Résumé
The confinement of colloidal particles in thin liquid films is typically accompanied with interfacial deformations which give rise to long-range capillary forces, attractive in most of the cases [1]. In simple fluids, these capillary forces compete mostly with electrostatic interactions. In thin complex fluid films such as a liquid crystal, other long-range interactions related to bulk distortions of the surrounding fluid appear. We have recently shown that the presence of a nematic order drastically changed the long-range interactions between particles [2] via an original elasto-capillarity coupling. In this work, we focus on the variety of distortions produced by the colloidal inclusions. In nematic films much thicker than the particles' size, a particle gives rise to elastic multipoles formed by the particle and nearby topological defects [3]. In thin nematic films with hybrid anchoring conditions at free surfaces, capillary effects strongly modify the behaviour of the elastic dipoles. For a certain range of film thickness values, two new interesting patterns emerged around isolated particles: the giant dipole [2] and the " butterfly " texture (see Fig.1). In the giant dipole, a micron-sized sphere is accompanied by a point defect located at a distance up to several hundreds microns. The situation is quite different in the " butterfly " texture: the particle still produces an accompanying defect in its close neighbourhood, but a pi-wall is formed on the opposite side. Using spatially resolved retardation and easy-axis maps, we analysed quantitatively and separately the 2D interfaces deformation and the nematic textures. Both behaviours are due to the same axisymmetric capillary deformation of the thin film around the beads but with different boundary conditions for the polar 2D c-director. Using numerical simulations, we were able to reproduce the patterns found in the films. Although capillary interactions between inclusions in a thin film are always attractive [4], these new spontaneous organizations in thin nematic films offer new ways to self-assemble complex colloidal systems in 2D. Figure 1: The two different birefringence patterns formed by microparticles trapped in a thin nematic film, observed a) between crossed polarizers and b) with an Abrio birefringence measurement system.[1] P.A. Kralchevsky, K. Nagayama K, Adv Colloid Interface Sci , 2000, 85, 145.[2] H.Jeridi, M.A.Gharbi, T.Othman, C.Blanc, Proc Natl Acad Sci USA, 2015, 112,14771.[3] I. Musevic, M.Skarabot, U.Tkaler, M.Ravnik and S.Zumer, Science, 2006, 313,954.