shear thinning

Some fluids get runnier the harder you stir them. Ketchup will not budge from the bottle until you shake it, then suddenly gushes; paint is thick on the brush but spreads thin under a stroke. Ceramic slurries do the same: their viscosity is high when barely disturbed but drops as you shear them faster. This is shear thinning, also called pseudoplasticity — the opposite of the (rarer) shear-thickening fluids that stiffen when you hit them.

It happens because fast shearing breaks up and lines up whatever structure the particles have built. At rest, or under gentle stirring, weakly attracting particles link into loose flocs and open networks that trap liquid and resist flow, so the slurry seems thick. Shear it hard and those flocs are torn apart, the freed liquid lubricates, and elongated or plate-like particles swing around to line up with the flow — all of which lets it slide more easily, so the measured viscosity falls. Push the shear rate higher and the viscosity keeps dropping toward a lower limiting value; ease off and, if the effect is purely shear-driven, it recovers the instant you stop. A common way to describe it is a power law, eta proportional to (shear rate)^(n-1) with n less than 1, so the bigger the exponent gap below 1, the more strongly it thins.

Shear thinning is a gift for ceramic processing, because it lets one slurry be two things at once. In tape casting the slip must flow thin under the high shear beneath the doctor blade so it spreads into a smooth even sheet, then thicken again the moment it passes out from under the blade so it holds its shape and does not run off the carrier. Extrusion and screen printing rely on the same behaviour: fluid while being forced through the die or mesh, stiff again as soon as the force stops. It is closely tied to yield stress and thixotropy, and formulators deliberately tune the degree of flocculation to dial in just how much a slurry thins under shear.