Delayed Yielding in Creep, Time - Stress Superposition and Effective Time Theory for a Soft Glass

TL;DR

This paper studies the creep behavior of a soft glassy material, showing how time-stress superposition and effective time theory can predict long-term rheological responses and delayed yielding phenomena.

Contribution

It introduces the application of effective time superposition to soft glasses, revealing conditions for superposition and explaining delayed yielding through aging-rejuvenation interplay.

Findings

Time-stress superposition holds over a range of conditions.

Delayed yielding occurs at low aging times and high stresses.

Critical yielding time increases with aging and decreases with stress.

Abstract

In this work we investigate creep flow of aqueous suspension of Laponite, a model soft glassy material, at different aging times and stresses. We observe that this system shows time - aging time - stress superposition over a range of aging times and stresses when real time scale is transformed into effective time scale. Existence of superposition in an effective time domain facilitates prediction of long and very short time rheological behavior. Analysis of the observed behavior from effective time approach suggests that superposition is possible only when the shape of a relaxation time spectrum is preserved at various aging times and stresses. We also observe that creep curves at low aging times and greater stresses demonstrate delayed but sudden yielding. The critical time, at which material yields, increases with increase in aging time and decrease in stress. We argue that local rejuvenation of part of the glassy material causes variation in the rate of evolution of relaxation modes. The resulting interplay between aging and rejuvenating modes lead to delayed yielding as observed experimentally.