Profile refractometry is a laser-based technique for measuring the index of refraction of a fluid as a function of time and position in a fluid. The technique was developed for use in quantifying the Soret effect in a binary fluid subject to an applied thermal gradient. (The Soret effect is the mass diffusion of chemical species due to an imposed thermal gradient.) More precisely, profile refractometry enables measurement of both dynamic and steady-state local gradients in the index of refraction of the fluid. These gradients are related in a known way to gradients in the composition of the fluid and thus to the Soret coefficient.
Profile refractometry overcomes some of the weaknesses of a steady-state beam-deflection (SSBD) technique used heretofore to obtain index-of-refraction data for calculating the Soret coefficient. In the SSBD technique, one transmits a laser beam parallel to a thermal gradient in a fluid contained in a narrow gap. The SSBD technique yields only a single measure that is averaged over the applied temperature range. The SSBD technique does not yield spatially resolved data, and is limited to a very small applied thermal gradient because a large thermal gradient would refract the laser beam by more than the few milliradians allowed by the narrow gap geometry.
In profile refractometry, a wide laser beam with an initially planar wavefront is made to propagate along an axis perpendicular to a thermal gradient in a bulk fluid. Unlike in SSBD, the beam samples a cross section of the fluid, yielding spatially resolved index-of-refraction data for all positions of interest along the thermal gradient. There is no need to keep refraction angles small and thus no need to limit the applied thermal gradient because the laser beam is not obscured at any refraction angle. In addition, because profile refractometry samples bulk fluid, the spurious refraction caused by capillary action, surface tension, and edge effects is less than that in a fluid sampled within a narrow gap as in the SSBD technique.
The image formed by the refracted laser beam contains information on the continuous refraction profile over the entire span of the fluid. When corrected for thermal effects, this profile represents a continuous measure of the concentration gradient in the fluid at every point along the axis between the thermal boundaries.
When a thermal gradient is applied to a fluid, the Soret effect (if it occurs in that fluid) gives rise to a transient fluid phase in which molecular migration occurs. Under some conditions, there develops a steady-state fluid structure that contains a stable concentration gradient. Methods of analysis that take these effects into account, and software that implements these methods, have been developed to process refraction-profile data to obtain values for each of the terms that define the Soret coefficient. The end result of the Soret analysis performed by the software is a three-dimensional matrix that contains data on the Soret coefficient as a function of time, position in the fluid, and temperature. Depending on the experimental conditions, convection and dynamic effects may also be represented in the data.
This work was done by Larry W. Mason of Lockheed Martin for Marshall Space Flight Center. For further information, contact the company at (303) 971-9067.
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