Why the surface always faces outwards

From the outside instead of the inside

By Frank Grotel├╝schen

The old theory assumed that a drop of water began to freeze inside. The crystal nucleus was therefore suspected to be in the drop. My colleagues and I have now discovered that this germ is much more likely to form on the surface of the drop. So we believe that the drop freezes from the outside in and not from the inside out, as the 60-year-old theory had assumed.

Azadeh Tabazadeh works at NASA's Ames Research Center south of San Francisco. Together with her colleagues, she had viewed and evaluated vast amounts of laboratory data. She noticed that in some experiments the water droplets turned to ice very quickly, in other experiments, however, quite slowly. A discrepancy that cannot be explained by the theory of the drop freezing from within, says Tabazadeh.

For example, the speed at which water droplets freeze in the air is significantly faster than the speed at which droplets freeze that are dissolved in oil. If the droplets were to freeze from the inside out, then it shouldn't matter what substance they are surrounded by. Only when one assumes that the droplets freeze from the outside inwards, i.e. that the crystallization nuclei form on the droplet surface, can one understand that freezing takes place very differently in air than in oil.

The new hypothesis could be of particular importance for what is happening high up in the atmosphere, i.e. in the clouds. Clouds consist of countless supercooled water droplets, some of which freeze to ice more or less quickly. For several decades, however, humans have been changing this balance: Through industry and agriculture, they bring more and more aerosols into the atmosphere, in other words, the finest liquid droplets. According to Tabazadeh, this is not without consequences:

There is no more water in the clouds than before. But it is distributed in more and more and smaller and smaller droplets. And many small droplets have a larger surface overall than a few large ones. Well, according to the old theory, that wouldn't make a difference with freezing. But if, according to our theory, freezing should start on the surface of the droplets, then the water in the cloud, because there is more surface, will turn to ice more quickly in the future.

Accordingly, the man-made emissions would have a significantly greater impact on the clouds than assumed. If you will, the clouds are icing up more than expected. The experts from NASA fear consequences for the world climate.

I think there will be less rainfall in the future. Because ice forms in the clouds faster than before, the water droplets can no longer get bigger and therefore stay longer in the cloud instead of going down as precipitation. But we still have to study how this effect will work in detail.

To this end, Tabazadeh and Co. want to incorporate their results into a computer climate model soon. However, they do not expect tangible results for another two years.