Shear localisation with 2D Viscous Froth and its relation to the Continuum Model 5 tails on the numerics of this calculation are best found in [11]. It should be noted that for high rates of strain, one would expect surface tensions in the soap films to vary to the point that this equilibrium condition would no longer apply (for example, because of the Marangoni effect). At least for lower rates of strain, the 2 π 3 rule is reasonable. The VF model may be conveniently incorporated into a Surface Evolver [4] script (as pioneered by Cox [7]), thus allowing for the use of various SE features. The procedure for performing (T1) topological changes in the Surface Evolver is as follows, and is illustrated in Fig. 1. The distance along the film between neighbouring three- fold vertex points is calculated at each timestep. When this film length becomes smaller than a predefined criti- cal cut-off length, l$_{c}$ then it is deleted using the Evolver’s ‘edgeweed’ command. A four-fold vertex is temporarily formed to maintain the topology of neighbouring cells (see Fig. 1(b)). The Evolver’s ‘pop’ command is then employed, which scans the foam for vertices which do not have a legal topology and replaces the four-fold ver- tex with a proper local topology. This results in a new film of effectively negligible length oriented in the per- pendicular direction to the old film (see Fig. 1(c)). Further details on the implementation of the VF Model can be found in the papers by Kern et al. [21] and Green et al. [11]. Throughout the implementation of the model, film segments adjacent to the three-fold vertex points are held an angle of 2 π 3 radians relative to each other, in accordance with Plateau’s rules for a soap froth. De- where λ is the wall drag coefficient, v $^{⊥}$( s ) is the velocity of a point s in the direction of the normal vector N ( s ) to the soap film, ΔP represents relative pressure differ- ences between neighbouring cells, γ is a constant surface tension force (in 2D), and K ( s ) is local film curvature calculated from the relative positions of adjacent (dis- crete) film segment points. See Fig. 2 for an illustration of the forces involved. Setting λ = 0 in eq. 2 recovers the Young-Laplace law, corresponding to soap films that are arcs of circles. Fig. 2 A diagram illustrating the various forces involved in film motion with the two-dimensional VF Model. B and B ′ indicate the two bubbles the central soap film is separating. Note that this film is in contact with a surface (in the plane of the page) which results in a drag force when the film is moving.