Yield-stress materials — toothpaste, mayonnaise, concrete, microgels — behave as solids at low stress and flow as liquids above a critical stress, the yield stress. What happens below the yield stress was contested. One theory: the material deforms elastically and recovers. Another: it flows, but very slowly — a creep that accumulates over time, eventually producing irreversible deformation even below yield.

Careful rheometry on a microgel and an emulsion, with slip effects corrected, shows bounded periodic strain responses below yield. The material deforms, oscillates, and returns. It does not flow. The sub-yield region is nonlinear viscoelastic, not plastic. There is deformation, and the deformation is nonlinear (it does not scale proportionally with stress), but it does not accumulate. The material remembers its shape.

The slip correction matters. Without it, apparent creep shows up in the data — but it is the material slipping against the measurement surface, not flowing internally. This artifact made the sub-yield regime look like slow flow when it was actually bounded deformation with a slippery measurement boundary. The instrument was measuring its own contact with the material, not the material’s internal dynamics.

The structural point is about the distinction between nonlinearity and yielding. These are independent properties. A material can respond nonlinearly to stress without flowing. The nonlinearity means the stress-strain relationship curves; yielding means the strain grows without bound. Confusing the two — assuming nonlinear response implies incipient flow — is a category error that this measurement corrects. Below yield, the material is strange (nonlinear, hysteretic, dissipative) but not flowing. Strange and flowing are different things.

(arXiv:2603.18302)