise the foam disorder. We suggest applications in the analysis of other systems.