During “cloud seeding” small particles are deliberately introduced into the clouds so that the water droplets condense and fall to the ground as rain. Silver iodide is usually used because its structure promotes ice nucleation particularly effectively. However, how exactly this works at the atomic level was previously unclear. A study now shows that the tiny silver iodide crystals have two different surfaces, only one of which contributes significantly to condensation. The results can help to specifically develop new materials for generating precipitation.
Clouds form when warm, moist air rises and cools. Since cold air can store less moisture, the water condenses on small particles, including pollen, dust or soot. Starting from these condensation nuclei, the droplets grow until they are finally so heavy that they fall to the ground as snow or rain. In so-called “cloud seeding”, also known as “cloud seeding”, people try to specifically influence this process by spraying small particles into the clouds using aircraft or rockets. In dry regions, the technology is used to make clouds rain down; some countries use it to ensure good weather during major events. German winegrowers also occasionally rely on cloud seeding in the hope of preventing crop damage from hail.
Complex mechanism revealed
The sprayed particles are usually silver iodide, which is considered a particularly effective condensation nucleus. “However, the mechanism at the atomic level that underlies its effectiveness is still unclear,” reports a team led by Johanna Hütner from the TU Vienna. Until now, only the rough mechanism was known: “Silver iodide forms hexagonal structures with the same hexagonal symmetry that is also known from snowflakes,” explains Hütner’s colleague Jan Balajka. “The distances between the atoms are also similar to those in ice crystals. For a long time it was assumed that this similarity in structure explained why silver iodide is such an efficient crystallization nucleus for ice. However, closer examination shows that the mechanism is far more complex.”
To investigate this mechanism, the researchers carried out experiments in ultra-high vacuum and at very low temperatures. They split silver iodide crystals and used high-resolution atomic force microscopy to observe how water condenses on these freshly created surfaces. As Hütner and her colleagues discovered, the two sides of the broken crystal are different: one side is rich in silver ions at the break edge, the other is rich in iodine ions. “We discovered that both surfaces rearrange themselves, but in completely different ways,” says Hütner.
Hexagons and squares
The silver-rich side therefore maintains a hexagonal arrangement, which forms an ideal template for the growth of an ice layer. Ice crystals accumulate here layer by layer. The iodine-rich side, on the other hand, forms a rectangular structure that no longer fits the hexagonal symmetry of the ice crystals. Instead of uniform layers, three-dimensional structures made of condensed water occasionally form here, the growth of which is very limited. “Only the silver-terminated surface contributes to nucleation,” explains Balajka. “The ability of silver iodide to trigger ice formation in clouds cannot be explained solely by the structure inside the crystal. What is crucial is the atomic arrangement on the surface – an effect that has been completely overlooked so far.”
Computer simulations confirm the results. “With these simulations we were able to calculate which arrangements of atoms are energetically most favorable,” explains Hütner’s colleague Andrea Conti. “By accurately modeling the interface between silver iodide and water, we were able to observe how the first water molecules arrange themselves on the surface to form a layer of ice.” From the researchers’ perspective, the results can help develop new materials for producing precipitation. “Ice nucleation is a phenomenon of central importance for atmospheric physics and an understanding at the atomic level is essential to find out whether other materials could be suitable as effective nucleators,” says Hütner’s colleague Ulrike Diebold.
Source: Johanna Hütner (TU Vienna, Austria) et al., Science Advances, doi: 10.1126/sciadv.aea2378