Résumé:

In a seminal work, [1] D. R. Nelson proposed that nematic coatings could be used to promote localized functionalization on colloidal particles. Topological defects are indeed necessarily present on genus-0 surfaces and it could be possible to attach ligands to them. Such a realization, however, still remains an experimental challenge. As a first step, the fabrication of thin nematic shells from double emulsions drops was reported a few years ago in Ref. [2]. Since then, the equilibrium structure of the topological defects on a sphere has been studied in details both theoretically and experimentally. It not only depends strongly on the nematic elastic constants [3] but also on the homogeneity of the shell thickness [4-5]. The defects structure on a nematic shell can therefore be finely controlled. As a next step, we have recently examined how a small number of solid particles can be trapped on nematic shells. We have used water/5CB/water double emulsions produced by microfluidics techniques. The anchoring of the nematic at the water-5CB interface is strongly planar due to the presence of polyvinyl alcohol in aqueous solutions. The colloids are silica beads treated with silane to provide a homeotropic alignment. We especially have examined how the particles interact with the topological defects, where they get trapped, and how the equilibrium textures depend on their size. In our experiments we have observed a great diversity of defect structures and patterns, different from the ones observed in simple nematic shells. I will describe their main features and explain how they result from the competition between elasticity and capillarity effects, during the formation of the shells. I will show how this last mechanism can be used to control the position of the ligands on the shell. References: [1] D.R. Nelson, Toward a Tetravalent Chemistry of Colloids, Nanoletters, 2, 1125 (2002). [2] A. Fernandez-Nieves et al., Novel Defect Structures in Nematic Liquid Crystal Shells Phys. Rev. Lett. 99, 157801 (2007). [3] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Nematic-Smectic Transition in Spherical Shells, Phys. Rev. Lett. 106, 247802 (2011). [4] T. Lopez-Leon,V. Koning, K. B. S. Devaiah, V. Vitelli and A. Fernandez-Nieves, Frustrated nematic order in spherical geometries, Nature Physics, 7 391 (2011). [5] D.Sec et al., Defect trajectories in nematic shells: Role of elastic anisotropy and thickness heterogeneity , Phys. Rev. E, 86, 020705(R) (2012).