From solid to liquid piezoelectric materials

The history and evolution of piezoelectricity are reviewed, starting from the discovery of pressure ("piezo" in Greek) electricity by the Curie brothers in a ferroelectric crystal until today, when liquid piezoelectricity is being observed in polar anisotropic fluids, namely, the ferroelectric nematic liquid crystal (N_F) materials. As effects analogous to the piezoelectricity have been observed in various crystals, polymers and biomaterials with lack of inversion symmetry, the definition of piezoelectricity has evolved to describe a linear coupling between mechanical stress and electric polarization. Mechanical stress-induced electric polarization is called direct piezoelectricity, and electric field-induced mechanical stress is called converse piezoelectricity. Soon after the discovery of ferroelectricity in chiral liquid crystals with two- and one-dimensional fluid order, owing to the lack of their inversion symmetry, linear electromechanical effects analogous to direct and converse piezoelectricity have also been observed in those materials. While these materials in certain directions can sustain static stress, the N_F phase is truly a three-dimensional fluid, and a steady stress can only be sustained by surface tension. The review concludes with a summary and analysis of direct and converse piezoelectric measurements on several N_F materials, followed by the challenges and possible future applications of liquid piezoelectricity.