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Liquid-crystal-mediated self-assembly at nanodroplet interfaces

Abstract

Technological applications of liquid crystals have generally relied on control of molecular orientation at a surface or an interface1,2. Such control has been achieved through topography, chemistry and the adsorption of monolayers or surfactants2,3. The role of the substrate or interface has been to impart order over visible length scales and to confine the liquid crystal in a device. Here, we report results from a computational study of a liquid-crystal-based system in which the opposite is true: the liquid crystal is used to impart order on the interfacial arrangement of a surfactant. Recent experiments on macroscopic interfaces have hinted that an interfacial coupling between bulk liquid crystal and surfactant can lead to a two-dimensional phase separation of the surfactant at the interface4, but have not had the resolution to measure the structure of the resulting phases. To enhance that coupling, we consider the limit of nanodroplets, the interfaces of which are decorated with surfactant molecules that promote local perpendicular orientation of mesogens within the droplet. In the absence of surfactant, mesogens at the interface are all parallel to that interface. As the droplet is cooled, the mesogens undergo a transition from a disordered (isotropic) to an ordered (nematic or smectic) liquid-crystal phase. As this happens, mesogens within the droplet cause a transition of the surfactant at the interface, which forms new ordered nanophases with morphologies dependent on surfactant concentration. Such nanophases are reminiscent of those encountered in block copolymers5, and include circular, striped and worm-like patterns.

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Figure 1: Model of the nematic LC droplet.
Figure 2: Representative configurations of nanodroplets.
Figure 3: Director contour lines and defect core as a function of temperature, corresponding to a surfactant concentration of x surf = 0.5.
Figure 4: Phase diagram showing the ordered patterns adopted by the surfactant at the droplet interface, as a function of temperature and composition.

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Acknowledgements

The continuum analysis of liquid crystal interfaces and the development of the corresponding theory were supported by the Department of Energy, Basic Energy Sciences, Biomaterials Program (DE-SC0004025). The calculations of surfactant organization at nanodroplet interfaces were supported by the National Science Foundation (DMR-1121288). The calculations reported here were performed on computational facilities supported by the National Science Foundation (DMR-1121288).

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J.A.M.-R. performed the molecular dynamics and Monte Carlo simulations presented in this work. E.J.S. provided critical analysis of droplet structures. N.L.A. was involved in study design. J.P.H.-O. performed the continuum calculations presented in this work. J.J.d.P. designed the study, analysed data, and wrote the paper. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to J. J. de Pablo.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data 1- 4, Supplementary References and Supplementary figures 1-7. (PDF 4772 kb)

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Moreno-Razo, J., Sambriski, E., Abbott, N. et al. Liquid-crystal-mediated self-assembly at nanodroplet interfaces. Nature 485, 86–89 (2012). https://doi.org/10.1038/nature11084

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