A General Control Strategy to Micropattern Topological Defects in Nematic Liquid Crystals Using Ionically Charged Dielectric Surface

Abstract

materials such as colloids, [1][2][3] blockcopolymers, [4][5][6] and liquid crystals. [7][8][9][10][11] Especially the structural and optical anisotropy in nematic liquid crystals (NLCs) are useful for stimuli-responsive applications. NLCs are characterized by a spontaneous orientational order of elongated molecules, called the director, which is denoted by a unit vector n. The director field is influenced by the surface boundary (anchoring) effect and by external fields such as electric, magnetic fields, and temperature. In recent years, many efforts have been made to fabricate microstructures with complex and periodic director fields using various approaches. [9,10,[12][13][14][15] In patterning the director field in two dimensions, the geometrical limitation often induces topological defects which are promising for optoelectronic devices [16][17][18][19] and soft actuators. [20][21][22][23][24][25] Thus, the stabilization and the precise control of n are essentially required for engineering defects toward further development. [26][27][28] Conventionally, it is known that defects can be stabilized by curved surfaces such as colloids [29][30][31][32] otherwise they annihilate over time. [33][34][35] When the NLC sample is sandwiched between two parallel glass substrates, the local director field must be controlled by surface modification techniques. [14,[36][37][38][39] Coating substrates with a polymer layer is a common process to designate n at the surface either to orient in the surface normal direction or to lie in the surface plane. In controlling n using the surfaces between parallel substrates, so far, attentions are mostly paid to the boundary condition of the coating materials on which NLC molecules contact, but meanwhile the thickness of the alignment layer, i.e., the electrical insulation of the alignment layer is not a main concern. The influence of the electrical resistivity of the alignment layer remains unwatched. On the other hand, recently, we have found a micropattern formation of an ion-doped NLC which generates a square array of topological defects under an AC electrical voltage. The pattern formation is spontaneously derived using a perfluoro polymer surface. [40] The phenomenon can be extended to optical applications by combining surface fabrication technique. [13,41] Contrary to the conventional understanding, the obtained results suggest that

Lensless Wavefront Parallel Processing of Vector Beams by Self‐Images of a Self‐Organized Q‐Plates Microarray

“…It should be noted that the required voltage to the sample cell, , for the director reorientation is practically influenced by the resistances and capacitances of the NLC ( and ) and the polysiloxane layer ( and ), which must be considered particularly for . [ 53 ] Here, is the effective AC voltage applied to the sample cell. By assuming an equivalent circuit of the sample cell, the frequency dependence of at the threshold is schematically illustrated in Figure 2b.…”

Reconfigurable spatially-periodic umbilical defects in nematic liquid crystals enabled by self-organization

“…The advantage of these compounds is that they are readily available and can easily ensure vertical anchoring for various NLCs. As with polydimethylsiloxane (PDMS), SOG surface allows the use of a silane coupling agent by modifying the surface with ultraviolet ozone (UVO) treatment [49]. Also, SOG can be substituted by a commercial silicone resin such as KR-251 (Shin-etsu).…”

Geometric theory of topological defects: methodological developments and new trends

“…Geometry of disclinations networks has been theoretically investigated in the literature, sometimes allowing for analytical expressions for the metric tensor [150][151][152]. Several authors have shown the possibility to design arrays of topological linear defects from photopatterning techniques [153][154][155][156][157] and even to manipulate them [158,159]. If this last point opens the possibility to emulate collisions between particles in the 2+1 model, it is even more interesting for the extension of the Deser, Jackiw and 't Hooft model to 3+1 dimensions [70]: matter particles are represented by a gas of piecewise straight string segments that are likely to collide with a higher frequency.…”