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Memory and topological frustration in nematic liquid crystals confined in porous materials

Abstract

Orientational ordering is key to functional materials with switching capability, such as nematic liquid crystals and ferromagnetic and ferroelectric materials. We explored the confinement of nematic liquid crystals in bicontinuous porous structures with smooth surfaces that locally impose normal orientational order on the liquid crystal. We find that frustration leads to a high density of topological defect lines permeating the porous structures, and that most defect lines are made stable by looping around solid portions of the confining material. Because many defect trajectories are possible, these systems are highly metastable and efficient in memorizing the alignment forced by external fields. Such memory effects have their origin in the topology of the confining surface and are maximized in a simple periodic bicontinuous cubic structure. We also show that nematic liquid crystals in random porous networks exhibit a disorder-induced slowing-down typical of glasses that originates from activated collisions and rearrangements of defect lines. Our findings offer the possibility to functionalize orientationally ordered materials through topological confinement.

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Figure 1: Patterns of DLs in porous media with various topologies.
Figure 2: Memory effects in NLCs confined in a bicontinuous cubic porous medium (BC).
Figure 3: Relaxation of the nematic order Q after turning off the field.
Figure 4: ℓ and T dependencies of the parameters characterizing the order-parameter relaxation.
Figure 5: Snapshots of defect lines (red lines) in RPM at selected times during their relaxation after removal of the field.
Figure 6: Functionalization of NLC by confinements for memory applications.

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Acknowledgements

We thank C. P. Royall for a critical reading of the manuscript. This work was supported by a Grant in Aid from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and by the Cariplo Foundation (grant 2008-2413), Italy.

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T.B. and H.T. conceived the project, T.A. carried out numerical simulations, M.B. carried out experiments and all authors analysed the data and wrote the manuscript.

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Correspondence to Tommaso Bellini or Hajime Tanaka.

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The authors declare no competing financial interests.

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Araki, T., Buscaglia, M., Bellini, T. et al. Memory and topological frustration in nematic liquid crystals confined in porous materials. Nature Mater 10, 303–309 (2011). https://doi.org/10.1038/nmat2982

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