Our research is focused on  smectic liquid crystal systems, mainly chiral antiferroelectric liquid crystal compounds and their mixtures. Liquid crystals are systems that have symmetries intermediate between the crystalline solid and the isotropic liquid. Thus they are often called "Partially ordered systems".

"Smectic" comes from the Greek word "σμηγμα", which means soap. Indeed smectic liquid crystals have layered structure just like soap solution. 


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RL12F

c-alphaThese systems exhibit a rich array of phase behavior, as a result of delicate competition between different kinds of interactions which have comparable strength as thermal energy kBT.

It has been a long puzzle as why these different phases appear, and also why they are observed in the particular sequence. This problem is the center theme of our research.




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Surface induced ordering effect is commonly observed in thermotropic liquid crystals. Usually the surfaces are found to be in a more ordered state than the interior/bulk material in soft material systems, contrary to what is common observed in solid state, i.e. surface melting.

Showing in the figure to the right is a cartoon representing a 4-layer smectic film with the two outermost layers in the synclinic SmC arrangement, while the interior remains in the uniaxial SmA packing.

In free standing film geometry, we are able to obtain films with thickness ranging from 2 to several hundred molecular layers, allowing us to study carefully the effect of surface in smectic liquid crystals.



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s12f

banana
Bent core molecules have attracted a considerable amount of research efforts since the discovery of their ability to form chiral structures without chirality on the molecular level. They have high spontaneous polarization values and are great candidates for future display applications.

However, in this type of materials the interactions are even more complex; resulting known at this time 7 bent core phases (Heated debate remains on whether some of them should be called "phases".).

Some of the bent core phases are smectic, thus allowing us to study their properties on our in-house systems, i.e., the structure of B2 phase and the biaxial SmA phase.



                                                               

Phenomenological models are very useful in helping us understand the different chiral smectic phases. Although they do not provide microscopic mechanism for the interactions, by assuming some types of interactions exist, and use the phenomenological free energy expansion, we obtain some key ideas about the conditions for different phases to be stabilized. Moreover, the resulting phase diagram can provide a valuable guide for future experiments.

Figure to the right is the phase diagram from our molecular dynamics program. The model we use treats each smectic layer as a spin. Including up to 3rd nearest neighbor interaction, a chiral interaction and a steric term, we are able to establish all the smectic C* variant phases in the phase diagram with a physical sound set of parameters.


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phase diagram