by Adrien Bourgoin
R&D Engineer at Nextflow Software

Isaac Newton assumed that the viscosity of a fluid can only be altered by varying its temperature. Chocolate melts much more easily when heated. Yet the viscosity of some fluids also depends on a stress applied to them. When the force is removed, the fluid returns to its original properties; we are talking about non-Newtonian fluid.

Shear thinning is the most common kind of non-Newtonian behavior of fluids. The viscosity decreases under shear strain: this happens with paint, glue, ketchup or even blood, which enables self-regulation of blood pressure. Conversely, the viscosity of a shear thickening material increases with the rate of shear strain. In other words, when a force is applied to this type of fluid, it becomes more and more viscous. The following video shows injection of 3 fluids, from left to right: shear thinning fluid, Newtonian fluid, and shear thickening fluid. Each fluid is defined with an identical base viscosity . It points out the flow variations occurring near the fluid impact on the rigid body.

For industrial applications, the flow of synthetic resins and glues raise drawing and nozzle issues. Motor designers have to ensure that a lubricant does not leak from gears while guaranteeing an effective action on all high-speed surfaces. Numerical simulations of such industrial processes involving polymers, paint, adhesive or sealing material require the use of the adequate viscosity constitutive equation (Carreau fluid, Cross fluid, Bingham fluid or others).

Illustrating video here