Playing with liquid crystal bubbles and Molecular wires from discotic liquid crystals

The experimental study of liquid crystal shells and droplets prepared by capillary microfluidics was pioneered in the David Weitz group at Harvard in the last decade [1,2]. Since then the technique has been adopted by a few groups—including ours—and complemented with computer simulations and theoretical efforts, together demonstrating that a very rich field of soft matter physics is opened by this new configuration for exploring liquid crystals [3-10]. In our own work we have focused particularly on the dynamic pattern formation during the nematic-smectic transition in shells under varying boundary conditions [6-7], on the pump-like actuation mechanism of nematic liquid crystal elastomer shells [8] and on the complex dynamic patterns forming from a large number of cholesteric shells or droplets arranging into a colloidal crystal arrangement [10]. In this talk I will give an update on these activities and also introduce some new directions, which further illustrate the wealth of interesting, and potentially applicable, phenomena that occur in liquid crystal shells and droplets.

References
[1] A. Fernandez-Nieves et al., Phys. Rev. Lett. 98, 087801 (2007)
[2] A. Fernandez-Nieves et al., Phys. Rev. Lett. 99, 157801 (2007)
[3] G. Skacej and C. Zannoni, Phys. Rev. Lett. 100, 197802 (2008)
[4] T. Lopez-Leon et al., Nat. Phys. 7, 391 (2011)
[5] S. Kralj et al., Soft Matter, 7, 670 (2011)
[6] H.-L. Liang et al., Phys. Rev. Lett., 106, 247801 (2011)
[7] H.-L. Liang et al., Soft Matter, 8, 5443 (2012)
[8] E. Fleischmann et al., Nat. Commun. 3, 1178 (2012)
[9] D. Sec et al., Phys. Rev. E, 86, 020705R, (2012)
[10] J. Noh et al., J. Mater. Chem. C, 2, 806 (2014)

J. H. Park¹ and G. Scalia^2
1Graduate School of Convergence Science and Technology, Seoul National University, Suwon, Korea
²Institute of Cybernetics, CNR, Pozzuoli (Napoli), Italy

Discotic liquid crystals are attractive organic semiconductors due to their unique self-organization behavior resulting in long range one-dimensional assemblies of pi-pi stacked molecules along which charge transport occurs. Due to the strong anisotropy of the columnar geometry it is very important to control the columnar alignment direction and the large scale uniformity of their orientation. Since structural defects strongly deteriorate the charge transport, their minimization is also fundamental for the use of the molecular wires.

We have realized very long and well aligned molecular nanowires from hexapentyloxytriphenylene (HAT5) discotic liquid crystals, resulting from the self-organization of the molecules in solvent. Very thin structured films were realized by simply spin coating the HAT5 solutions on different substrates. In general the interaction between discotic and solvent molecules strongly affects the final morphologies of the molecular assemblies after solvent evaporation. In particular, toluene allows the formation of very long molecular nanowires. The chemical structure of the solvent appears to have an important role contributing in the self-assembly by entering in the final structure, matching parts of HAT5 molecules, as also suggested by Raman spectroscopy investigations. In addition, the evaporation rate of the solvent, but also the surface properties of the substrates, play an important role in the formation of the wires. Optical, atomic force microscopy and polarized Raman spectroscopy were performed for elucidating the structural organization of the molecular wires on substrates. In summary, the final organization of the discotic molecules is a result of the combined action of the spontaneous liquid crystal self-assembly and external factors, such as solvent molecules. Thanks to this interplay, it becomes possible the tuning of the structural assembly of the molecules and the formation of well ordered structures for efficient charge transport.