= Couette rheometer
{c}
{title2=$\mathcal T=4\pi\eta L\Omega R_i^2R_o^2/(R_o^2-R_i^2)$}
Concentric cylinders impose shear while <torque> and <rotation measure> a material's response. The displayed calibration holds for Newtonian laminar flow, inner-cylinder <rotation> and a stationary outer cylinder. In a narrow gap, the rate is approximately $\Omega R_i/(R_o-R_i)$ and inner-wall <stress> is $\mathcal T/(2\pi LR_i^2)$. End effects, <wall slip> and inertial <secondary flow> require control.
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