From slow coarsening to catastrophic collapse: ageing of vibrated viscoelastic foams 

Many soft matter materials evolve through surface tension driven phase separation. A particular example is foams, which coarsen as gas diffuses between bubbles due to differences in Laplace pressure. In aqueous foams coarsening leads to characteristic bubble growth, and accompanying rearrangement dynamics. However, in many industrial products and processes the continuous phase is not a Newtonian fluid, but a viscoelastic one, such as a paste or a gel. Such foams typically have longer life-times, as the foam ageing processes are slowed down.

We study the temporal evolution of foams made from concentrated emulsions. The behaviour of our foams is no longer dominated by capillary effects, as it is impacted by the rheological properties of the emulsion between the bubbles. We show that the yield stress of the continuous phase changes both the foam dynamics and its structural evolution leading to spatially heterogeneous coarsening. If the foams are mechanically vibrated, the stabilising impact of the yield stress fluid disappears, and coalescence cascades can be observed. The competition between coarsening and coalescence can be modulated through vibration amplitude and foam liquid fraction, which can be used to create unusual structural motifs. The resulting patterns are unlike those observed in aqueous foams. Beyond their importance in the design of foamy materials, the results give a macroscopic vision of phase separation in a viscoelastic medium.