Abstract
Electric field induced dynamic reorientation phenomenon of graphene/graphitic flakes in homogeneously aligned nematic liquid crystal (NLC) medium has been demonstrated by optical microscopy. The flakes reorient from parallel to perpendicular configuration with respect to boundary plates of confining cells for an applied field strength of as low as tens of millivolt per micrometer. After field removal the reoriented flakes recover to their initial state with the help of relaxation of NLC. Considering flake reorientation phenomenon both in positive and negative dielectric anisotropy NLCs, the reorientation process depends on interfacial Maxwell-Wagner polarization and NLC director reorientation. We propose a phenomenological model based on electric field induced potential energy of graphitic flakes and coupling contribution of positive NLC to generate the rotational kinetic energy for flake reorientation. The model successfully explains the dependence of flake reorientation time over flake shape anisotropy, electric-field strength, and flake area. Using present operating scheme it is possible to generate dark field-off state and bright field-on state, having application potential for electro-optic light modulation devices.
Highlights
Electric field induced reorientation of particles such as polymer cholesteric liquid crystal flakes (PCLC) due to their unique wavelength and polarization selective reflection properties has been widely studied for electro-optic devices and flexible, reflective display applications [1,2,3,4,5]
As the graphitic flakes are freely suspended in positive dielectric anisotropic NLC medium while they are subjected to oscillating external electric field, we assume a proportionate propagation of electric field induced elastic distortion from host nematic liquid crystal medium to graphitic flakes
The electric field induced reorientation process of opaque ~1 μm thick, 20-200 μm2 area, and irregular shaped graphitic flakes in nematic liquid crystal medium has been demonstrated. Construction mechanism of such flake in nematic liquid crystal medium has been proposed based on intuitive arguments
Summary
Electric field induced reorientation of particles such as polymer cholesteric liquid crystal flakes (PCLC) due to their unique wavelength and polarization selective reflection properties has been widely studied for electro-optic devices and flexible, reflective display applications [1,2,3,4,5]. Two-dimensional graphitic flakes [10,11,12] (layers of pi-stacked graphene sheets) are optically opaque, and possess anisotropic shape and electrical properties Such unique combination of interesting physical properties instigates electric field induced switching behaviour investigation of graphitic flakes for future display application. Research based on laser tweezer has explored optical trapping induced realignment of anisotropic nonspherical particles in isotropic fluid (water) [15] and anisotropic liquid crystal phase [16,17], which have opened up the possibility of manipulating flake reorientation through external optic stimulus This laser tweezer-realigning microrheological method leads to irreversible and arbitrary rotation and unexpected spinning dynamics during optical trapping process, which makes it hard for uniform flake rotation control as well as further display switching application. The relationship of the reorientation time of flakes with electric field strength, frequency, flakes size, and shape anisotropy is further discussed
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