Ice is found almost everywhere on Earth – in glaciers, snow, and clouds. Despite its widespread presence, it still holds secrets about its physical properties.
A long-standing puzzle concerns its electrical behavior. Each water molecule is polar, meaning it has a positive and a negative end. But when water freezes into the usual hexagonal ice (known as ice IH), the overall crystal exhibits no polarity. The reason lies in the rules for how hydrogen atoms arrange themselves. Each oxygen must contact two nearby hydrogen atoms, but throughout the lattice, the orientation of the hydrogen atoms is random. This disorder prevents charges from forming in an organized way and instead cancels them out. As a result, ice is not piezoelectric, unlike quartz or certain ceramics. Piezoelectric materials generate an electric charge when they are compressed; ice doesn’t.
However, nature has often suggested a different story. Thunderstorms produce lightning when ice particles and sleet (soft hail) collide. Cracking ice sheets and avalanches release electromagnetic bursts. It is clear that ice can produce electricity when stressed, but the physical explanation remains unclear. Traditional models have invoked freezing potential, surface ions, or temperature differences between collisions. However, these explanations have often been short-sighted, failing to account for the magnitude of the charge or polarity reversals within the storms.
Big bets
The concept of flexoelectricity is important here. Flexoelectricity is a universal coupling between mechanical bending (strain gradients) and electrical polarization. Unlike piezoelectricity, flexoelectricity does not require a specific crystal symmetry: it can occur in any material. When a solid is bent, compressed unevenly, or otherwise deformed in a non-uniform manner, charges can appear. The effect is usually small, but it can grow in materials with high dielectric constants, such as ceramics.
Could this also happen on ice?
That’s what a new study in Nature Physics, led by teams in China, Spain and the US, set out to investigate. Before this study, no one had directly measured flexoelectricity in ice. The prospect of confirming it is lucrative. It would mean that ice, which is not piezoelectric, is electromechanically active when bent. It would also suggest a new physical mechanism for charging in thunderstorms, potentially supplementing or even correcting an older theory.
The stakes are actually high: The electrification of thunderstorms is one of the oldest unsolved problems in atmospheric science. For more than a century, scientists have debated how colliding ice particles create the vast electric fields that produce lightning. Solving this mystery is crucial to meteorology, aviation safety, and even climate science, because lightning affects atmospheric chemistry (and climate change is also making lightning strikes more frequent).
The researchers conducted the first systematic tests, trying to answer a few questions. Two of them were: is the ice IH actually flexoelectric, and if so, what is its coefficient? And can flexoelectricity explain the charging of ice particles during thunderstorms?
Viņu eksperimenti un simulācijas sniedza pārliecinošus pierādījumus par abiem skaitļiem.
Meklējiet anomālijas
Lai pārbaudītu ICE elektromehāniskās īpašības, pētnieki izveidoja “ledus kondensatorus”. Viņi ieslīdēja ultrapure, starp diviem metāla elektrodiem, pēc tam iesaldēja to ar apkārtējā spiedienu, veidojot polikristāliska ledus plātnes, kas ir pāris milimetru biezas. Alumīnija folijas tika uzklāti zelta vai platīna pārklājumi, lai kalpotu kā elektrodi. Rentgenstaru difrakcija un Ramana spektroskopija apstiprināja, ka paraugi atrodas normālā sešstūra ledus fāzē (IH), nevis kādā eksotiskā variantā.
Eksperimenta kodolā tika izmantots dinamisks mehāniskais analizators. Šī ierīce izmantoja kontrolētu trīspunktu lieces kustību: ledus plāksne balstījās uz diviem balstiem, kamēr zonde tika nospiesta pa vidu. Ledus izliekoties, pētnieki izmērīja gan mehānisko pārvietojumu, gan no tā izrietošos elektriskos lādiņus. Lādēšanas pastiprinātājs, kas savienots ar elektrodiem, kas notverti signāli, bet osciloskops sinhronizēja datus. Analizējot sakarību starp celma gradientiem un polarizāciju, viņi ieguva fleksoelektrisko koeficientu – skaitu, kas saka, cik spēcīgi saliekot ledu rada lādiņš.
Mērījumi tika veikti plašā temperatūras diapazonā no 143 K līdz 273 K. Tas ļāva komandai meklēt anomālijas, kas saistītas ar fāžu pārejām vai virsmas efektiem. Paralēli viņi uzstājās ab initio Kvantu mehāniskās simulācijas, lai modelētu, kā ledus-ūdens saskarnes ar dažādiem metāliem-zelts, platīns, alumīnijs-ietekmē virsmas pasūtīšanu. Šie aprēķini palīdzēja izskaidrot eksperimentālās anomālijas.
Visbeidzot, komanda uzcēla teorētisko modeli ledus gravaupelu sadursmēm pērkona negaisa laikā. Izmantojot klasisko kontaktu mehāniku un to izmērītos fleksoelektriskos koeficientus, viņi aprēķināja iespējamo lādiņa atdalīšanas daudzumu daļiņu sadursmju laikā. Viņi salīdzināja savas prognozes ar gadu desmitiem ilgiem laboratorijas datiem par ledus uzlādi vētrām līdzīgos apstākļos.
Rezultāti bija pārsteidzoši. Pirmkārt, komanda pirmo reizi parādīja, ka ICE patiešām ir fleksoelektrisks. Laikā no 203 K līdz 248 K, efektīvais fleksoelektriskais koeficients konsekventi bija ap 1,01-1,27 nanokoulombas uz metru. Tā nav triviāla vērtība: tā ir salīdzināma ar labi izpētītai dielektriskai keramikai, piemēram, stroncija titanātam un svina cirkonam. Citiem vārdiem sakot, ledus, kas ilgi tiek uzskatīts par elektromehāniski inertu, var izraisīt ievērojamu elektrisko polarizāciju, kad saliekts.
Slēptie pārsteigumi
Importantly for meteorology, the team showed that ICE flexoelectricity could play a major role in thunderstorms. Their collision-induced polarization calculations matched the range of charges measured in previous laboratory studies of the effects of ice gravPel. In addition, the model certainly explained the elusive features of thunderstorm electrification, such as the reversal of charge polarity with temperature. When the flexoelectric coefficient is positive, the Gaupel tends to become negatively charged; when it becomes negative at higher temperatures, the polarity reverses. This was consistent with observations of the trifol structures of thunderstorms, where regions of opposite charge exist.
The researchers cautioned that flexoelectricity is probably not the only mechanism. Storm electrification is complex, involving surface ions, melting, fractures, and impurities. However, flexoelectricity is universal: it should be produced by any inhomogeneous deformation. This makes it a powerful contributor to storm charging, but not the only one. Their work has potentially added a major new piece to a centuries-old puzzle.
The study thus potentially transformed our understanding of ice. It showed that ordinary ice IH, despite lacking piezoelectricity, is flexoelectric with a strength similar to ceramics. And it has suggested that flexoelectricity provides a natural, quantitative mechanism for the charging of ice particles in thunderstorms, potentially helping to explain how lightning is born.
Finally, even the most familiar material, water ice, still has surprises in store. A snowflake is not just frozen water: when folded and collided, it can act like a tiny generator. And in the turbulent dance of storm clouds, these tiny generators can light up the sky.
Published – September 16, 2025 11:07 AM IST
