During my Ph.D. I studied the thermal diffusion of molten salts (Silver/Potassium-Nitrate mixtures in a near eutectic composition) with holographic interferometry. Thermal diffusion is a mass transfer due to an external applied temperature gradient. The mass transfer leads to a concentration gradient causing diffusion in the opposite direction. The describing coefficient is the Soret-coefficient s. To determine the Soret-coefficient a sandwich cell is used in most cases. A sandwich cell has an upper and a lower (theoretical) infinite heating plate with a small measurement volume inbetween. The applied temperature gradient has the same direction as the gravitational accerleration, i.e. upper temperature higher than lower temperature.
The main problem of thermal diffusion measurements is natural convection. Natural convection occurs as soon as a temperature gradient perpendicular to the gravity vector is present. It is almost impossible to avoid horizontal temperature gradients when optical measurement methods are used, because windows are required for those techniques. The different thermal conductivities of cell walls, windows and investigated liquid produce horizontal temperature gradients and thus, convection.

In our work we tried to minimize convectional effects by heating the cell windows from the outside to the calculated average temperature of the measurement volume.
The experiments were carried out with real-time holographic interferometry by using a thermoplastic holographic camera. First a hologram of the isothermal state was taken, then a temperature gradient was applied. Establishing the steady state took about 24 hours with the used cell geometry. However, the thermal equilibrium was reached after few minutes already. A second hologram was taken after the thermal equilibrium was reached to a) determine the exact temperature gradient in the test cell and b) to seperate the interference pattern due to the temperature gradient from the interference pattern due to the concentration gradient. Knowing the dependence of the refractive indices of temperature and composition (these measurements were performed with a new developed setup for this purpose) is was possible to calculate the Soret-coefficient with the distance between the interference fringes due to the concentration gradient.