Converse Flexoelectric Effect

As in the piezoelectricity of solids, flexoelectricity also manifests as a direct effect [f, Eq. (6.4)] and a converse effect [f, Eq. (6.5)], with an electric field-induced curvature ... 

As has been shown, the splay and bend distortions of a nematic create electric polarization. There is also a converse effect the external electric field causes a distortion due to the flexoelectric mechanism. For example, if the banana-shape molecules with transverse dipoles are placed in the electric field, the dipoles are partially aligned along the field and their banana shape induces some bend. This effect takes place even in nematics with zero dielectric anisotropy. 

Converse flexoelectric effects (i.e. voltage-generated curving) have been demonstrated in uranyl-acetate-stabilized phosphatidylserine BLMs by real-time stroboscopic interferometric measurements the obtained satisfactory agreement between the converse and the direct (i.e. curvature-generated voltage) flexoelectric coefficients have been in accord with the Maxwell relationship [8]. 

Converse flexoelectric studies of lyotropic liquid crystals, such as vesicles, is still an active subject. Notably, the sensory mechanism of outer hair cell composite membranes " can be understood by the flexoelectric properties of the lipid bilayer. The converse of this effect, i.e., a voltage-generated curvature, has also been observed and was discussed by Todorov et Another related phenomenon is the ferroelectricity which results from the tilted layered structures of chiral molecules, which has been discussed extensively since the 1980s.Ferroelectric phases are called... 

We note here that, though the substrate curvature due to the converse giant flexoelectric effect could be observed, detection of the converse flexoelectric effect of calamitic nematics using the same geometry cannot be... 

A.T. Todorov, A.G. Petrov and J.H. Fendler, First observation of the converse flexoelectric effect in bilayer lipid membranes, J. Phys. Chem. 98(12), 3076-3099, (1994). doi 10.1021/jl00063a004... 

J.Y. Fu, Z. Zhu and L.E. Cross, Experimental studies of the converse flexoelectric effect induced by inhomogeneous electric field in a barium strontium titanate composition, J. Appl. Phys. 100(2), 024112/1-6, (2006). 

The theory and experiments of lyotropic and biomembrane flexoelectricity are reviewed. Flexoelectricity is a reciprocal relation between electricity and mechanics in soft lyotropic systems, i.e., between curvature and polarization. Experimental evidence of model and biomembrane flexoelectricity (including the direct and the converse flexoelectric effects) is reported. The biological implications of flexoelectricity are underlined. Flexoelectricity enables membrane structures to function like soft micromachines and nanomachines, sensors and actuators, thus providing important input to nanoionics apphcations. Nanobio examples include membrane transport, membrane contact, mechanosensitiv-ity, electromotility, hearing, nerve conduction, etc. 

A. Todorov, Experimental Investigations of Direct and Converse Flexoelectric Effect in BUayer Lipid Membranes, PhD Thesis, Syracuse University, 1993. 

The second application uses the converse flexoelectric effect, i.e. a field-induced splay-bend distortion, to generate a fast, symmetric and thresholdless linear electro-optic effect in a cholesteric liquid crystal. 

Fig. 11.27 Converse flexoelectric effect (a) Structure of the electrooptical cell, (b) Distribution of the director angle over the cell thickness pictured by lower straight lines for zero (solid line) and finite (dot line) anchoring energies, respectively. The upper curves show spatial dependence of two principal refraction indices no (dash line) and (z) (solid line)...

Fig. 11.29 Conversed flexoelectric effect in cells with homeotropic (a) and homogeneous (b) director alignment and electric field applied along the cell normal. Weak anchoring energy at the bottom plate allows the flexoelectric deflection of the director 3 at the surface propagating up in the vertical direction (e = 0)...

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