rheogram

A rheogram is a graph that acts as a flow 'fingerprint' for a material. You plot the force used to make it flow (shear stress) on one axis against how fast it flows (shear rate) on the other, and the shape of the resulting line instantly tells you what kind of flow behaviour the material has. One picture summarises what no single viscosity number could.

The shape is diagnostic. A straight line through the origin means Newtonian flow (constant viscosity). A curve bending toward the shear-rate axis means pseudoplastic, shear-thinning behaviour; a curve bending the other way means dilatant, shear-thickening behaviour. A line that crosses the shear-stress axis at a positive value rather than at zero indicates a yield value and therefore plastic flow.

Rheograms can also reveal time-dependence. When the curve obtained while increasing the shear rate does not retrace itself as the shear rate is brought back down, the two branches enclose a hysteresis loop, the signature of thixotropy; the loop's area indicates how much structure was broken down and how slowly it recovers. Reading rheograms is therefore a core skill for characterising and comparing pharmaceutical liquids and semisolids.