Rising Shear Stress Can Still Mean Falling Viscosity

A stress curve and a viscosity curve can rise or fall differently. Read the quantity on each axis before classifying the material.

Read the Reported Quantity

Apparent viscosity is the ratio of shear stress to shear rate. Anton Paar’s viscometry reference presents that relationship. A higher shear stress at a higher rate does not by itself prove higher viscosity, because the denominator has changed as well.

Retain the stress unit Pa and the rate unit s⁻¹. A stress-versus-rate plot is a flow curve; a viscosity-versus-rate plot presents the corresponding ratio. Their labels answer different questions even when they come from the same observations. Primary rheology reference.

Illustrative Ratio Calculation

Invented observations are 10 Pa at 10 s⁻¹ and 30 Pa at 100 s⁻¹. The corresponding apparent viscosities are 1 Pa·s and 0.3 Pa·s. Stress has increased, while the ratio has decreased. These are hypothetical arithmetic values, not syrup test data.

Calling this two-point illustration shear thickening simply because stress rises would confuse the axes. Two points also do not characterize every region of a real formulation. The responsible laboratory should identify the complete measured range and any method limitations.

Carry Both Plots into the Discussion

Where a supplier receives only a cropped graph, restore the legend, axes, units and condition references. Ask whether the displayed values are raw stress, apparent viscosity or a fitted model. Keep calculated ratios recognizable as calculations instead of new measurements.

Use the worksheet to reconcile a material description with the supplied data. For a liquid-filling enquiry, this prevents a graph-label error from becoming an equipment selection claim. It does not supply a universal shear-rate target or authorize changes to the product formulation. HM application context: Syrup Liquid Filling Machine.