Giant Flexoelectricity of Bent-Core Nematic Liquid Crystals

Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC’s bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.

Figure 1

(a) Molecular structure of the bent-core material and its simplified geometrical model. (b) Schematics of the experimental setup.

Figure 2

Model of the sample geometry and deformation of the flexible cell during the periodic vibration driven by the speaker. (a) The geometry of the cell in the $xy$ plane. (b) The cell structure during the deformation.

Figure 3

(a) Displacement dependence of flexoelectric current (in rms values) of ClPbis10BB at $74°\mathrm{C}$; (b) the temperature dependence of $|e_3|$ of a well-aligned cell measured at 5 Hz in $15\mathrm{mm}\times 15\mathrm{mm}$ active area plastic cells.

Figure 4

Variation of the flexoelectric coefficient on a relative temperature scale $T-T_{N-I}$ for the bent-core liquid crystal ClPbis10BB and for the calamitic liquid crystal 5CB measured in cells of $A=1{\mathrm{cm}}^2$ active areas. The inset shows part of the figure ($|e_3|$ of 5CB) at a magnified scale.