shear rate

Shear rate describes how fast one layer of a fluid is sliding past the next. Picture a deck of cards lying flat: if you push the top card so the whole stack fans out, the difference in speed between neighbouring cards, spread over the thickness of the deck, is the shear rate. It is the 'how fast it flows' side of the story, the response to applied shear stress.

Formally, shear rate is the velocity gradient across the flowing layers, with the symbol gamma-dot and units of reciprocal seconds (per second, s⁻¹). A high shear rate means rapid, vigorous flow; a low shear rate means slow, gentle movement.

Different pharmaceutical operations occur at wildly different shear rates, and this is the whole reason shear rate matters. Gentle settling in a bottle happens near zero shear; pouring is moderate; rubbing a cream into skin or forcing a liquid through a syringe needle reaches very high shear rates. For a non-Newtonian product, viscosity changes across this range, so the relevant shear rate must always be considered when predicting real-world behaviour.