Large-scale self-organization of reconfigurable topological defect networks in nematic liquid crystals

Abstract

Topological defects in nematic liquid crystals are ubiquitous. The defects are important in understanding the fundamental properties of the systems, as well as in practical applications, such as colloidal self-assembly, optical vortex generation and templates for molecular self-assembly. Usually, spatially and temporally stable defects require geometrical frustration imposed by surfaces; otherwise, the system relaxes because of the high cost of the elastic energy. So far, multiple defects are kept in bulk nematic liquid crystals by top-down lithographic techniques. In this work, we stabilize a large number of umbilical defects by doping with an ionic impurity. This method does not require pre-patterned surfaces. We demonstrate that molecular reorientation controlled by an AC voltage induces periodic density modulation of ions accumulated at an electrically insulating polymer interface, resulting in self-organization of a two-dimensional square array of umbilical defects that is reconfigurable and tunable.

Tunable Dynamic Topological Defect Pattern Formation in Nematic Liquid Crystals

“…The system we employ, a 2D array of umbilical defects, is realized by two‐important factors: a homeotropic alignment layer of high resistivity and a negative dielectric anisotropic LC mixed with ions for high conductivity, as first demonstrated by Orihara and colleagues . When an AC field is applied, the high conductivity of the LC and high resistivity of the alignment layer create a short‐range ordered 2D array of umbilical defects.…”

“…The spacing between the neighboring defects, and hence the unit size of the pattern, depends on the frequency and the amplitude of the applied AC field, as well as on the cell gap and on the material properties of the LC. By patterning the electrodes or using optical tweezer, arrays created in this way have been made sufficiently regular to generate well‐defined 2D diffraction patterns . However, there are crucial shortcomings to overcome in order for this system to be useful as a diffraction grating.…”

Standing wave-mediated molecular reorientation and spontaneous formation of tunable, concentric defect arrays in liquid crystal cells

“…13 The LCs in their experiment contain large amounts of ionic compounds, and this effect cannot be the origin of the pattern formation in our study using pure LCs without ions (Supplementary Figure 2). To verify the lack of connection between the periodic pattern and the ionic effect, we redesigned the pulse-train signal to have a higher frequency within the pulse period, as shown in Figure 3, where the S L and S H signals are the standard and modified pulse-train signals, respectively.…”

“…Interestingly, the director profile around the +1 defects has a concentric arrangement, that is, a vortex structure. 16 Although the +1 defects with radial director profiles are frequently found in LC cells, 13 the concentric director profiles are rarely observed. To reconfirm the director field structure, we examined the director profiles near the +1 and − 1 defects using two methods, an optical compensator technique and an FCPM analysis.…”

“…In fact, pattern generation is a popular topic in LCs and can be implemented via various methods, such as self-assembly, 9 confinement effect, 10 defect-assisted assembly, 11 electrohydrodynamic convection, 12 ionic effect, 13 surface effects 14 and dielectrophoresis 15 ; however, standing nematodynamic waves have not been used. Additionally, tunable concentric defect arrays have not been achieved via another method.…”