Résumé:

The fascinating idea of mimicking molecular chemistry with colloids (nano-objects) has motivated the research of scientists in many fields. However, colloids typically lack the ability to bond along specific directions as atoms and molecules do, which limits the complexity of the structures that they can spontaneously form. In a seminal paper, it was suggested that liquid crystal properties could be exploited to tackle this problem [1]. The idea is to coat spherical colloidal particles with a thin layer of nematic liquid crystal and add ligands to the unavoidable defects or bald spots that arise when the nematic order is established on the surface of a sphere. Such ligands would act as linkers between particles and direct their interactions along specific directions, which are determined by the defect positions. Despite the big impact that these nano-structured systems could have in photonics or material science, this topic has been barely explored experimentally, mainly because of the experimental difficulties associated to the fabrication of a nematic shell. In the FP7-PEOPLE-2008-IEF-236091 project, we have overcome this problem by taking advantage of a novel microfluidic technique that enable a controlled fabrication of multiple emulsions, i. e., droplets containing smaller droplets inside [2,3]. Specifically, our nematic shells are double emulsions that consist of droplets of nematic liquid crystals containing a single aqueous droplet inside. When the size of the inner droplet is comparable to the size of the outer one, the aqueous core is coated by a thin nematic shell. With these experimental shells, we have not only corroborated theoretical predictions, but we have been able to systematically control the number and position of the defects formed. For thin shells, we have shown that these defects can be engineered to emulate the linear, trigonal and tetrahedral directionalities of sp, sp2 and sp3 carbon bonds [4,5]. References: [1] D. R. Nelson, Nano Lett., 2 (2002) 1125. [2] A. S. Utada, E. Lorenceau, D. R. Link, P. D. Kaplan, H. A. Stone and D. A. Weitz, Science, 308 (2005) 537. [3] A. Fernandez-Nieves, V. Vitelli, A. S. Utada, D. R. Link, M. Marquez, D. R. Nelson and D. A. Weitz, Phys. Rev. Lett., 99 (2007) 157801. [4] T. Lopez-Leon, V. Koning, K. B. S. Devaiah, V. Vitelli and A. Fernandez-Nieves, Nature Phys., 7 (2011) 391. [5] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Phys. Rev. Lett. 106 (2011) 247802.