The linear stability analysis of a layer of a magnetic fluid with a deformable free surface, which is heated from below and exposed to a uniform, vertically applied magnetic field is presented. In this configuration the temperature dependence of the surface tension, the buoyancy and the focusing of the magnetic field due to surface fluctuations act as destabilising effects.We show that this system has for thin layers a stationary codimension-2-point, which can be reached for experimentally relevant values of the material parameters.We also analyse the transition from thin to thicker layers for which there is no codimension-2-point and we show how the codimension-2-point disappears.Finally we demonstrate that there is no oscillatory instability in the regions of parameter space considered here.1.

The dynamics of two different side-chain liquid crystalline elastomers (SCLCE) exhibiting a smectic A phase is investigated by low-frequency shear and compression experiments. We find that both the isotropic and the smectic A phase have their own characteristic viscoelastic behavior, which seems to be independent of the compound under investigation and thus universal. The relaxation in the isotropic phase shows a distribution of relaxation times, which gives rise to a scaling law of the elastic moduli with an exponent of 0.5. The viscoelastic response in the smectic A phase, however, exhibits a much broader spectrum of relaxation times containing very long-lived modes. It appears that there exists a low-frequency scaling law with an exponent of 0.3 characterizing the smectic A phase. We propose that this result stems from a transient smectic network with a very long lifetime that was set up by the smectic domains. At the phase transition, this transient smectic network disappears, which leads to a sharp decrease of the dynamic shear as well as the compression modulus.

We investigate the rheological properties of polydomain smectic A Side-Chain Liquid Crystalline Elastomers (SCLCE) by dynamic shear and compression measurements and find a very similar behavior for both experiments. We show that the dynamic shear modulus G is independent of a precompression applied to the sample. In addition, we present the first dynamic measurements of the anisotropy of G observed for the corresponding Liquid Single Crystal Elastomers (LSCE). We find that these monodomains show dynamically a dramatic difference depending on whether the shear is in a plane parallel or perpendicular to the layer normal, demonstrating the in-plane fluidity of the smectic layering.

As has been shown recently by Kundler and Finkelmann, a sample of nematicliquid-single-crystal elastomers subject to a mechanical stress perpendicular to the initial director orientation shows a reorientation of the director reminiscent of that observed in lowmolecular-weight nematic liquid crystals in a magnetic field. Here we present a simple model, which captures all the essential features. We calculate the threshold stress and show that the director reorientation occurs over the entire sample. A weakly nonlinear analysis gives a forward bifurcation in agreement with the experimental results. We also discuss the origin of the domain walls in the director observed experimentally and give an expression for the thickness of these walls.