Structural Relaxation in Simple Yield Stress Materials Influences Their Rheology

Abstract

Simple yield stress materials are composed of soft particles, bubbles, or droplets with purely repulsive forces. The constituent elements are typically too large to undergo thermal fluctuations, suggesting that the internal structure of the material, and therefore the rheology, should not change over time. We explore the rheology of Carbopol, a prototypical simple yield stress material, and show that gradual structural relaxation of the material results in a small yet significant reduction in the dynamic yield stress. This relaxation process can lead to a non-monotonic creep deformation rate under constant stress, culminating in delayed fluidization of the material. These findings show that the yield stress is not merely a static material property but may be a function of the internal structure of the material.