  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 material
internal structure, and therefore its rheology, should not change over time.
Here, we explore the rheology of Carbopol, a prototypical simple yield stress
material, and reveal that gradual, spontaneous relaxation of residual stresses,
induced when loading the sample into rheometer, results in a small yet
significant reduction in the material 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
highlight that yield stress is not merely a static material property but a
function of the material internal stress state.
