Slow Relaxation and Landscape-Driven Dynamics in Viscous Ripening Foams

TL;DR

This study reveals that the slow relaxation and complex dynamics in viscous ripening foams originate from the self-similar energy landscape and viscous forces, differing fundamentally from glassy physics.

Contribution

It demonstrates that foam dynamics are driven by a landscape-based mechanism rather than glassy physics, highlighting the role of the energy landscape's geometry in slow relaxation.

Findings

Bubbles exhibit intermittent movement at low viscosities.

Higher viscosities lead to continuous, fractal-like bubble trajectories.

Foams recover slowly due to kinetic trapping in high-energy landscape regions.

Abstract

Foams and dense emulsions display complex mechanical behavior, 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 study such anomalous mechanics in a simulated wet foam driven by ripening and find behavior that has a different physical origin than that in glasses. Rather, the dynamics is due to a balance of forces from the system's self-similar potential energy landscape and viscous stress. At the lowest viscosities, bubbles move intermittently, with the system shifting abruptly between shallow potential energy minima. For higher viscosities, in contrast, the bubbles move continuously and the system follows a tortuous, fractal path through high-dimensional configuration space, at higher mean energy than the lower viscosity case. The long-time dynamics and power-law rheology are the direct consequence of the potential energy landscape's self-similar geometry. Lastly, we find that the slow recovery of perturbed foams is due to the foam being kinetically rather than energetically trapped in high-energy portions of the energy landscape. Overall, viscous ripening foams follow a biased energy minimization pathway that explores regions of the energy landscape that are qualitatively different (flatter and smoother) than those corresponding to well-annealed glasses.