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.

A key difference between Newtonian and Non-Newtonian fluids is their reaction to the forces they experience during processing. 

WHAT IS VISCOSITY?

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.

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.

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.

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.

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.

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.

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.