Elastocapillarity in thin nematic films
Résumé
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.
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[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).