Shear thinning

Classification of fluids with shear stress as a function of shear rate: Pseudoplastic, Bingham and Bingham pseudoplastic all show reduction in apparent viscosity with increasing shear rate.

Shear thinning is a term used in rheology to describe non-Newtonian fluids which have decreased viscosity when subjected to shear strain. The term is sometimes considered to be a synonym for pseudoplastic behaviour, and is usually defined as excluding time-dependent effects, such as thixotropy. Shear-thinning behaviour is generally not seen in pure liquids with low molecular mass, or ideal solutions of small molecules like sucrose or sodium chloride, but is often seen in polymer solutions and molten polymers, and complex fluids and suspensions like ketchup, whipped cream, blood, paint, and nail polish.

Relationship with thixotropy

Some authors consider shear-thinning to be a special case of thixotropic behaviour, because the recovery of the microstructure of the liquid to its initial state will always require a non-zero time. When the recovery of viscosity after disturbance is very rapid however, the observed behaviour is classic shear-thinning or pseudoplasticity, because as soon as the shear is removed, the viscosity returns to normal. When it takes a measurable time for the viscosity to recover, thixotropic behaviour is observed. When describing the viscosity of liquids, however, it is therefore useful to distinguish shear-thinning (pseudoplastic) behaviour from thixotropic behaviour, where the viscosity at all shear rates is decreased for some duration after agitation: both of these effects can often be seen separately in the same liquid.

Everyday examples

Modern paints are examples of pseudoplastic materials. When modern paints are applied the shear created by the brush or roller will allow them to thin and wet out the surface evenly. Once applied the paints regain their higher viscosity which avoids drips and runs.

Ketchup is a shear-thinning fluid, caused by the addition of a relatively small amount of Xanthan gum - usually 0.5%.

See also

• Non-Newtonian fluid
• Power-law fluid
• Thixotropy
• Dilatant
• Rheology
• Kaye effect

External links

• The Great Ketchup Mystery
• NASA - The Physics of Whipped Cream NASA April 25, 2008

References: