Based in Switzerland, the Frontiers team are pretty familiar with snow. On clear days we can already see snow gathering on the nearby mountains and many of us are have started planning our winter ski, snowboarding and snowshoeing trips. However, in lots of places, including my native country, Scotland, snowfall is occasionally desired but unreliable.
In order to guarantee or enhance snow coverage, winter sports resorts sometimes use snow machines to make fake snow. These machines take advantage of the freezing process of water, either by forcing a mixture of water and compressed air out of nozzles to form snow, or by spraying a mist of water into a fan that blows the water droplets upwards into the air, where they transform into snow.
However, the creation and use of artificial snow is not restricted to winter sports. We often see artificial, although seemingly real, snow in movies and adverts, as well as in shop displays. This snow may be made of paper, plastic or even foam, and the type is selected based on the location of use, appearance and length of time it is required.
In the office today we are making snow with a sodium polyacrylate superabsorbent polymer. There are various types of sodium polyacrylate superabsorbent polymers, the most famous ones being those used in nappies (or diapers). However, unlike the polymer in nappies which absorbs water to become a gel, the snow-like superabsorbent polymer will become fluffy when we add water.
Superabsorbent polymers are made of long chains of molecules which are attached to each other at various points along the strands (cross-linked). They are named superabsorbent polymers as they have the ability to absorb very large volumes of water, sometimes even several hundred times their mass.
When water is added to a superabsorbent polymer, the water enters the polymer by osmosis (water molecules pass through a barrier from one side to the other). The polymer then starts to expand as water fills the spaces between the cross-linked polymer strands. As a result, the cross-links between the polymer strands eventually block further absorption (and expansion).
The degree of cross-listing of a superabsorbent polymer determines how much water it can absorb and its resulting physical properties. For example, if a polymer has a low degree of cross-linking then it will have high absorption capability and will become a gel when water is added. This is because there is plenty of space within the three dimensional structure of the polymer to accommodate water. I see why these types of superabsorbent polymers are popular for nappies!
In the case of the snow-like polymer there is a much higher degree of cross-linking and so the strands are more tightly held together. Less water is absorbed when it is added to this type of polymer but its network unfolds to create fluffy snow (whether the snow is suitable for making snow angels is an experiment for another day!).
In the meantime, happy National Chemistry Week!