Simulation of cholesteric systems in bulk and in confined settings using a coarse-grained model on pitch level
Final Report Abstract
Liquid crystals are materials that exhibit phases between the typically known liquid and crystalline phases. This intermediate state is characterized by a gradual transition from a disordered, liquid-like arrangement of molecules to a more ordered, crystalline structure. The different liquid crystalline phases display distinct behaviors (e.g., with respect to fluidity, conductivity, or interaction with light). The formation of liquid crystalline phases is primarily attributed to the anisotropic properties of the constituent particles, which may arise from their shape or their interactions with one another and with external fields. If the particles are not only anisotropic but also of a chiral nature, a so-called cholesteric phase can form 1. This phase was one of the first liquid crystal phases that was found because the material appears in specific or iridescent colors. The color depends on the pitch of the cholesteric helix, by which the ordering of the molecules is described: The molecules align with their neighbors but rotate on a larger scale around an axis; a full rotation determines the pitch length, and, with that, the characteristic optical properties. How the molecular properties translate to the macroscopic liquid crystalline properties is still not fully understood. With our project, we wanted to study this gap in further detail and develop a coarsegrained model on the cholesteric pitch level. Coarse-graining involves simplifying the description of a system by grouping individual atoms or molecules into larger units, thus facilitating more efficient simulations – our primary research tool. This method enables the observation of material behavior on larger length scales compared to traditional fine-grained models. We found that previous models for an intermediate coarse-graining step can be combined and made more general and flexible. With the right combination of parameters, different phase behavior can be achieved. For the coarse-grained model on the pitch level, we envisioned using prisms that contain the pitch information and form liquid crystalline phases themselves. Based on a previous study about concave twisted prisms2, we chose a similar model and studied the phase behavior of convex twisted prisms. Various phases were found depending on the geometry of the prisms‘ cross-section. The overall phase behavior, however, is determined by the prisms‘ effective aspect ratio, that changes upon twisting. With these studies and observations, we took a few steps towards perfect control of (cholesteric) liquid crystalline systems.
Publications
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Excluded volume interactions and phase stability in mixtures of hard spheres and hard rods. Physical Chemistry Chemical Physics, 24(19), 11820-11827.
Opdam, Joeri; Gandhi, Poshika; Kuhnhold, Anja; Schilling, Tanja & Tuinier, Remco
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Phase behavior of the generalized chiral Lebwohl-Lasher model in bulk and confinement. Physical Review E, 105(5).
Elsässer, P. & Kuhnhold, A.
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Structure of nematic tactoids of hard rods. The Journal of Chemical Physics, 156(10).
Kuhnhold, Anja & van der Schoot, Paul
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The effect of particle geometry and initial configuration on the phase behavior of twisted convex n-prisms. Soft Matter, 20(27), 5351-5358.
Gandhi, Poshika & Kuhnhold, Anja
