shear stress

Shear stress is the sideways force you apply to a fluid to make it flow, measured per unit of the area you are pushing on. Imagine spreading butter with a knife: the force of the knife sliding across the surface, divided by the area of contact, is the shear stress. It is the 'effort' side of the flow story.

More precisely, shear stress acts parallel to a fluid layer rather than squeezing it head-on. When you stir, pour, pump, or rub a product, you impose shear stress on it, and the fluid responds by flowing at some shear rate. The symbol is usually the Greek letter tau (τ), and the SI unit is the pascal (Pa), the same unit as pressure but applied tangentially.

In rheology, shear stress is one of the two fundamental quantities (the other being shear rate) whose relationship defines a material's flow type. Plotting shear stress against shear rate produces the rheogram, the characteristic 'fingerprint' that tells you whether a material is Newtonian, shear-thinning, shear-thickening, or plastic.