cone-and-plate viscometer

A cone-and-plate viscometer measures viscosity by trapping a small sample between a flat plate and a shallow, slowly rotating cone whose tip just touches the plate. Spinning the cone drags the sample into flow; the instrument records the force (torque) needed to keep it turning at a chosen speed. The clever part is the geometry: the cone shape makes every part of the sample experience the same shear rate at once.

That uniform, well-defined shear rate is the whole point. Because you can set the rotation speed to give exactly the shear rate you want and read off the corresponding shear stress, you can sweep across a range of shear rates and build a complete rheogram. This is precisely what is needed to characterise non-Newtonian materials, distinguishing pseudoplastic from plastic from thixotropic behaviour.

Cone-and-plate instruments need only a small sample, reach equilibrium temperature quickly, and are easy to clean, making them the workhorse for creams, gels, ointments, and structured suspensions. Their limitations are the need for careful gap setting and the fact that very large suspended particles can be crushed in the narrow cone gap; for such samples a wider-gap geometry is used instead.