A major requirement for industrial mixer design is a thorough understanding of the mixed material viscosity.  This is especially true for high viscosity fluids.  Most viscous fluids are also non-Newtonian.  This means the fluid does not have a fixed value for viscosity.  There is an “apparent viscosity” at the mixing impeller that is related to how fast the material is stirred.

The following provides an overview of Newtonian and non-Newtonian fluids including videos demonstrating the mixing characteristics of each.

 

Newtonian Fluids 

A Newtonian fluid is a fluid whose rate of flow is proportional to the stress applied to it. The viscosity of a Newtonian fluid is therefore constant and independent of shear rate.

The video below shows a lab scale, baffled tank with a Newtonian fluid (corn syrup).  The viscosity is 80cps. The mixer has an axial flow (45 degree pitched blade) turbine impeller.  The video demonstrates that under these conditions, the fluid has good top to bottom flow and fluid movement out the tank walls.

 

 

Non-Newtonian Fluids 

A non-Newtonian fluid is a fluid whose rate of flow is not proportional to the stress applied. The viscosity of a non-Newtonian fluid is variable and may increase or decrease with stress, with time, or with a combination of both. View our mixology terminology resource for definitions of related terms Pseudoplastic, Thixotropic, and Dilatant.

Most non-Newtonian fluids are shear thinning, which means the viscosity in the region of the mixing impeller decreases as the rotational speed of the impeller is increased.  As an example, the following chart depicts the viscosity curve for a specific shear thinning fluid.

 

 

The following video demonstrates the mixing characteristics of a non-Newtonian fluid. The setup utilizes the same lab scale tank, mixing impeller and operating speed as in the Newtonian fluid video. The non-Newtonian fluid in the tank is carbopol based.

 

 

The “apparent” viscosity for the non-Newtonian fluid in this video is represented by:

The apparent viscosity for this fluid, in the region of the impeller, is also 80cps.  But as demonstrated in the video, the impeller “cuts a hole” in the fluid with top to bottom circulation closer to the region of the impeller.  The material farther away from the impeller has poor circulation with little or no movement.

For More Information about Newtonian and New-Newtonian Fluids

A wide variety of online resources provide general information about Newtonian and non-Newtonian fluids including several YouTube videos. If you need assistance with Newtonian or non-Newtonian fluids as it relates to your industrial mixing application, email ProQuip at applications@proquipinc.com or call us at 330-468-1850.