When talking about a fluid, what's the difference between Newtonian and Non-Newtonian?

Answer 1

Newtonian: (water)
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Non-Newtonian: (milk)
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Answer 2

Sir Isaac Newton showed how stress and the rate of change of strain are very close to linearly related for many familiar fluids, such as water and air. These Newtonian fluids are modeled by a coefficient called viscosity, which depends on the specific fluid.

However, some of the other materials, such as emulsions and slurries and some visco-elastic materials (eg. blood, some polymers), have more complicated non-Newtonian stress-strain behaviours. These materials include sticky liquids such as latex, honey, and lubricants which are studied in the sub-discipline of rheology.

Remember, as the name applies, Newtonian fluids are like water and air, the problems are solved with the famous f=ma type equations. Non-Newtonian fluids, I always remember honey, do not follow those equations. Blood is another example. Honey and blood have things going on within themselves that change their properties as they're flowing.

Answer 3

In a Newtonian fluid, viscosity is independent of strain rate. In other words, if you stir the fluid the viscosity stays the same - it gets neither easier nor harder.

In a Non Newtonian fluid viscosity changes with strain rate. In other words, if you stir the fluid it gets either thinner (examples include ball point ink, non drip paint) or thicker (examples include custard, egg white, quick sand).