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.