The Laboratory for Chemo-Mechanics Coupling and Intelligent Materials at Xi'an Jiaotong University (XJTU), along with a number of collaborators, has designed a special capacitive beam structure in which ultrapure water is filled between two metal electrodes, and surface tension is used to confine it within the electrode area to prevent diffusion. It is then frozen at low temperatures to form an ice beam.

A three-point bending deformation is applied to the sample using a dynamic mechanical analyzer (DMA), while simultaneously measuring the polarization response induced by the strain gradient, thereby obtaining the flexoelectric coefficient of ice. The results show that ice does indeed exhibit flexoelectricity, with its flexoelectric coefficient in the ~nC/m range, comparable to that of typical oxide ceramics such as SrTiO₃ and TiO₂.

The research team further measured the change in the flexoelectric coefficient of ice with changes in temperature. Three intervals were observed: i) 203~248 Kelvin, the coefficient is relatively stable, belonging to the intrinsic response interval; ii) above 248 K, the signal diverges, accompanied by a 180° phase reversal and significant creep, which stems from the influence of the ice surface premelting effect; iii) below 203 K, the coefficient increases significantly and forms a peak near 160 K, followed by a decrease.

This peak was quite unexpected, as flexoelectric peaks typically appear near phase transitions. However, the bulk phase transition of ice from paraelectric Ih to ferroelectric XI at atmospheric pressure occurs at 72 K, far below the experimental temperature range. Simultaneously measured elastic modulus data also ruled out the possibility of bulk phase structural changes.

The research team further established a flexoelectric model for the generation of an electrical charge through the collision of ice particles and graupel in thunderclouds, and calculated the polarization generated by the strain gradient at the collision interface.

The results show that the flexoelectric polarization charge generated by a single collision is comparable to the amount of charge transfer measured in a series of previous wind tunnel experiments. In addition, the 180-degree phase reversal exhibited by the flexoelectric coefficient in the near-melting point region is also consistent with the typical tripolar charge structure formed along the altitude direction in thunderclouds.

These pieces of evidence suggest that the flexoelectricity of ice may play an important role in the process of lightning origination. At the same time, the authors emphasized in the paper that the expansive topic of lightning’s origin cannot be fully answered by one study, and many questions remain to be explored, but the flexoelectricity of ice undoubtedly provides a new important clue to understanding this natural mystery.

The research team has revealed the flexoelectric effect of ice for the first time, and discovered that ice undergoes a surface ferroelectric phase transition at approximately 160 K. This discovery not only provides a new perspective for understanding the intrinsic properties of ice, but also opens up new directions for exploring the potential role of ice in natural charging phenomena. On Aug 27, the above research results were published online in Nature Physics under the title Flexoelectricity and Surface Ferroelectricity of Water Ice.