Thin films and interfaces

The properties of matter in a reduced geometry like thin films and interfaces drastically differ from the properties in the bulk state. The physics of such system is fascinating. In our research we investigate properties of freely suspended smectic films with a thickness of several molecular layers. Such films present an excellent system to study behaviour and phase transitions in quasi two dimensions. The projection of the director (local mean orientation of the molecular long axis) on the smectic layer defines a vector order parameter c(x,y) which describes the deformational state of the system. The orientation of c(x,y) can be experimentally determined using polarising microscopy. Due to anisotropy of the liquid crystalline order, different types of deformations of the director field are described by several elastic constants given by the symmetry of the phase. Additional symmetries and couplings of the order parameter to the external field or the geometric constrains given by the boundary of the film may considerably complicate the system. The competition between different interactions often favours incompatible ground states and leads to frustrated states. In those states the order parameter is not homogeneous any more, but follows a pattern. One of the most interesting elements which initiate pattern formation in freely suspended films are chiral symmetry and polarity. These two properties often result in formation of striped patterns of the director field (striped phases). Several examples in bent core LCs are formation of labyrinthine patterns of the layer dislocations and strings. A bit more complicated and fascinating example of pattern formation caused by chirality is a so called “Dark Conglomerate Phase” which structure is similar to a structure of a sponge. In this case, the modulation of the director field is coupled to the curvature of the smectic layers. In such phases not only do the layer fragments play a role of the building blocks but also the topological defects which are organized in a periodic fashion.
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Thick film in a polar smectic-C phase
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Transition into a higher ordered smectic phase.
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Modulated ferroelectric smectic-C phase

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