Delayed yielding in soft glasses and gels, and the role of critical strain

Soft materials exposed to constant stress during creep tests exhibit a deformation response that transitions from solid-like behavior (e.g., Andrade creep) to a flowing state. This transition is called fluidization, and for stresses near to the fluid yield stress it may occur suddenly after a delay in time, particularly apparent in thixotropic materials due to their time-sensitive microstructures. The onset of fluidization does not have a clear definition, and is often only apparent after full fluidization has occurred.

Focusing on tomato ketchup as a model thixotropic fluid, we introduce a parameter to locate the initiation time of yielding events from bulk rheological signatures. "Spurt", Sp, is defined as the log time derivative of the instantaneous shear rate. This term is nondimensional, and mathematically represents the relative change in shear rate, appearing as a peak that becomes positive and increasing at the onset of fluidization. Using spurt, we cleanly delineate avalanche onset and duration, and reveal a power-law dependency in delay time as the applied stress approaches the creep yield stress from above, while the same onset occurs at a single critical yield strain (per material). This behavior is consistent for ketchup, a colloidal gel, and a bentonite solution, and we find direct agreement between fluidization times predicted using Sp and those made by local ultrasonic velocity profiles, for a range of creep stresses. Finally, we show the absence of the typical spurt response in two yield stress fluids shown to not exhibit thixotropy (carbopol and an aqueous foam), thus revealing the importance of thixotropy in yielding behavior.

Spurt allows for the mathematical definition of fluidization time and critical yield strain from bulk rheological measurements during creep experiments, which can advance our rheological understanding of this important phenomenon, and may be used to anticipate critical material failures more generally.