(149c) Anisotropic Nanoparticles Immersed in Nematic Liquid Crystals

Liquid crystals are finding increasing applications in a wide variety of fields. One example is the use of liquid crystals as tunable solvents for microemulsions and particle dispersions [1]. The inclusion of the particles induces elastic distortions in the liquid crystal, giving rise to long-range interparticle interactions that are absent when isotropic solvents are used. These interactions will depend on the size and shape of the colloids, the orientation of the liquid crystal bulk director field with respect to the particles, and the local ordering of the liquid crystal molecules at the surfaces of the particles. Distinct, controlled particle assemblies can be achieved by engineering these physical properties. Such systems have potential applications for development of new composite materials with improved physical properties [1]. Another potential application is the use of nanoparticles-liquid crystal systems in optical sensors for chemicals and biomolecules [2]. Analytes can perturb the local ordering of a liquid crystal around the nanoparticles, triggering the formation of inhomogeneous textures that can be detected by simply using a microscope and polarized light.

The main objects responsible for the response of a sensor, or the structure of a microemulsion or suspension, are the defects that arise around a specific nanoparticle or a collection of nanoparticles. Optimization of the applications mentioned above therefore requires a fundamental understanding of the structure and dynamics of topological defects around the particles, as well as the liquid crystal-induced interparticle interactions that arise due to elastic distortions. A number of studies have considered the case of spherical particles immersed in a nematic liquid crystal; in contrast, the case of anisotropic particles has received little attention. In this work [3], we report numerical calculations for the defect structures and potential of mean force (PMF) that arise when elongated, spherocylindrical nanoparticles are immersed in a nematic liquid crystal. A Landau-de Gennes free energy in terms of the tensor order parameter Q is used to represent the nematic solvent. Different configurations with one, two and three elongated nanoparticles with strong homeotropic anchoring are considered, and their relative stability is analyzed. The attractive interactions between elongated particles are also compared to those observed for nanometer-sized spheres of similar diameters [4]. The interparticle energies in arrays of spherocylinders were up to three times stronger than those observed for spherical particles of comparable diameters.

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[2] J. M. Brake, M. K. Daschner, Y. Y. Luk and N. L. Abbott, Science 302, 2094 (2003); R. R. Shah and N. L. Abbott, Science 293, 1296 (2001); V. K. Gupta, J. J. Skaife, T. B. Dubrovsky and N. L. Abbott, Science 279, 2077 (1998)

[3] F. R. Hung, O. Guzmán, B. T. Gettelfinger, N. L. Abbott and J. J. de Pablo, Phys. Rev. E, accepted (2006)

[4] O. Guzmán, N. L. Abbott and J. J. de Pablo, J. Polym. Sci. Pol. Phys. 43, 1033 (2005); O. Guzmán, E. B. Kim, S. Grollau, N. L. Abbott and J. J. de Pablo, Phys. Rev. Lett. 91, 235507 (2003)