14 Joseph D. Barry et al. quasi-static calculations using the Surface Evolver (of the type mentioned in in Sec. 1) for 29 foam samples of disorder µ$_{2}$ ( A ) = 0 . 13 ± 0 . 03 with N$_{b}$ = 100 bubbles. This effectively sets the viscous stress c$_{v}$ ˙ ϵ ( y ) a to zero, thereby allowing us to obtain an accurate estimate of the magnitude of the stress overshoot, Δ . The foam samples are created using the process outlined in Sec. 3. The shear stress σ$_{xy}$ (defined in [22]) is recorded for each simulation and subsequently averaged; see Fig. 10. As there is localisation in these simulations (at either the moving or stationary boundary) which affects our stress measurements, the limit stress σ$_{l}$ reported here must be treated as an approximate measurement. The value of the yield stress σ$_{y}$ , however, is exact, as up to a strain of unity, the foam is in the elastic regime and the bubble motion has not yet localised. We measure the magnitude of the stress overshoot to be Δ = 0 . 1 γ/ ¯ A 1 / $^{2}$, which corresponds to a 17% overshoot. In the calculation shown in Fig. 12, a 20% overshoot is used. The drag force per unit area for the Continuum Model is given by eq. 10 and acts in the direction of shear. We wish to relate this to the the drag force of the VF Model, λv $^{⊥}$, which is a force per length and acts in the normal direction to a soap film (see Fig. 2). Trivially, the drag exponent, b = 1. The numerical prefactor c$_{d}$ may be calculated analytically for a 2D hexagonal honeycomb structure, which serves as a reasonable approximation. We also take into account the direction in which the drag In the VF simulations, it is observed that bubbles move on average only in the direction of shear. The mag- nitude of this ‘apparent’ velocity is denoted by v$_{app}$ in Fig. 11. However, the drag force for the VF model by definition points in the direction of the normal to a soap film, and so we project v$_{app}$ in this direction (see Fig. 11(i)). This results in | v $^{⊥}$| = | v$_{app}$ | cos θ , where θ is The drag force per unit area must be proportional to the total length of the soap films in that area. For the honeycomb, this yields force is defined and the orientation of soap films in the foam. Fig. 10 Averaged shear stress data for 29 foam samples of disorder µ$_{2}$ ( A ) = 0 . 13 ± 0 . 03. The stress overshoot is clearly evident. The yield stress σ$_{y}$ is taken to be the maximum stress value, which occurs at a strain of unity. The limit stress σ$_{l}$ is the stress average from a strain of 2 to 10. Calculation performed using quasi-static simulations. c$_{d}$ ∝ √ 2 √ 3 ¯ A . (23) 0 2 4 6 8 10 Strain 0.2 0.4 0.6 Stress  [  / A 1/2$^{]}$ y l