Delayed elastic contributions to the viscoelastic response of foams

TL;DR

This paper reveals that the slow viscoelastic behavior of foams is due to a reversible, delayed elastic response characterized by a power-law fluid model, which explains non-Maxwellian features and aligns with soft glassy rheology theories.

Contribution

It demonstrates that foam viscoelasticity includes a reversible power-law elastic component, linking experimental foam mechanics to soft glassy rheology models and bubble dynamics.

Findings

The foam's slow response is fully reversible and power-law in nature.

The power-law spectrum aligns with soft glassy rheology.

Bubble dynamics and stress diffusion underpin the observed behavior.

Abstract

We show that the slow viscoelastic response of a foam is that of a power-law fluid with a terminal relaxation. Investigations of the foam mechanics in creep and recovery tests reveal that the power-law contribution is fully reversible, indicative of a delayed elastic response. We demonstrate how this contribution fully accounts for the non-Maxwellian features observed in all tests, probing the linear mechanical response function. The associated power-law spectrum is consistent with soft glassy rheology of systems with mechanical noise temperatures just above the glass transition [Fielding et al., J.Rheol. 44, 323(2000)] and originates from a combination of superdiffusive bubble dynamics and stress diffusion, as recently evidenced in simulations of coarsening foam [Hwang et al., Nat.Mater. 15, 1031 (2016)].