The temperature dependence of the thickness of thick free-standing films is studied using a high-resolution film thickness measurement technique. A small discontinuity in the temperature dependence of the smectic layer thickness at every phase transition between ferro-, ferri-, and antiferroelectric phases is observed. We show that the major contribution to it arises from a change in the smectic tilt angle.

Symmetry arguments show that a flow of matter through a chiral medium can induce a collective molecular rotation. We report the study of such a linear coupling between a gas flow and molecular motion in thick free standing films of SmC* liquid crystal. This effect could be used to develop liquid crystalline micromotors potentially useful for microfluidics and micromechanics. Typical evolving target patterns have been recently observed by Tabe et al. [1] in a Langmuir monolayer of chiral liquid crystal molecules deposited on glycerol. We designed a more robust system composed of a free standing film of chiral molecules, without any liquid substrate. The flow of gas through the film is tuned by controlling the gas chemical potential. Direct pattern observation needs no Brewster angle microscope and is done under optical polarizing microscope. The whole set-up allows a quantitative study of the flow/rotation coupling phenomenon. The system shows a very rich spatio-temporal behavior. Here we focus on patterns obtained in a film suspended on a circular frame. In this geometry, a steady-state is attained where a spiral pattern is formed (see fig.1). We obtain the set of equations which govern the dynamics of the film under the gas flow. In a particular case, the equations involve only the c-director dynamics [2]. In the steady-state regime, they reduce to the competition of the elastic torque and an external torque induced by the flow. The observed static spiral patterns confirm the existence of such an external torque.

We present the X-ray diffraction, polarizing optical microscopy and electrooptic response studies of the chiral liquid crystal siloxane DSi3-MR11 [1] with a de Vries-type behavior. In a wide temperature range (b.Delta T sim 50°C) the material shows no layer shrinkage but a weak nearly linear layer spacing variation with temperature. Neither discontinuity nor slope change occurs at the SmA*-SmC* transition. In addition, using the X-ray data, the unusual electrooptic response in both 1st and 2nd harmonics together with known thermal behavior of birefringence, we evidence the cross-over from the classical orthogonal to the de Vries-like SmA* regime in DSi3-MR11 siloxane.

The role of dislocations in the deformation or flow of lamellar-phase liquid crystals is critically examined in the light of two studies. In the first, dislocation climb is directly observed under a compression normal to the layers. Two regimes of climb are evidenced. In the second study, lamellar phases are sheared in a rotating rheometer. Gliding screw dislocations carry a vorticity tensor component, which counterbalances plastically the applied vorticity. The similarities and differences between dislocation processes in solid and lamellar liquid crystals are discussed.