🧪 Newtonian or Not?
A fluid appears with its shear-stress curve. Read how it flows and drop it in the right bin — Newtonian, Non-Newtonian, Ideal plastic (needs a yield stress) or Thixotropic (thins with time). Build your streak.
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The four behaviours
Newtonian: τ = μ·(dv/dy) with constant μ — a straight line through the origin (water, air, oil, kerosene, glycerine, mercury). Non-Newtonian: the τ–(dv/dy) relation is non-linear, so the apparent viscosity changes with the rate of deformation (paints, slurries, blood, polymer solutions) — the study of these is rheology. Ideal (Bingham) plastic: needs a finite yield stress before it flows at all, then responds linearly (toothpaste, drilling mud). Thixotropic: beyond a yield stress its apparent viscosity drops with time under shear (printer's ink, ketchup).