On the elusive nature of Carbopol gels: "model", weakly thixotropic, or time-dependent viscoplastic materials?

The question of whether Carbopol gels always behave as "model" yield stress material is addressed experimentally. Prompted by several simple hydrodynamic experiments performed with Carbopol gels that can not be fully understood within the commonly accepted "model" picture, we revisit the yielding behaviour of a Carbopol gel. When subjected to a loading/unloading process, the yielding of the Carbopol is gradual and exhibits a rheological hysteresis. By in-situ visualisation of the microstructure it is demonstrated that these features do not originate from a micro-structural damage of the micro-gel. A systematic description of the role of the rate at which the material is forced (loaded or unloaded) on the yielding scenario is given. In closing, the question of how simple a scalar model can be and yet accurately describe the experimentally observed yielding scenario of a Carbopol gel in a rheometric flow is addressed. It is concluded that simple scalar models may do such job as long as they are not too simple and include a minimal amount of physical ingredients.