Résumé
by solid substrates, the particle creates a hyperbolic hedgehogdefect, usually located near the bead (at a distance of order of its size) and the pair particle-defectforms a neutral unit, stable in time. Here we show theoretically and experimentally how capillaryeffects strongly modify the behaviour of particles trapped in a thin nematic film with hybrid anchoringconditions at free surfaces. For a certain range of thickness values (films thinner than the particles’size) two new interesting patterns are formed by isolated particles: the giant dipole [3] and the“butterfly” texture (see Fig.1). In the giant dipole, a micron-sized sphere is accompanied by a pointdefect which is located at a distance up to several hundreds microns. The situation is quite different inthe “butterfly” texture: the particle still produces an accompanying defect in its close neighbourhood,but a p-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 thebeads but with different boundary conditions for the polar 2D c-director. Using a simple 2D Ansatz,we were able to reproduce the c-director patterns found in the films. The local 3D textures have beenalso investigated in the framework of the Landau-de Gennes theory. Although capillary interactionsbetween inclusions in a thin film are always attractive [4], these new spontaneous organizations in thinnematic films offer new ways to self-assemble complex colloidal systems in 2D.Figure1: The two different birefringence patterns formed by microparticles trapped in a thin nematicfilm, observed a) between crossed polarizers and b) with an Abrio birefringence measurement system.[1] I. Musevic, M.Skarabot, U.Tkaler, M.Ravnik and S.Zumer, Science, 313,954 (2006).[2] P.Poulin, H.Stark, T.Lubensky and D.Weitz, Science, 275,1770 (1997).[3] H.Jeridi, M.A.Gharbi, T.Othman, C.Blanc, Proc Natl Acad Sci USA, 112,14771 (2015).[4] P.A. Kralchevsky, K. Nagayama K, Adv Colloid Interface Sci , 85, 145–192 (2000).