Non-Newtonian fluid
A non-Newtonian fluid is one whose viscosity changes with the shear applied to it, so it has no single viscosity figure. Most media that give pumps trouble are non-Newtonian. Sewage sludge, slurries and many food products thin out as they are worked, so the viscosity you measure at rest is not the viscosity the pump sees.
In more detail
A Newtonian fluid holds the same viscosity whatever you do to it. Water is Newtonian, and so is a thin oil: shear it quickly or slowly and the resistance per unit of shear does not move. Everything else is non-Newtonian. The distinction matters on a pump enquiry because viscosity is normally handled as one number, and on these media one number does not exist.
The four kinds you meet in practice
Four behaviours cover nearly everything a pump sees, and they are not interchangeable.
| Behaviour | What it does | Where you meet it |
|---|---|---|
| Shear-thinning (pseudoplastic) | Gets runnier the faster it is sheared | Sewage sludge, slurries, many food products |
| Shear-thickening (dilatant) | Stiffens the faster it is sheared | Dense particle suspensions; cornflour and water is the schoolroom version |
| Bingham plastic | Sits still until the applied stress passes its yield stress, then flows | Thickened sludge, toothpaste, stiff pastes |
| Thixotropic | Thins with time under steady shear and recovers at rest | Some sludges, gels and paints |
Shear-thinning is the one that dominates in wastewater and food work. Almost all sludges, slurries and semi-solid wastewater are non-Newtonian and shear-thinning, and the degree of it shifts with temperature as well as with shear rate.
Why a centrifugal selection goes wrong on these media
The industry method for correcting a water-tested pump curve onto a viscous duty, ANSI/HI 9.6.7, is written for Newtonian media. On a shear-thinning medium there is no single viscosity to feed into it, so the correction has nothing solid to stand on. Measure a sample at rest and you get the stiffest figure the medium will ever show you. The medium moving through a pipe is being sheared, so it is thinner than that sample, and the duty point is not where you drew it. Measure it in motion instead and you have flattered the start-up condition, which is the moment the pump actually has to break the medium loose. Either way the number you selected on was never the number the pump would meet.
Yield stress is what catches people out
Some media do not move at all until the applied stress passes a threshold. That threshold is the yield stress, broadly defined as the stress that causes significant flow, and below it the medium sits in the pipe as a plug. A centrifugal pump develops pressure by moving the medium, so its shut-off head is the most it can offer against a plug that has not yielded. If the yield stress needs more than that, the pump runs, draws power and shifts nothing. A positive displacement pump keeps raising pressure until the plug gives or the relief valve lifts, which is why it is what goes on thickened sludge and stiff pastes.
There is no typical case to design against
Our rule on any thick medium is to select against the worst case rather than the usual one, and on a non-Newtonian medium the usual one does not exist. Take the stiffest condition the medium reaches: coldest, longest stood still, first start of the week. Shear-thinning helps you once the pump is turning and works against you every time it starts, so start-up is the condition that sizes the drive. Send us the medium, the temperature, the dry solids content if it is sludge, and how long the line stands between runs.
What we put on these duties, and what it costs you
Positive displacement, near enough always. A progressive cavity pump moves thick, shear-thinning sludge at a near-constant flow rate and will keep building pressure until the medium gives. A rotary lobe pump does much the same in a smaller footprint with easier maintenance access, and it suits shear-sensitive food product where the medium has to be handled gently. Both hold their flow rate as the discharge pressure moves, which is what you want when the medium's resistance will not stay put. The cost is that neither forgives dry running. On a progressive cavity pump it burns through the stator and damages the rotor, and that failure is not a cheap one. Get the suction conditions right first, or you will pay for the pump twice.
Frequently asked
What is the difference between a Newtonian and a non-Newtonian fluid?
A Newtonian fluid keeps the same viscosity no matter how hard it is sheared. Water and thin oils behave that way. A non-Newtonian fluid's viscosity changes with the shear applied, so it has no single figure, and a pump selected against one measurement will not behave as the curve says.
Why is toothpaste a non-Newtonian fluid?
Toothpaste has a yield stress. It sits in the tube as a solid until you squeeze hard enough, then it flows like a liquid, then it stops again on the brush instead of running off. Thickened sludge behaves the same way in a pipe, which is why it needs a pump that can build pressure against a plug.
How is ketchup a non-Newtonian fluid?
Ketchup is shear-thinning. Tip the bottle and little happens; shake or tap it and the shear drops its viscosity until it pours, sometimes all at once. Sewage sludge and many food products do the same thing on a process line, which is why a sample measured at rest overstates what the pump will see.
Is custard a non-Newtonian fluid?
It depends which custard. Cornflour mixed with water, the version used for the classroom demonstration, is shear-thickening: hit it and it stiffens. Cooked egg custard behaves more like an ordinary thick liquid. The distinction matters, because shear-thickening and shear-thinning media want opposite things from a pump.
What causes non-Newtonian behaviour?
Structure in the medium. Particles, fibres, polymers and flocs form a loose network that resists flow while it is intact. Shear breaks the network down, so the medium thins; leave it alone and the structure rebuilds. In dense particle suspensions the particles jam together instead, and the medium stiffens under shear.