Measurement of the viscosity coefficient eta3 in free-standing smectic films.

Thinner parts (the so-called holes) and thicker parts (islands) move in free-standing smectic films due to gravity if the film is inclined. The velocity of the movement depends on the viscosity coefficient eta3 of the film. Therefore, this measurement is a direct method to determine eta3 in smectic phases. This paper contains the corresponding hydrodynamic calculations for the flow patterns in and around a circular hole in a circular two-dimensional film under various conditions. The viscous force on the hole is calculated. Experiments are performed with the liquid crystal n-octyl-cyano-biphenyl (8CB) in the smectic phase. The influence of the disclination which surrounds the hole and the influence of the gas which surrounds the film on the movement is studied. It is shown that the influence of these effects on the determination of eta3 can be neglected if the film is thick and the difference in the thicknesses between film and hole is small. The temperature dependence of eta3 is determined for 8CB.