Water Freezes Differently on Positively and Negatively Charged Surfaces of Pyroelectric Materials

Abstract

Although ice melts and water freezes under equilibrium conditions at 0 degrees C, water can be supercooled under homogeneous conditions in a clean environment down to -40 degrees C without freezing. The influence of the electric field on the freezing temperature of supercooled water (electrofreezing) is of topical importance in the living and inanimate worlds. We report that positively charged surfaces of pyroelectric LiTaO3 crystals and SrTiO3 thin films promote ice nucleation, whereas the same surfaces when negatively charged reduce the freezing temperature. Accordingly, droplets of water cooled down on a negatively charged LiTaO3 surface and remaining liquid at -11 degrees C freeze immediately when this surface is heated to -8 degrees C, as a result of the replacement of the negative surface charge by a positive one. Furthermore, powder x-ray diffraction studies demonstrated that the freezing on the positively charged surface starts at the solid/water interface, whereas on a negatively charged surface, ice nucleation starts at the air/water interface.

Mechanism of supercooled droplet freezing on surfaces

“…T w can be determined from the ∆T estimated earlier. The water droplet static contact angle, θ, the nominal size of surface roughness, R s , and the critical ice nucleus radius (that is, the minimum size an incipient ice crystal needs to reach to maintain a stable freezing process), r c 19 , are the principal parameters determining the factor f in equation (3). The roughness parameter can be better appreciated by seeing the scanning electron microscope image in Supplementary Figure S4.…”

Externally Applied Electric Fields up to 1.6 × 105 V/m Do Not Affect the Homogeneous Nucleation of Ice in Supercooled Water

“…Experimentally, electrically-induced freezing, or an enhanced tendency to nucleate ice, were claimed to be observed at the electrostatically-charged surfaces of conductors 12,15,16,35 , polar amino acid crystals 14 , and pyroelectric crystals 21 . Water freezing experiments carried on charged surfaces suggested that electric fields much smaller than the predictions of simulations influence the nucleation of ice; nevertheless, careful freezing experiments carried under external electric fields [11][12][13]20 did not observe any effects of the electric field on the nucleation of ice.…”

“…Electric fields are often encountered in water near the surface of particles, molecules, and ions; they might play an important role in the heterogeneous nucleation of ice by impurity particles 9,14,21,27 .…”

“…Previous experimental investigations [9][10][11][12][13][14][15][16][17][18][19][20][21] have provided only inconclusive evidence that electric fields affect the homogeneous nucleation of ice. (iv) A new electric-field-based method to control of the nucleation of ice could improve technological processes as freeze-drying 22 , cryopreservation 23,24 , food manufacture 25 , and cold-energy storage 26 .…”

Unipolar assembly of zinc oxide rods manifesting polarity-driven collective luminescence

“…Unique applications, such as developing a transiently charged crystal assembly due to the pyroelectric nature of ZnO (such as those in Fig. 1C) can now be envisioned (12,13).…”