The crystal that makes clouds rain

The experiments have to be performed in the dark
Photo Credit: Technische Universität Wien

No one can control the weather, but certain clouds can be deliberately triggered to release rain or snow. The process, known as cloud seeding, typically involves dispersing small silver iodide particles from aircraft into clouds. These particles act as seeds on which water molecules accumulate, forming ice crystals that grow and eventually become heavy enough to fall to the ground as rain or snow.

Until now, the microscopic details of this process have remained unclear. Using high-resolution microscopy and computer simulations, researchers at TU Wien have investigated how silver iodide interacts with water at the atomic scale. Their findings reveal that silver iodide exposes two fundamentally different surfaces, but only one of them promotes ice nucleation. The discovery deepens our understanding of how clouds form rain and snow and may guide the design of improved materials for inducing precipitation.

The structure of the surface determines whether the material acts as a condensation nucleus.
Image Credit: Technische Universität Wien

Surface structure holds the key to ice formation

“Silver iodide forms hexagonal structures with the same sixfold symmetry familiar from snowflakes,” says Jan Balajka from the Institute of Applied Physics at TU Wien, who led the research. “The distances between atoms also closely match those in ice crystals. For a long time, the structural similarity was believed to explain why silver iodide is such an effective nucleus for ice formation. A closer examination, however, reveals a more complex mechanism.”

The atomic structure of the surface where ice nucleation occurs differs from that inside the crystal. When a silver iodide crystal is cleaved, silver atoms terminate one side and iodine atoms the other. “We found that the silver-terminated and iodine-terminated surfaces both reconstruct, but in completely different ways,” says Johanna Hütner, who performed the experiments. The silver-terminated surface retains a hexagonal arrangement that provides an ideal template for the growth of an ice layer, whereas the iodine-terminated surface reconstructs into a rectangular pattern that no longer matches the sixfold symmetry of ice crystals.

“Only the silver-terminated surface contributes to the nucleation effect,” explains Balajka. “The ability of silver iodide to trigger ice formation in clouds cannot be explained solely by its bulk crystal structure. The decisive factor is the atomic-scale rearrangement at the surface, an effect that had been completely overlooked until now.”

The Team
Photo Credit: Technische Universität Wien

Unraveling ice nucleation through experiments and simulations

The TU Wien team investigated these effects using two complementary approaches. First, experiments were conducted under ultrahigh vacuum and at very low temperatures. Water vapor was deposited onto small silver iodide crystals, and the resulting structures were examined using high-resolution atomic force microscopy.

“One of the challenges was that all experiments had to be performed in complete darkness,” explains Johanna Hütner. “Silver iodide is highly light-sensitive, a property that once made it useful in photographic plates and films. We only used red light occasionally when handling the samples inside the vacuum chamber.”

In parallel, the team simulated the surfaces and the water structures covering them using density functional theory, an advanced computational method for quantum mechanical modeling of atomic-scale interactions. “These simulations allowed us to determine which atomic arrangements are energetically most stable,” explains Andrea Conti, who performed the calculations. “By accurately modeling the silver iodide–water interface, we could observe how the very first water molecules organize on the surface to form an ice layer.”

“It is remarkable that for so long, we relied on a rather vague, phenomenological explanation of silver iodide’s nucleation behavior,” says Ulrike Diebold, head of the Surface Physics Group at TU Wien, where the study was conducted. “Ice nucleation is a phenomenon of central importance for atmospheric physics, and the atomic-scale understanding provides a foundation for evaluating whether other materials could serve as effective nucleation agents.”

Published in journal: Science Advances

Title: Surface reconstructions govern ice nucleation on silver iodide

Authors: Johanna I. Hütner, Andrea Conti, David Kugler, Franziska Sabath, Kim Noelle Dreier, Hans-Georg Stammler, Florian Mittendorfer, Angelika Kühnle, Michael Schmid, Ulrike Diebold, and Jan Balajka

Source/Credit: Technische Universität Wien

Reference Number: phy110325_01

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