Transient convective effects on diffusion measurements in liquids

The transient effects of convective disturbances on self-diffusivity measurements are evaluated using numerical simulations. "Codastefano-type" diffusivity measurements, where continuous real-time measurements of the concentration of the diffusing species are made at two discrete locations using a radioactive tracer, are considered. Convection arises due to buoyancy caused by the interaction of temperature gradients with either gravity (terrestrial experiments) or residual acceleration (microgravitv experiments). For small deviations from ideal isothermal conditions, the system's response to residual microgravity disturbances is examined. The consequences of heating and cooling "temperature ramps" are also examined for both microgravity and terrestrial conditions. The frequency, amplitude, and orientation of the residual acceleration vector and the magnitude of heating or cooling rates all affect the fidelity of a given measurement. It is also shown that, for actual high-temperature experiments, it is important to assess the degree of isothermality that can be attained.