Elastocapillarity in thin nematic films

Haifa Jeridi (1) , Mohamed Amine Gharbi (2) , Tahar Othman (1) , Christophe Blanc (3)

1 LP3ME - Laboratoire de Physique de la Matiere Molle et de la Modélisation Electromagnétique [Tunis]
2 Department of Physics [Montréal]
3 L2C - Laboratoire Charles Coulomb

by solid substrates, the particle creates a hyperbolic hedgehog defect, usually located near the bead (at a distance of order of its size) and the pair particle-defect forms a neutral unit, stable in time. Here we show theoretically and experimentally how capillary effects strongly modify the behaviour of particles trapped in a thin nematic film with hybrid anchoring conditions 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 point defect which is 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 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 the beads 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 been also investigated in the framework of the Landau-de Gennes theory. 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. Figure1: 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] 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).

Domaines: Matière Molle [cond-mat.soft]

Haifa Jeridi, Mohamed Amine Gharbi, Tahar Othman, Christophe Blanc. Elastocapillarity in thin nematic films. 4th International Soft Matter Conference, Sep 2016, Grenoble, France. 2016. ⟨hal-01371506⟩