Research ArticleMATERIALS SCIENCE

Control of colloidal placement by modulated molecular orientation in nematic cells

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Science Advances  16 Sep 2016:
Vol. 2, no. 9, e1600932
DOI: 10.1126/sciadv.1600932
  • Fig. 1 Elasticity-directed placement and assembly of normally anchored polystyrene spheres in regions with splay deformations of the patterned director.

    (A) A sphere with perpendicular surface anchoring placed in a uniform nematic causes an appearance of a hyperbolic hedgehog that can be located on either the left- or right-hand side; the resulting director deformations are of a dipolar type. (B) Periodic splay-bend stripe pattern; the normalized splay energy density is labeled by colors according to the scale on the right-hand side. A sphere is placed in the bend region by optical tweezers and released there, and migrates toward the splay region. The dark blue curve is the typical experimental trajectory of the sphere’s center. (C) Self-assembly of spheres into linear chains in the regions of maximum splay with p > 0 [bright-field microscope with unpolarized light, and nematic liquid crystal pentylcyanobiphenyl (5CB)]. (D) Polarizing microscopy images of a sphere moving from one splay region to the next splay region, with concomitant reorientation of the structural dipole from p < 0 to p > 0; the white arrows point toward the hyperbolic hedgehog. (E) Typical experimental trajectory of the sphere within the director configuration mapped in the PolScope mode of observation of the experimental cell. (F) Experimentally measured x(t) dependence for a sphere moving from y = l to y = 0 and its theoretical fit by Eq. 5. (G) Experimentally measured y(t) dependence for a sphere moving from y = l to y = 0 and its theoretical fit by Eq. 6. All spheres have a radius of R = 2.5 μm; the period of the patterned director is l = 80 μm. The nematic material corresponding to parts (D) to (G) is MLC6815.

  • Fig. 2 MSD of a dipolar colloid in uniformly aligned LC cell.

    Homeotropically anchored sphere with a radius of 2.5 μm in a nematic MLC6815 cell with uniform alignment Embedded Image, which is achieved by unidirectional rubbing of the aligning layer of PI2555 deposited on the bounding plates.

  • Fig. 3 Elasticity-directed placement and assembly of tangentially anchored polystyrene spheres in regions with bend deformations of the patterned director.

    (A) Director deformations around a tangentially anchored sphere in a uniform nematic are of quadrupolar symmetry, with two point defects, called boojums, at the poles. (B) Periodic splay-bend stripe pattern; the normalized bend energy density is labeled by colors according to the scale on the right-hand side. A sphere placed and released in the splay region using optical tweezers migrates toward the bend region. The dark red curve is the typical experimental trajectory of the sphere’s center. (C) Self-assembly of spheres into chains in the regions of maximum bend (bright-field microscope and unpolarized light). (D) Experimentally measured x(t) dependence for a sphere moving from y = l to y = l/2 and its theoretical fit by Eq. 9. (E) Experimentally measured y(t) of a sphere moving from y = l to y = l/2 and its theoretical fit by Eq. 10. All spheres have radius of R = 2.5 μm; periodicity of the director pattern is l = 80 μm. The LCs used are 5CB (C) and MLC6815 (D and E).

  • Fig. 4 Preferential placement of colloids with different surface properties in a cell with periodically patterned splay-bend director field.

    (A) Polarizing microscope texture of polystyrene colloids with homeotropic and tangential anchoring located in the alternating bands of splay and bend, respectively. (B) Scheme of preferential placement with the experimentally determined director pattern (PolScope observations). The homeotropically anchored spheres reside in the region of splay (y = 0), whereas tangentially anchored spheres localize in the regions of bend (y = ±l/2). Note the different orientations of chains formed by homeotropic spheres (along the x axis) and by the tangential spheres (perpendicular to the x axis). The colloids are placed in an LC mixture with zero dielectric anisotropy. Polarizer is parallel to the x axis and analyzer along the y axis.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1600932/DC1

    fig. S1. Self-assembly of colloids with homeotropic anchoring in the regions of photopatterned splay distortions during the filling of the LC into the cell.

    movie S1. Colloidal sphere with dipolar director is transported from one splay region to the next splay distortion due to elastic interaction.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Self-assembly of colloids with homeotropic anchoring in the regions of photopatterned splay distortions during the filling of the LC into the cell.
    • Legend for movie S1

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.avi format). Colloidal sphere with dipolar director is transported from one splay region to the next splay distortion due to elastic interaction.

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