Antagonistic flexoelectric response in liquid crystal mixtures of bent-core and rodlike molecules.

We report the measurements of the temperature variations of the flexoelastic coefficient (e(*)/K) of a host calamitic liquid crystal (RO) and its mixture with two guest bent-core (BC-120 and BC-60) liquid crystals. The bent-core (BC) molecules have different core structures and bend angles; namely, θ=/~120° and =/~60°, respectively. We find that e(*)/K is independent of temperature and decreases rapidly with increasing concentration of BC-120 molecules and changes sign from positive to negative. In mixtures with BC-60, e(*)/K is always positive and its concentration-dependent variation is not unique. At 7M% it is significantly large (three times) near the nematic-isotropic transition and decreases strongly with reducing temperature. Dielectric measurement suggests antagonistic orientation of the dipole axes (arrow axes) of the two BC molecules in the host liquid crystal, and based on this, the opposite sign of e(*)/K is explained.