6 Joseph D. Barry et al. is that the straight line boundaries at y = 0 and y = 1 naturally occur as a result of this process. A semi-periodic monodisperse sample is created using the standard method outlined in the Surface Evolver documentation (which is supplied with the software pack- age). Disordered semi-periodic samples are created by the following process (illustrated in Fig. 3). Using the above methods for foam sample creation, we create one monodisperse foam sample and five foam sam- ples of polydispersity µ$_{2}$ ( A ) = 0 . 13 ± 0 . 002, where the measure of polydispersity is defined as the second mo- ment of the area distribution, Points are placed at random in the unit cell using a uniform distribution to determine the x and y posi- tions; see Fig. 3(a). New points are added to the box if they are more than a predefined minimum distance r$_{min}$ from any other point. The process is continued until the desired number of points have been successfully placed. A lower value of r$_{min}$ results in more polydisperse sam- ples. These points are translated to boxes to the left and right, and reflected (see Fig. 3(a)) through the lines y = 0 and y = 1 to boxes above and below, based on the method of De Fabritiis and Coveney [10] (as indi- cated by the background triangles in Fig. 3(a)). With the software package Qhull [1], the Voronoi Diagram (a particular way of tessellating the plane into regions of convex polygons) of these points is calculated; see Fig. 3(b). This is then passed into the Surface Evolver. The box in the centre is isolated (see Fig. 3(c)) and keeping the areas of each of the cells fixed, the Surface Evolver performs line minimization on the structure. The result- ing structure is our final two-dimensional half-periodic (i.e. periodic in the x-direction only) foam data file; see Fig. 3(d). Of interest here (as a result of the reflection) We proceed to move the top boundary in the posi- tive x-direction with velocity V by incrementally moving vertices at y = 1 a distance V dt per timestep dt . The VF algorithm, as outlined in Sec. 2 is used to determine the dynamics of the foam during each timestep. Typical values for the displacement of the shearing boundary per The foam samples consist of N$_{b}$ = 100 bubbles in a square unit cell of area 1; it is too computationally expensive to run larger samples in a VF simulation. In our dimensionless simulation units, our system size L = 1 and mean bubble area ¯ A = 0 . 01. We define a new length scale ¯ A 1 / $^{2}$, the square root of the mean bubble area. In these new units, L = 10 ¯ A 1 / $^{2}$. The width W$_{l}$ of one layer of bubbles in our square sample is given by Here A denotes the area of a bubble, and ¯ A the mean bubble area. µ$_{2}$ ( A ) = ( 1 − A ¯ A$^{)}$$^{2}$ . (3) W$_{l}$ = L/ $^{√}$N$_{b}$ = ¯ A 1 / 2 (4) 4 Simulation Details and Results 3 Sample Creation