In rheology, shear thinning is the non-Newtonian behavior of fluids whose viscosity decreases under shear strain. It is sometimes considered synonymous with pseudo-plastic behaviour, and is usually defined as excluding time-dependent effects, such as thixotropy.

Shear thinning is the most common type of non-Newtonian behavior of fluids and is seen in many industrial and everyday applications. Although shear thinning is generally not observed in pure liquids with low molecular mass or ideal solutions of small molecules like sucrose or sodium chloride, it is often observed in polymer solutions and molten polymers, as well as complex fluids and suspensions like ketchup, whipped cream, blood, paint, and nail polish.

Though the exact cause of shear thinning is not fully understood, it is widely regarded to be the effect of small structural changes within the fluid, such that microscale geometries within the fluid rearrange to facilitate shearing. In colloid systems, phase separation during flow leads to shear thinning. In polymer systems such as polymer melts and solutions, shear thinning is caused by the disentanglement of polymer chains during flow. At rest, high molecular weight polymers are entangled and randomly oriented. However, when undergoing agitation at a high enough rate, these highly anisotropic polymer chains start to disentangle and align along the direction of the shear force. This leads to less molecular/particle interaction and a larger amount of free space, decreasing the viscosity.

At both sufficiently high and very low shear rates, viscosity of a polymer system is independent of the shear rate. At high shear rates, polymers are entirely disentangled and the viscosity value of the system plateaus at η_∞, or the infinite shear viscosity plateau. At low shear rates, the shear is too low to be impeded by entanglements and the viscosity value of the system is η₀, or the zero-shear-rate viscosity. The value of η_∞ represents the lowest viscosity attainable and may be orders of magnitude lower than η₀, depending on the degree of shear thinning.

Viscosity is plotted against shear rate in a log(η) vs. log(${\displaystyle {\dot {\gamma }}}$) plot, where the linear region is the shear-thinning regime and can be expressed using the Ostwald and de Waele power law equation:

The Ostwald and de Waele equation can be written in a logarithmic form:

The apparent viscosity is defined as ${\displaystyle \eta ={\tau \over {\dot {\gamma }}}}$ , and this may be plugged into the Ostwald equation to yield a second power-law equation for apparent viscosity:

This expression can also be used to describe dilatant (shear thickening) behaviour, where the value of n is greater than 1.