Viscosity of Newtonian and Non-Newtonian Fluids

VISCOSITY OF NEWTONIAN AND NON-NEWTONIAN FLUIDS

Viscosity describes a fluid’s resistance to flow and is a fundamental material property in any system involving fluid motion. However, fluid characterization goes far beyond a single viscosity value. Each fluid responds differently to applied shear stress, shear rate, deformation, and flow history. As a result, viscosity must be evaluated as a function of operating conditions relevant to the application.

Based on how viscosity changes with shear rate, fluids are broadly classified as Newtonian or non-Newtonian

As a result:

Shear stress increases linearly with shear rate
Viscosity remains constant over the entire shear rate range
Viscosity is typically dependent only on temperature

Examples of Newtonian Fluids

Common Newtonian fluids include:

Water
Organic solvents
Low-molecular-weight oils
Honey

A plot of shear stress versus shear rate produces a straight line, showing a linear
increase in stress with increasing shear rates with the slope equal to the fluid’s viscosity (see Figure 1). Similarly, a viscosity versus shear rate plot shows a constant value regardless of shear rate (see Figure 2)

Bingham plastics are non-Newtonian fluids that require a minimum yield stress before flow begins. Once this yield stress is exceeded, they exhibit an approximately linear relationship between shear stress and shear rate, resembling Newtonian behavior. Common examples include mayonnaise, toothpaste, and ketchup. Although their post- yield flow is linear, they are still classified as non-Newtonian due to the presence of yield stress.

Molecular Origins of Newtonian Behavior

Newtonian fluids are typically composed of small, isotropic molecules (symmetric in shape and properties) that do not align under flow. However, Newtonian behavior can also be observed in dilute polymer or protein solutions, particularly at low shear rates. In these cases, a low shear viscosity plateau may be observed before non Newtonian effects appear at higher shear rates.

Non-Newtonian Fluids

Most real-world fluids exhibit non-Newtonian behavior, meaning their viscosity depends on shear rate (Shear Thinning or Thickening). Unlike Newtonian fluids, non-Newtonian fluids may display a non-linear relationship between shear stress and shear rate (see Figure 1).

Shear Thinning and Shear Thickening

Shear thinning fluids decrease in viscosity as shear rate increases. These
are extremely common in industrial and biological systems.

Examples include:

Monoclonal Antibodies & Protein solutions
Ketchup
Paints and coatings
Polymer solutions
Blood

Shear thickening fluids increase in viscosity with increasing shear rate. A classic example is a suspension of cornstarch in water, which behaves like a solid under sudden stress but flows when shear is reduced. You have probably seen examples on TV or online where people can run across this type of solution, yet they sink if they stand still.

Non-Newtonian behavior in fluids arises from microstructural changes that occur under flow. In polymer solutions, shear flow can stretch and align the long, anisotropic polymer chains in the direction of deformation, which typically reduces resistance to flow and leads to shear thinning behavior. In colloidal suspensions, shear thinning arises when hydrodynamic forces dominate over Brownian motion and interparticle forces, causing flow induced microstructural rearrangements that reduce viscosity.

Why it Matters.

Fluid flow behavior strongly depends on viscosity. For non-Newtonian fluids, however,
viscosity is not a single constant value. It depends on the applied shear rate or
deformation history. As a result, the velocity profile in a channel or pipe can differ significantly depending on whether the fluid is Newtonian, shear-thinning, or shear-thickening. Looking at Figure 3, you can observe three very different velocity profiles depending on the fluid behavior. In pressure-driven flow, the shear rate is highest near the walls and lowest at the centerline. For all these fluids, the shear rate at the walls (i.e. the slope of the velocity profile near the wall) is going to determine viscosity. For non-Newtonian fluids, the local viscosity varies with this local shear rate, which in turn alters the overall velocity distribution and flow resistance. Proper rheological characterization is therefore essential to determine whether a fluid behaves as Newtonian or non-Newtonian and to identify the relevant shear-rate range for a given application.

Many conventional viscometers report a single apparent or “index” viscosity at an unspecified or poorly defined shear rate. However, accurate modeling and process design require measurements of true (absolute) viscosity as a function of well-defined rate. Reliable viscosity characterization must therefore be performed over the shear-rate range relevant to the intended process conditions. Learn more about RheoSense viscometers and how they allow measurements of true viscosity over a wide range of shear rates.

Viscosity of Newtonian and non-Newtonian Fluids