Thursday 23 November 2017

Snow and ice: How do you make fake snow?

Teresa Tierney and Sarah Hayes
Teresa Tierney and Sarah Hayes

TERESA TIERNEY AND SARAH HAYES

We all love a white Christmas; it is the subject of many songs and stories, - but what if we don't get one? Luckily, we can still make our own white Christmas with a polymer called sodium polyacrylate.

What is a polymer? A polymer is basically a repeating unit. We see polymers in our everyday lives. Plastics are made of a synthetic polymer called polystyrene, while the DNA and proteins in our bodies are made up of natural polymers.

How does sodium polyacrylate work to create fake snow? This polymer is a superabsorbent material and has the ability to absorb 200-300 its own mass in water. It is routinely used as an absorbent in baby's nappies (it is the small little white spherical pieces in the nappy with the cotton).

Superabsorbent polymers (SAPs) are materials that have the ability to absorb and retain large volumes of water. If we think of these polymers as a frame in which to hold the water molecules we can start to get an idea of how they work.

The polymer is hydrophilic (water loving), non-toxic and partially cross-linked. The cross-links are ionic bonds that link one polymer chain to another, putting our 'frame' together. This porous framework causes the material to act like a sponge, allowing it to absorb and hold water in its structure (frame), which does not dissolve itself in the water.

When the sodium polyacrylate is placed in water, the water molecules move into the 'pores' and fill them up. It expands in volume as more and more water is absorbed. For this to work properly an attractive force is needed between the water molecules and the polymer. A special bond, known as a hydrogen bond, forms between the two and allows the polymer to hold the water in place in its 'pores'.

The fake snow is made up almost entirely of water, and the water will evaporate over time, making it feel cold to the touch, just like real snow! So, once you have your sodium polyacrylate all you need to do is add water, and hey presto - instant snow!

This type of fake snow was used in filming Steven Spielberg's hit TV series 'Band of Brothers'. To clean it up, salt will break it back down into water and polymer, ready for re-use.

Sometimes snow for television or film is made in a different way, if they need a lot of it. This snow is not artificial, it is real snow, but a machine makes it rather than weather conditions. Real snow comes from water vapour.

*Teresa Tierney is a PhD candidate at the University of Limerick (UL), whose work is funded by the Irish Research Counciil. She is an associate member of the Synthesis and Solid State Pharmaceutical Centre (SSPC). Dr Sarah Hayes is Education and Outreach Officer, SSPC, whose academic oarents are UL, UCD, TCD, NUI Galway, UCC , DCU, National Institute for Bioprocessing Research and Training, Waterford IT and Athlone IT.

Irish Independent

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