The Laboratory for Chemo-Mechanics Coupling and Intelligent Materials at Xi'an Jiaotong University (XJTU) and collaborators have discovered that doping ice with salt can increase the flexoelectric coefficient by approximately three orders of magnitude.

They revealed that the mechanism is the transport of salt ions along grain boundaries under bending deformation, generating an electric current. This discovery brings the development of ice energy closer to becoming a reality and provides new clues for understanding the electrical activity of ice-covered ocean worlds such as Europa and Enceladus.

This study chose the most common solute, sodium chloride (NaCl). The flexoelectric coefficient of NaCl itself is ~10⁻³ nC/m, far lower than that of ice. Therefore, if doping were simply a linear superposition of the individual flexoelectric polarizations, the significance would be limited. Fortunately, water ice systems often do not follow simple linear superposition rules but exhibit more complex phenomena.

In polycrystalline ice, grain boundaries are wetted by nanometer-thick quasi-liquid layers due to the premelting effect. During the freezing process, salt ions struggle to enter the ice crystal lattice and are pushed to the grain boundaries, further enhancing the premelting effect and broadening and increasing the grain boundary channels.

As a result, polycrystalline salt ice retains a solid framework while forming a continuous nanoscale transport network. When bending occurs, the premelted salt water layer is squeezed along the grain boundaries from the compressed side to the tensioned side.

Since there is an electrical double layer structure at the ice-water interface, this transport process carries a net charge, thereby generating an electric current. The research team named this mechanism the "streaming flexoelectricity effect".

The research team prepared ice beams with different salinities and performed flexoelectric tests using a standard three-point bending apparatus. The results showed that the equivalent flexoelectric coefficient increased significantly with increasing salinity: it reached approximately 3 μC/m at 25 wt%, approximately three orders of magnitude higher than that of pure ice. The team established a theoretical model and derived an analytical expression for the equivalent flexoelectric coefficient:

Y is Young's modulus, f is the loading frequency, ε and η are the dielectric constant and viscosity of the brine, respectively, d is the channel width, g is the grain size, and ζ is the Zeta potential. This concise formula simultaneously covers solid, liquid, and solid-liquid coupling factors and can qualitatively and quantitatively reproduce the experimental results without fitting parameters.

In addition, this formula reveals the action mechanism of NaCl: as salinity increases, the grain size g decreases, the channel thickness d increases, and the transport network expands. At the same time, NaCl activates the translation and rotation of confined water molecules by disrupting hydrogen bonds, thereby reducing the viscosity η and increasing the dielectric constant ε. These factors work together to cause a strong flexoelectric response in saline ice.

The related research results, titled Streaming Flexoelectricity in Saline Ice, were published online in Nature Materials on Sept 15, and were selected as the cover paper of that issue.