Nematic twist–bend phase in an external field

Abstract

The response of the nematic twist-bend (N TB ) phase to an applied field can provide important insight into structure of this liquid and may bring us closer to understanding mechanisms generating mirror symmetry breaking in a fluid of achiral molecules. Here we investigate theoretically how an external uniform field can affect structural properties and stability of N TB . Assuming that the driving force responsible for the formation of this phase is packing entropy we show, within Landau-de Gennes theory, that N TB can undergo a rich sequence of structural changes with field.For the systems with positive anisotropy of permittivity we first observe a decrease of the tilt angle of N TB until it transforms through a field-induced phase transition to the ordinary prolate uniaxial nematic phase (N ). Then, at very high fields this nematic phase develops polarization perpendicular to the field. For systems with negative anisotropy of permittivity the results reveal new modulated structures. Even an infinitesimally small field transforms N TB to its elliptical counterpart (N TBe ), where the circular base of the cone of the main director becomes elliptic.With stronger fields the ellipse degenerates to a line giving rise to a nonchiral periodic structure, the nematic splay-bend (N SB ), where the two nematic directors are restricted to a plane. The three structures, N TB , N TBe and N SB , with a modulated polar order are globally nonpolar. But further increase of the field induces phase transitions into globally polar structures with nonvanishing polarization along the field's direction. We found two such structures, one of which is a polar and chiral modification of N SB , where splay and bend deformations are accompanied by weak twist deformations (N * SBp ). Further increase of the field unwinds this structure into a polar nematic (N p ) of polarization parallel to the field.

Soft modes of the dielectric response in the twist–bend nematic phase and identification of the transition to a nematic splay bend phase in the CBC7CB dimer

“…Assuming the mode frequency is proportional to q 2 [10], the drop of mode frequency at about 0.2 V/μm coincides with jump of wave number of the helical structure. Due to the model [33] this is again an indication of electric-field induced transition from N TB to N SB phase similar like for bent core sample [25].…”

“…However such a transition may also be an indication of the negative dielectric anisotropy of CB7CB in the range of N TB phase. In such a case the transition from the N TB to the N SB phase is expected at sufficiently strong electric fields [25,33]. The analysis of the data (see Fig 4a) points towards the field induced formation N SB phase, created below the N phase at sufficiently high bias fields.…”

Biaxiality-driven twist-bend to splay-bend nematic phase transition induced by an electric field

“…A similar effect is also expected (31, 32) when a strong electric field, E, is applied parallel to h in the N TB phase of a compound with positive dielectric anisotropy, ∆ > 0. So far, the estimates of the field strength required for a measurable transition shift range from a few (31,33) to 100 V/m (32). For ∆ < 0 and/or E ⊥ h, the uniaxial symmetry of the N TB phase is broken and the N TB cone becomes elliptical, leading eventually, as expected, to an N TB to N SB transition (31)(32)(33).…”

Physics of liquid crystals of bent-shaped molecules

“…On the other hand, N SB structures can be triggered by applying a sufficiently strong electric field to the N TB phase perpendicularly to the helical axis, provided that 0 ε Δ < . (Pająk, Longa, and Chrzanowska 2017) Another potential instance of a forced N SB occurrence has been described by Meyer, Luckhurst and Dozov(C. Meyer, Luckhurst, and Dozov 2015), as a domain wall that separates the regions of left-and right-twisted regions of the N TB phase .…”