Viscosity Measurement From Microscale Convection At High Pressure And Temperature

Measurements of induced thermal convection have been used to study fluid viscosity at simultaneous high pressure and temperature conditions. Direct observations of flow were made by tracking entrained particles in samples melted by laser heating during high-pressure confinement. Finite element models confirmed thermal convection as the origin of the detected motions, and were refined to assess the fluid viscosity. Observations of flow in ethanol partially melted in the laser-heated diamond anvil cell at 2-3 GPa point to a sharply rising viscosity at room temperature above the equilibrium solidification pressure, similar to that seen previously in methanol. The analysis shows that measurement of viscosity from convective flow in laser-heated fluids under static pressure is a promising strategy to determine viscosity at ultrahigh pressures, where high melting temperatures and small samples preclude application of traditional viscometric techniques. The data confirm theoretical predictions of detectable natural convection at ultralow Rayleigh numbers (Ra << 1) in a microscopic system having sufficiently large temperature gradients.