When viscosity is a constant
The simplest, best-behaved liquids obey Newtonian flow: their viscosity is a fixed number that does not change no matter how gently or how hard you shear them. Double the shear stress and the shear rate doubles too; the ratio between them — the viscosity — stays put. Named after Isaac Newton, these are the liquids our everyday intuition fits best.
Pure water, simple oils, glycerol, light mineral oil and a plain sugar syrup are all close to Newtonian. So is a true solution of a drug in a single solvent, as long as nothing in it builds a structure. These liquids are easy to specify and easy to measure — which makes them the perfect starting point before we tackle the messier real-world behaviours.
Reading the rheogram
We plot flow behaviour on a rheogram: shear rate on one axis, shear stress on the other. For a Newtonian liquid the plot is a straight line through the origin. Its slope is fixed, and because viscosity is stress ÷ rate, a constant slope means a constant viscosity. The steeper the line, the more viscous the liquid. This clean straight line is the signature you will learn to recognise instantly.
Temperature: the big lever
Newtonian viscosity is constant with shear, but it is very far from constant with temperature. Warm a liquid and its molecules slip past one another more easily, so viscosity falls — often steeply. This is why honey loosens near a warm window and why an oily base softens in your hand. For measurement this is critical: report a viscosity without its temperature and the number is almost meaningless.
Why temperature is always quoted
A light oil measured in an Ostwald viscometer:
at 20 C -> eta = 90 cP
at 40 C -> eta = 45 cP
A 20 C rise has roughly HALVED the viscosity.
Lesson: "viscosity = 90 cP" is incomplete.
Always write "90 cP at 20 C".Because of this, an Ostwald viscometer — and indeed any instrument used to build a rheogram — is run in a water bath held at a stated temperature, typically 20 or 25 °C. Control the temperature, and a Newtonian liquid becomes one of the most reproducible things you can measure in the lab.