The diamond-anvil cell as a high-pressure viscometer

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Abstract
A high‐pressure viscometer based on a diamond‐anvil pressure cell is described and its use demonstrated. A sphere (typical diameter 50 μm) rolls down a diamond surface under the force of gravity and its position is tracked electronically. The resulting velocity is converted to viscosity through a modified Stokes equation that includes a term describing the increased drag due to the close proximity of the walls within the pressure chamber. A model for this wall correction is described and that the drag coefficient is constant throughout the data collection is demonstrated. The precision of the resulting viscosity data averages 4%. The accessible viscosity range is 0.5–107 cP using a diamond cell that is capable of pressures exceeding 10 GPa. However, most fluids exceed 107 cP at far lower pressures. The accuracy of the device is demonstrated by comparing the presented data on the pressure dependence of the viscosity for five fluids [chloroform, water, octamethyltrisiloxane, 1‐pentanol, and bis(2‐ethylhexyl)sebacate] with previous measurements.

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