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This behaviour is observed in the VF simulations that we will discuss. In the literature, the extent of this shear localisation effect is measured by a localisation length. In this paper, two different definitions of localisation length will be employed. 

Simulation results for polydisperse froths subjected to simple shear will be presented, where the localisa- tion length is found to vary with λV (viscous drag times driving velocity; why this is the important parameter to consider is explained in Sec. 4). For low λV , a wide range of localisation lengths is found; see Sec. 4. This effect is attributed to the existence of distinct yield and limit stresses. We give qualitative evidence of this in Sec. 5, where we extend the general Continuum Model to in- corporate such an effect. In Sec. 6 we proceed to relate parameters of the VF Model to the Continuum Model which is shown to accurately predict the observed be- haviour for a Herschel-Bulkley exponent of a = 0 . 3. The localisation length is found to be independent of λV in the monodisperse case. 

In experiments with 2D foams, often a single layer of bubbles is confined between two narrowly spaced glass plates (a Hele-Shaw cell). There are also other types of quasi-2D systems, such as the Bragg raft [3], where a sin- gle layer of bubbles floats on a liquid pool, and the con- fined bubble raft, where a Bragg raft is trapped under- neath a glass plate. The most important distinction be- tween these experimental realisations is concerned with the presence of viscous drag. When a foam is in contact 

with one or two confining plates, there is a drag force associated with any movement of the foam relative to the plate(s). As we shall see, the drag force in the VF Model (see eq. 2) plays a role in the localisation of flow in our foam samples. 

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When a 2D foam is subjected to an applied shear stress, after an initial transient, it yields and begins to flow. The foam yields locally when the yield stress is reached. A more detailed description of the stress-strain relation will be required when interpreting the simula- tion results presented in this paper; see Sec. 5. At the local level, yielding is due to plastic events, i.e. T1 topo- 

