The Importance of Newtonian and Non-Newtonian Fluid Dynamics in Processing

All plant and processing personnel understand that maintaining product integrity is critical. In food processing, handling can change the texture, taste, and appearance of product, and in pharmaceutical processing, maintaining delicate cell structure is vital to a medication's efficacy. As a result, the behavior of foods, beverages and pharmaceuticals under processing conditions is a key factor to consider when assessing a system's performance.

In this post — written for plant personnel, engineers, quality specialists, R&D departments, and other processing nerds —  we focus on products that fall into two categories — Newtonian Fluids and Non-Newtonian Fluids. Newtonian and Non-Newtonian fluid products respond differently to forces that come into play during processing. Those forces include system pressure and velocity and friction that occurs at pipe or tube surfaces. Pumps, for example, create pressure for moving fluids through systems, and that pressure acts as a force on the fluids as they move through pumps and pipes. 

However, a Newtonian fluid such as water does not change as much under processing conditions as a Non-Newtonian fluid such as ketchup, whose thickness and flow characteristics change dramatically during processing. We measure those characteristics as viscosity and shear.

Newtonian Fluid ExamplesNon-Newtonian Fluid Examples
AlcoholSoap Solutions
Motor OilToothpaste
A key difference between Newtonian and Non-Newtonian fluids is their reaction to the forces they experience during processing. 


Viscosity is a measure of a fluid’s resistance to flow. In a processing system, some fluids’ viscosity changes when they come in contact with, for example, a rotating impeller inside a pump. The pressure increase that the fluid encounters creates shear force on the fluid. Some liquids become less viscous (thinner) with increased shear force, while others become more viscous (thicker) with increased shear force. By comparison, a Newtonian fluid such as water, does not change its viscosity, no matter how much shear occurs.

What is shear?

Shear is the relative motion between adjacent layers of a moving fluid. Some of the simplest examples include spreading butter on bread or applying sunscreen. In each case, one level of the liquid (butter on the knife) is moving relative to the adjacent layer (butter on the bread).

Shear force acts in a direction parallel to pump and tube surfaces. Resistance to such forces in a fluid is correlated with its viscosity and interior pipe surface smoothness.

Shear matters to personnel in sanitary processing because system design — pump speed, flow rates, pipe diameter, among other factors — can affect the amount of shear and its effect on product integrity.
Newtonian and Non-Newtonian fluids

Fluids in motion develop shear stresses caused by particles moving in relation to each other. For a fluid flowing in a pipe, fluid velocity at the pipe wall is nearly zero, but the velocity of fluids increases with distance from the pipe wall.

Depending on the fluid’s viscosity and temperature within your system, the relative movement of fluid particles in relation to the pipe wall is faster or slower.

Shear rate and viscosity

Product moving through a system is subject to shear forces that may cause the product’s viscosity, or resistance to flow, to change. Shear occurs at different rates, with some fluids responding to shear rates in opposite ways.

Newtonian fluids such as water do not change in response to shear forces, but Non-Newtonian fluids behave in a variety of ways when subjected to different shear rates.

Higher shear rate = more viscosity (thicker)

Some fluids increase in viscosity with an increase in shear rate. These fluids are referred to as shear-thickening or dilatant and include things like cornstarch/water mixtures and silly putty.

When trying to press your hand quickly into a container of a cornstarch/water mixture, it is met with a lot of resistance, but pressing slowly gives the molecules time to get out of the way, and you can easily press through the liquid.

Pump Head

Higher shear rate = less viscosity (thinner)

Some fluids become thinner as the rate of shear increases. These fluids are referred to as shear-thinning or thixotropic. Fluids that experience shear-thinning include: ketchup, lotions, and blood, for example.

Shear stress is caused by the flow of fluid across a surface. The result of shear stress is a difference in fluid velocity at the tube surface and at the center of the tube.

Shear strain

Shear-thickening fluids act like solids under a fast-moving shear force. That’s why pumps running at high RPMs can generate fast-moving shear forces and high pressure causing an increase in the viscosity of product. Under high pressure, molecules in shear-thickening products crowd each other and resist flow.

However, with lower pressure and lower velocity processes, molecules have time to get out of each other’s way, so their viscosity decreases allowing shear-thickening products to act like liquids because molecules don’t bunch up.

Pump variables that affect fluids during processingPiping variables that affect fluids during processing
Pump speedPipe Diameter
Pressure generated by pumpInterior surface roughness

Fluid variables that affect processing systems

Fluid viscosity ranges from highly viscous (thick) fluids such as honey, which resists flow much more than low-viscosity (thin) fluids such as water. Processors use viscosity measurements to maximize production efficiency. In addition to affecting the flow rate of a fluid as it moves through pipes, viscosity also affects how long product takes to dispense for packaging. Because your process should be designed to optimize flow, accounting for viscosity is an important factor in maximizing efficiency.

All liquids and soft solids have some viscosity, and because food processing is all about moving products through systems by applying force to them, viscosity is an important factor in design and operation.

The degree of a fluid’s response to shear forces is its shear sensitivity. If all fluids were the same no matter how much stress they experience, system calibration would be more straightforward. But all fluids don’t act the same way under pressure. 

Ketchup: Shear-sensitive

Ketchup at rest is thick and it doesn’t pour very well.

But if you apply pressure to a squirt bottle of ketchup, the ketchup becomes less viscous and flies out of the bottle.

Honey: Not Shear-sensitive

Honey, on the other hand, is not affected by pressure.

Squeeze a honey bottle as hard as you like, and the stuff takes its sweet time anyway.

Shear-sensitive products such as ketchup, shampoos, and egg whites must be handled gently throughout the system to preserve product integrity. 

While a slower RPM pump can prevent product from being affected inside pump casings, pump speed is only one variable in your product handling. Pipe diameter and product temperature also have a role.

Processing problems associated with viscosity and shear

One common problem related to viscosity is excess power usage. More viscous fluids in your system require more power to move than less viscous ones. Not accurately accounting for viscosity can therefore result in potential replacement of power components.

Not accounting for a fluid’s shear sensitivity can similarly result in higher power consumption plus the possibility of product degradation.

Rheology testing is a common practice for determining a product’s viscosity and shear sensitivity before specifying pump requirements. Rheology testing measures the flow of fluids under processing conditions to see to what degree the fluid changes with variables such as pressure, flow, and temperature.

The results of testing help predict accurate power consumption requirements.

Food Industry

Shear in pumps

Shear-sensitive liquids change viscosity when under stress or pressure. When flowing by the impeller inside a pump, some liquids become less viscous, a phenomenon called shear thinning. Other fluids become more viscous with increased force, which is called shear thickening or dilatant. 

The distinctive feature of dilatants is that they get thicker in proportion to the shear forces they encounter. 

Churn water through a pump and it all flows through; churn a cornstarch/water mixture through a pump and, depending on the amount of shear stress the pump generates, you have a fluid of substantially higher viscosity.

Newtonian Fluids Under Shear Stress

If you were to place your hand in a bucket of water (a Newtonian fluid) at room temperature and squeeze the water as you make a fist, you would feel no change in water’s viscosity as it makes its way through your fingers.

Some Non-Newtonian Fluids Under Shear Stress

Thrust your hand into a bucket of dilatant fluid such as cornstarch and water, make a fist and withdraw it from the bucket, and you will wind up with a fistful of a much thicker fluid.

Newtonian and Non-Newtonian Fluids In a Pump

Churn water through a pump and it all flows through; churn a cornstarch/water mixture through a pump and, depending on the amount of shear stress the pump generates, you have a fluid of substantially higher viscosity.

In regards to friction loss, fluids passing through pipes similarly change viscosity depending on the amount of shear stress produced near pipe walls, with velocity and volume decreasing at and near pipe walls, and increasing at the pipe center.

Newtonian liquids such as water do not change viscosity when force is applied. Non-Newtonian fluids such as ketchup change viscosity when force is applied.

Pump efficiency at the given duty point is an important factor in achieving gentle product handling. Efficiency in your pump is measured as the ratio between the amount of fluid that enters the pump and the amount of it that exits.


With low efficiency, your product remains in the casing and recirculates, resulting in overhandling and changes to its viscosity. If those changes in viscosity become permanent due to overhandling, they compromise the integrity of the product.

Positive displacement pumps deliver a constant flow of fluid at a given pump speed. When viscosity increases, however, resistance to flow increases, so to maintain system flow at higher viscosities, positive displacement pumps require more horsepower.

The curve to the left illustrates the relationship between viscosity and pump shear. For more info, visit How to Read a Positive Displacement Pump Curve.

Shear in pipes

Fluids in pipes flow under the action of whatever force is applied within the system. A fluid in motion undergoes shear stress due to particles in the fluid moving relative to each other.

Other factors in your system, such as tube diameter, can cause damage to product. If velocity is too high while product travels through a small-diameter tube, turbulent flow results and friction between product and inner walls of the tubing creates shear of the product.

By contrast, laminar (smooth) flow is less likely to disturb product integrity.
Shear in Pipes

Factors such as velocity, energy, pump efficiency, and tube diameter all determine how product is handled. In some cases when product is sensitive to processing parameters, engineers can slow velocity by increasing line size and shortening distances to minimize pressure requirements.

The role of temperature

Temperature control is an important factor in maintaining optimum viscosity because product viscosity can vary significantly with temperature. 

Some food sauces, for example, have low viscosity when heated but thicken to high viscosity when cooled. Similarly, heat honey, and it pours faster than when it cools down to room temperature.

Next Steps

Maintaining product integrity in processing is critical and a key factor in ensuring this integrity is understanding the behavior of your product under processing conditions.

We're here to help! 

CSI is known as a market leader in the specification, sizing, and supplying of equipment for hygienic industry processes. If you have any questions concerning fluid dynamics in sanitary processing or need help with your specific application, contact us today at 417-831-1411.


Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at