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The development and use of the Rayleigh interferometer to study molecular diffusion in an applied magnetic field Howell, Steven Kelley


The purpose of this work was to investigate the effects of an externally applied homogeneous steady magnetic field on liquid phase diffusion through a porous membrane. A physico-chemical model was developed to describe the effect of a magnetic field on magnetically anisotropic molecules diffusing through a porous membrane. An applied magnetic field is expected to cause a reduction in the diffusivity of an anisotropic molecular system. Orientation of the molecule in the magnetic field (Cotton-Mouton effect) will change the effective cross-sectional area of the molecule, increasing the viscous drag between the molecule and membrane pore surface thereby reducing the diffusion coefficient. An optical interferometric technique was used to measure diffusion coefficients which offered advantage over other methods since concentration profiles could be locally observed adjacent to the membrane surface without disturbing the diffusive flows. A Rayleigh interferometer was designed and constructed to be placed between the pole pieces of a 30 cm electromagnet. The diffusion of an aqueous sucrose solution through General Electric Nucleopore membranes (pore diameters 0.8 μm and 8.0 μm) was measured in applied field strengths from 0 to 12.5 kGauss. This combination of membrane and solution was selected for this initial work because of potential applications to biological systems and to verify the validity of the measurement technique since widely accepted diffusion data for this system are available in the literature for comparison. A computer program was developed to account for errors introduced by wavefront deflection in a refractive index gradient and to numerically calculate mass fluxes and diffusivities from interference fringe data. Within the limits of experimental error a slight decrease (1% to 2%) in the diffusion coefficient of sucrose through the membrane has been observed for applied magnetic fields up to 12.5 KG. Free diffusion coefficients measured at no field conditions compared to accepted values measured at identical concentration and temperature to within ± 3%. While these results indicate some alignment of the sucrose-water clusters in an applied magnetic is taking place, further work is needed to improve the accuracy of the experimental technique and to study molecules possessing a higher degree of anisotropy than sucrose and water. Recommendations are made for modifications to the diffusion cell which should significantly reduce experimental errors. Magnetically anisotropic molecular systems are also recommended for study which should show a greater degree of magnetic orientation than sucrose and water.

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