Two constitutive models for yield-stress materials make contradictory claims about what happens below the yield point. One — the Bingham-type framework — says the material is solid below yield: apply stress less than the threshold and you get bounded, recoverable deformation. The other — continuous-flow models — says plastic flow occurs at all stress levels, just very slowly below yield. The material creeps forever, even under light load.

The experimental resolution requires extraordinary care. At low stresses, the strains are tiny. Any residual slip between the sample and the rheometer plates can masquerade as slow flow. Previous measurements that appeared to show sub-yield creep may have been measuring the boundary, not the bulk.

Using parallel superposition rheometry on a microgel and an emulsion — with slip artifacts properly identified and subtracted — the deformation below yield is bounded and periodic. The material oscillates. It does not flow. The bounded response means the energy stored in the material is recoverable, not dissipated. The material is a nonlinear viscoelastic solid in this regime, not a very slow liquid.

The through-claim: two models that agree above the yield point disagree below it, and the disagreement was hidden by an artifact that mimicked one model’s prediction. Slip at the boundary looks like creep in the bulk. The measurement that appeared to confirm continuous flow was actually confirming that the plates couldn’t hold the sample. Once the artifact is removed, the material picks a side: it is solid, not slow. The constitutive debate was not about the material. It was about the measurement.

Source: “Sub-Yield Dynamics in Yield-Stress Materials,” arXiv:2603.18302 (2026).