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

Before you select a pump for any service, you need to know and understand the characteristic properties of the fluid you wish to pump. Fluids fall into two classifications—Newtonion or non-Newtonion. Every time I review the characteristics of both fluids, I get caught up in the overwhelming array of nomenclature and technical terms. I call non-Newtonian fluids rascals because they do not follow the rules. I was thinking I might take a simpler approach for the purpose of this column.

A major portion of any discussion regarding the subject concerns viscosity, and I suggest you read my article on the subject (Pumps & Systems, November 2017). For a more in-depth, technical article, see my co-contributor’s article from Pumps & Systems, December 2011, by Dr. Lev Nelik.

Viscosity Definition

Viscosity is a fluid’s resistance to flow or pour, but not all fluids resist or react in the same way or even in the same time reference. With rare exceptions, the viscosity for every fluid will change indirectly with the temperature; if the temperature of the fluid goes up, the viscosity will decrease and vice versa. One way to visualize viscosity is by watching a metal ball fall through a glass container of the liquid at different speeds for various viscosities. The faster the ball falls, the lower the viscosity of the fluid. To take the viscosity concept another step, it is also that property of a fluid that resists a shearing force. Simply put, it pours either fast or slow. Pouring slow means a thicker fluid, which also means a higher viscosity, or you can think of it as possessing a higher resistance to the applied force. Lastly, an alternate method to visualize viscosity is the internal fluid friction resulting when one layer of fluid is forced to move in relation to another layer. Note that the opposite or inverse of viscosity is fluidity.

Terms

A set of terms we need to explain to understand viscosity are stress and strain and how they will affect the fluid properties. First, imagine you are pushing two or more flat plates or boards stacked on top of one another. In the example, you exert a fixed amount of force to the top plate and it consequently moves at a certain speed (velocity) over a specific distance in a measured amount of time. Note that in this imaginary stack of plates, the second, third and fourth plates in the stack will also move, but with lesser amounts of distance and speed. If the moving plate reaction is constant (linear), then the resulting action of the moving pieces represents approximately the same way a Newtonion fluid would react.

We call these fluids Newtonion because they act in a predictable (classic physics) sort of way. These classic fluid properties are named for scientist Sir Isaac Newton. For the simple and perhaps more natural fluids that existed during Newton’s lifetime (1642-1726), he figured out that the viscosity of most fluids changed only with temperature. In the modern era and the advent of polymers and other modern liquids, we now have fluids where this is no longer true. Consequently, this newer class of liquids/fluids is referred to as non-Newtonion.

If a material has a viscosity that is independent of the applied shear stress, then it is referred to as an ideal or Newtonion fluid. Water is considered a Newtonion fluid because it behaves in a classical way. If you stir a glass of water with a spoon, the viscosity does not change no matter how vigorously or how long you stir. The viscosity of a Newtonion fluid is only dependent on the temperature of the fluid (consequently, the requirements for pumping water through a pipe is an easily predictable evolution). Water, oil, alcohol, glycerin and gasoline are common examples of Newtonion fluids. Also note that if you were pumping a viscous liquid like oil at 100 centipoise (cP) with a pump operating at 1,750 revolutions per minute (rpm), the viscosity of a Newtonion fluid does not change even if you increase the speed to 3,550 rpm. If held at constant temperature and the shear rate doubles, the viscosity does not change.

A non-Newtonion fluid has a viscosity that varies with the shear stress and shear rate, so it becomes harder to predict the fluid behavior when it is pumped. Examples are latex paints, soaps, tars, glues, most slurries, colloids, polymers and peanut butter.

peanut butter

Newton's Law of Viscosity

Newton’s law of viscosity is simply:

Stress = [Viscosity] X [Rate]

Slightly more complex, the formula is really:

Shear Stress = [Viscosity] X [Shear Rate]

What is Shear Stress & Shear Rate?

Skip this section if you are looking for simplicity in this column.

Stress is how a force is distributed over or through a material. Shear (for a liquid) is defined as the relative motion between the adjoining layers of a moving fluid. Shear stress is a force per unit area in the simplest definition and applies when the stress is applied at right angles to the subject (for a solid). When you apply a shear stress to a fluid, it will deform at a constant rate, and that is called the shear rate. These types of stresses are calculated parallel to the subject fluid.

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