pascal second

The pascal second is what a rheometer prints, and its size makes it the unit of thick things. Water at 20 °C is 0.001 Pa·s, olive oil about 0.08, honey a few, and molten glass at working temperature around a thousand. Polymer melts entering an injection mould run from a hundred to ten thousand Pa·s, which is why mould filling is governed by viscosity long before strength enters the picture, and lubrication, coatings and food-texture work all specify in this unit or its thousandth, the mPa·s.

Its most famous measurement is the pitch drop experiment at the University of Queensland, running since 1927: a funnel of room-temperature pitch, a material that shatters under a hammer blow, has released nine drops in nearly a century, from which the viscosity comes out on the order of $10^{8}$ Pa·s, around a hundred billion times water. Solid and liquid are statements about timescale, and the viscosity in Pa·s is the number that sets the timescale.

Two things about the unit itself. It never received a name: the poiseuille was proposed for 1 Pa·s but no CGPM ever adopted it, so dynamic viscosity remains a workhorse quantity with a nameless SI unit. And it is the viscosity that shear stress wants, $\tau = \mu\,\dot{\gamma}$, the form the Hagen-Poiseuille and Stokes drag results are built on; the kinematic value in m²/s is this divided by density and must never be substituted for it.