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A Potter-Elvehjem homogenizer disrupts biological tissue between a fitted pestle and a surrounding vessel, often using a rotating drive. Flow and mechanical deformation in the small clearance transfer energy from the drive into the sample. ^[ref]

For an idealized narrow gap containing a Newtonian fluid, the shear rate scale is $U/\delta$, with relative surface speed $U$ and clearance $\delta$; viscous stress scales as $\eta U/\delta$. Actual tissue is more complex, but the relation explains why clearance, motion and dynamic viscosity affect disruption.