Ripening foams are not really analogous to glasses

Foams and dense emulsions display complex mechanics, including intermittent rearrangement dynamics, power-law rheology, and slow recovery after perturbation. These effects have long been considered evidence for glassy physics in these and other materials having similar mechanics, such as the cytoskeleton. Here we observe such anomalous mechanics in a simulated wet foam driven by ripening, and find behavior that is distinctly different from that in glasses and controlled by the strength of viscous damping. For a wide range of intermediate damping values, the system configuration moves continuously over a high-dimensional potential energy landscape devoid of energy minima, at an energetic height typically associated with equilibrium fluids, following a trajectory having a tortuous, fractal character. At the very lowest viscosities, the configuration moves similarly but at lower potential energy, while intermittently hopping between shallow energy minima producing avalanche-like rearrangements, as seen in previous studies. This tortuous motion over a fractal landscape leads the material to display power-law rheology at all but the very highest viscosities. Our model successfully captures the observed slow recovery of perturbed foams while suggesting that it is driven by the configuration being kinetically rather than energetically trapped in the high-energy portions of the landscape. Overall, our findings suggest these systems display a hierarchical fractal landscape which is stratified by energy, where different energy domains give rise to separate foam-like, intermittent, and glassy dynamics.

https://arxiv.org/abs/2301.13400