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Researchers Develop Model for Simulating the Interaction of Water Molecules

Ice is everywhere. Learning more about how water molecules in ice behave can help scientists analyze many processes such as how glaciers move and how lightning occurs.

Over the years, scientists have developed a number of models to simulate the interaction of water molecules. Some of these models have been simple and easy to work with but have not produced accurate results. More accurate models have been complicated and hard to work with.

A team of chemists, physicists, and materials scientists at the University of Washington—with collaborators from the University of North Carolina, Pacific Northwest National Lab, the Supercomputer Computations Research Institute, and Los Alamos National Lab—set out to find a better way. They started with everything we know about how water molecules interact and built a simulation model from the bottom up. This model can simulate how water molecules interact under different conditions—for example, as ice crystals or as liquid water. This new simulation model is simple, and its results are promising.

According to Fernando Vila, one of the investigators on this KDI project, "Now we are starting to use the model for real things. We are currently studying the behavior of water molecules interacting on the surface of ice and are just starting to get results."

When we think about ice, what comes to mind most often are familiar images—icicles hanging from the gutter or ice cubes in the freezer. Ice also plays a key role in a number of important phenomena such as lightning. Lightning occurs when ice particles in clouds collide, charge, and separate, creating a buildup of static electricity that is then discharged as lightning.

The new model will be able to simulate this process that results in lightning. The additional information generated by the model about how ice particles behave in clouds—especially, about the transfer of charge from one ice particle to another—should help meteorologists predict weather.

The new model also can teach us about space and about the origins of life on Earth. The water ice that we have on Earth is different from the water ice found in space. Because it is very cold in space and there is no air, instead of regular ice, you have what is called amorphous solid water. It is not in crystal form, like the ice we know. According to Vila, "Since it is very difficult to generate this kind of ice in the lab, we are trying to understand it using simulations. This new model is perfect for that since it can simulate very different phases of water."

These kinds of ice found in space are interesting because they probably form the surface of Jupiter's moon Europa and because they coat interstellar dust particles. Such particles likely serve as catalysts for the formation of organic molecules in space.

For life to develop, you need at least some organic molecules. There are several hypothesis of how the molecules came to be on Earth—they were either formed here or they came from space loaded in comets. If they came from space, then how were they formed out there? One possibility that scientists are starting to check is that the molecules were formed as a result of reactions on the surface of these dust particles.

Ice also has an important role in the global energy balance. Most climate, weather, and biological processes on Earth are fueled by the intake of light (i.e., energy) that comes from the sun. The global energy balance is determined by how much of that light is retained and how much is reflected into space.

Two of the most important factors affecting the global energy balance are the amount of ice on the surface of the Earth and the Earth's cloud cover. Ice on the ground and water vapor in clouds are mostly white and are very reflective. The creation of more clouds and glaciers increases the amount of light that is reflected into space. Fewer clouds and glaciers increase the amount of energy that is retained. Therefore, the balance of energy depends crucially on the chemical and physical processes that create or destroy both clouds and glaciers.

This balance of energy is regulated by feedback processes: the more energy you let in, the higher the temperature and the more clouds you have. The clouds then start reflecting more energy out into space, which causes the temperature to drop. At the same time, the greenhouse effect tends to raise the temperature.

These feedback processes are complicated. The new model can simulate all the reactions that occur in its different phases. Simulations of these complex chemical reactions can be done in a matter of minutes!

As Vila explained, "All these chemical and physical processes have energy requirements. Knowing exactly how much energy is required allows you to create models that help predict both weather and climate. What we want to do is to provide accurate energy values for as many processes as possible, in that way helping create the best possible global models."

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