Learn the difference between viscosity and shear.
by Jim Elsey
May 23, 2018

Shear rate is the fluid velocity divided by the distance moved. Another way to describe shear rate is as the speed of relative motion between layers of a liquid that is moving. Note that just the simple action of pumping certain fluids can irreversibly damage them, while others require the shear rate to increase to a certain level to change the viscosity and get them moving in the first place. Shear sensitive fluids are usually pumped with positive displacement pumps such as progressive cavity or internal gear pumps. Latex and glue are good common examples of shear sensitive fluids.

Non-Newtonion Fluids

The viscosity of a non-Newtonion fluid changes with temperature, but more importantly how it is agitated (how a force is applied) or with the amount of pressure; these are classed as shear stresses. Non-Newtonion fluids fall into three categories:

  • time dependent
  • time independent
  • viscoelastic

From these three, there are five main subclasses of non-Newtonion fluids:

  • plastic (time independent)
  • pseudo-plastic (time independent)
  • dilatant (time independent)
  • thixotropic (time dependent)
  • rheopectic (time dependent)

Dilatant & Thixotropic Fluids

When the viscosity of a fluid changes with the shear rate, the fluid can be either dilatant or thixotropic. Note there are categories where time dependency or independency comes into play and subcategories.

Dilatant and pseudo-plastic fluids are both types of non-Newtonian fluids where the relationship between viscosity and shear rate is not time dependent.

When the viscosity increases with an increase in agitation (shear rate) these fluids are known as dilatant. Dilatant properties are not normally found in pure materials and are usually suspensions, mixtures or compounds. Examples are clay, slurries and silly putty.

If the viscosity decreases as the agitation rate (shear rate) increases, this is typically known as a thixotropic fluid. Examples include soaps, tar and vegetable oil. In a thixotropic fluid, the viscosity decreases with stress over time. At a constant shear rate, the shear stress decreases monotonically. Note that a function that decreases monotonically does not exclusively have to decrease, it simply must not increase.

My favorite example for a thixotropic fluid is ketchup. Every hungry human has tried to get cold ketchup out of the bottle and on to hot french fries as fast as they can, but the ketchup will not cooperate. So you shake or smack the bottle and the product eventually comes out, and then sometimes perhaps too fast. When you shake the bottle, you are applying a shear stress to the fluid. That stress causes the fluid to become less viscous. This a simple example of a shear sensitive fluid—in this case, a thinning fluid. An industrial example of a thixotropic fluid from the oil and gas market is drilling mud.

The initial viscosity may be too high to get the fluid flow started, but the viscosity decreases as the shear rate increases, and then it flows very well as long as there is stress. Other fluids similar to ketchup are gels, latex paints and lotions.

As an example, when you squeeze suntan lotion from the tube, it may be difficult to get the flow started, but when you rub the lotion on your hands, the added sheer stress causes the lotion to be more of a liquid and quickly soak in. A similar example would be shaving cream or toothpaste.

Another instance is paint. You want the liquid to adhere to the brush when you slowly dip the brush into the can, but you also want the paint to flow when you are applying it to the surface to be painted.

Quicksand, while technically a valid fluid, is probably more of a movie and urban myth example. But if you ever get stuck in quicksand, it is best not to struggle vigorously as you will sink deeper and faster. You can possibly float to the top if you simply stop moving.

From a pumping perspective with thixotropic fluids, once you overcome the initial resistance, the fluid will flow similar to a standard Newtonian fluid. It is also important to realize the time dependent aspect. Note: You will require more horsepower to get this fluid started (initiated) to flow and you need to keep it flowing.

For dilatant fluids, although there are shear thinning fluids like above, there are also shear thickening fluids. These fluids behave in the opposite way, which means the more you agitate them (shear stress), the more they will thicken and resist flow (higher viscosity). The most common dilatant liquid for this example is mixing cornstarch with water.

If you agitate it quickly, it will act as a solid, but if you move an object through the fluid medium very slowly, it reacts more like a liquid.

There are dozens of videos on the web that will entertain and educate you on this subject of thickening fluids.

The properties of dilatant fluids are being studied for use in body armor and football helmets.

Summary

For most non-Newtonion fluids, it will be wise to consider the type and speed of pump you choose to move the fluid. Understand that viscous fluid behavior changes with different flow rates. The more you try to force a dilatant fluid through a pipe, the more it will resist in what seems like a pushback on the system.

The size of the pipe will also be a factor since the smaller pipes will have increased velocity with associated higher shear rates. Additional consideration may be required and focused on the pump clearances.

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