Powered by Indico
v3.2.8
Recent advances in microfluidic techniques has allowed the production of thin liquid crystal (LC) shells suspended in and containing isotropic liquid phases, displaying a number of unique soft matter physics phenomena that are deeply fascinating as well as useful [1]. By selecting the compositions of the inner and outer phases we can tune the liquid crystal alignment at the inner and outer boundaries, thereby controlling the topological constraints imposed on the LC on the two sides. Planar alignment in combination with spherical topology forces the LC to develop defects in the director field, similar to the poles of the meridional field on the earth. However, while the earth’s poles have integer topological defect strength, the LC can, in case of planar alignment on both sides, also split each integer defect into two s = +1/2 defects or - in principle - merge the two integer defects into a single s = +2 defect, leading to shells with a total of 1, 2, 3 or 4 defects depending on the geometric details of the shell [1]. If, on the other hand, only one side is planar while the other is homeotropic, then a bent director field develops in the nematic phase which breaks director sign invariance and thus forbids non-integer defects [2]. In our research we have paid particular attention to the time evolution of the defect configuration as a nematic phase is cooled into a smectic state for hybrid [2] as well as uniformly planar [3] alignment. We find a number of complex textures in the smectic phase as well as interesting pretransitional phenomena in the nematic phase (top series of photos) which can be explained by considering the peculiar elasticity of the nematic phase and the evolution of the three main elastic constants on cooling towards a smectic state. Since the defects are high energy points that could anchor appropriately designed linker molecules, the LC shells may also provide a path to developing particles that self-assemble into colloidal crystals with tetravalent symmetry (thus forming a colloid scale analog to sp3-hybridized carbon in diamond), highly desirable for photonic crystals but very difficult to realize, [4]. While this application is still quite far in the future, we recently demonstrated that polymerized LC elastomer shells can be produced with high throughput using the microfluidic process, and that these shells can act like small self-assembled one-piece “micro hearts” that upon induction of the nematic-isotropic phase transition can pump a liquid (bottom figure), useful e.g. for the development of 3D microfluidic devices [5].
References
[1] A. Fernandez-Nieves et al. Phys. Rev. Lett., 99, 157801 (2007); T. Lopez-Leon, V. Koning, K.B.S. Devaiah, V. Vitelli, & A. Fernandez-Nieves, Nat. Phys., 7, pp. 391-394 (2011)
[2] H.-L. Liang, R. Zentel, P. Rudquist, J. Lagerwall, Soft Matter, 8, p. 5443 (2012)
[3] H.-L. Liang, S. Schymura, P. Rudquist, J. Lagerwall, Phys. Rev. Lett. 106, 247801 (2011)
[4] D.R. Nelson, Nano. Lett., 2, 1125 (2002)
[5] E. Fleischmann et al., Nat. Commun., 3, DOI: 10.1038/ncomms2193 (2012)