Systems of particles immersed in nematic liquid crystals (NLCs) have potential applications for development of composites, nanostructured materials and colloidal crystals [1,2,3]. The inclusion of colloids induces elastic distortions in the NLC, giving rise to long-range, strong interparticle interactions (up to several thousands of kT) that can induce the formation of a number of ordered colloid structures. In this work, we use computer simulations to predict the behavior of systems of one or several nanoparticles in NLCs, when they are close to or confined in nanochannels of different geometries. Micron-sized channels have been used recently to elongate DNA molecules in genomic analysis [4], and to induce reversible changes in the topological defects of NLC droplets in water [5]. The aim of our work is to explore the use of nanochannels and NLCs to manipulate nanoparticles (as opposed to the micron-sized particles typically used in experiments) and direct their assembly into controlled, ordered structures. Experimental work for these nanoscale systems is expected to face more difficulties as compared to their micron-sized counterparts. Therefore we turn to computer simulation as a way to explore and obtain rapid predictions of the behavior of systems of nanoparticles in NLCs close to nanochannels. We use a continuum mesoscale theory in terms of the tensor order parameter Q to model the nematic solvent. Systems containing one and three nanoparticles with strong homeotropic anchoring are considered. Our results [6] suggest that strong NLC-mediated interactions between the particles and the side-walls of the channels, on the order of hundreds of kT, arise when the colloids are inside the channels. The magnitude of the channel-particle interactions is dictated by a combination of two factors, namely the type of LC defect structures that develop when a nanoparticle is inside a channel, and the degree of ordering of the nematic in the region between the colloid and the nanochannel. The channel-particle interactions become stronger as the nanoparticle diameter becomes commensurate with the nanochannel width. Nanochannel geometry also affects the channel-particle interactions; among the different geometries considered, a cylindrical channel seems to provide the strongest interactions. Our calculations suggest that small variations in geometry, such as removing the sharp edges of the channels, can lead to important reductions in channel-particle interactions. Our calculations for systems of several nanoparticles indicate that linear arrays of particles with Saturn ring defects, which are not stable in a bulk system, can be stabilized inside the nanochannels. This result suggests that nanochannels and NLCs could be used to direct the assembly of nanoparticles into ordered arrays with unusual morphologies. Experiments are currently underway to determine the validity of the predictions put forth in this work. [1] P. Poulin, H. Stark, T. C. Lubensky and D. A. Weitz, Science 275, 1770 (1997); P. Poulin, V. Cabuil and D. A. Weitz, Phys. Rev. Lett. 79, 4862 (1997); J. C. Loudet, P. Barois and P. Poulin, Nature (London) 407, 611 (2000); J. C. Loudet, P. Barois, P. Auroy, P. Keller, H. Richard and P. Poulin, Langmuir 20, 11336 (2004); [2] I. I. Smalyukh, S. Chernyshuk, B. I. Lev, A. B. Nych, U. Ognysta, V. G. Nazarenko and O. D. Lavrentovich, Phys. Rev. Lett. 93, 117801 (2004); I. I. Smalyukh, O. D. Lavrentovich, A. N. Kuzmin, A.V. Kachynski, and P. N. Prasad, Phys. Rev. Lett. 95, 157801 (2005) [3] I. Muševic, M. Škarabot, U. Tkalec, M. Ravnik and S. Žumer, Science 313, 954 (2006) [4] Y.-L. Chen, M. D. Graham, J. J. de Pablo, K. Jo and D. C. Schwartz, Macromolecules 38, 6680 (2005); K. Jo, D. M. Dhingra, T. Odijk, J. J. de Pablo, M. D. Graham, R. Runnheim, D. Forrest and D. C. Schwartz, Proc. Natl. Acad. Sci. U. S. A. 104, 2673 (2007) [5] A. Fernández-Nieves, D. R. Link, M. Márquez and D. A. Weitz, Phys. Rev. Lett. 98, 087801 (2007) [6] F. R. Hung, B. T. Gettelfinger, G. M. Koenig Jr., N. L. Abbott and J. J. de Pablo, J. Chem. Phys., accepted (2007)

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