Sinking into Snow

snow
Image: FAU/Celina Henning

Materials scientists at FAU have demonstrated for the first time in an experiment that collapsing connections between ice crystals are responsible for snow giving way suddenly – and repeatedly – under weight. Their results could be used in avalanche research. The study has recently been published in the renowned journal Nature Physics.

If you have ever taken a walk in the snow, you know the phenomenon: carefully placing one foot in front of the other, trying to land evenly with each step. The snow crumbles a bit underfoot, but it holds – up until the moment when you suddenly find yourself standing up to your hips in snow. ‘Most people assume that this happens because the snow has a crust on its surface that they break through if they put too much weight on it or step in the wrong spot,’ said Prof. Dr. Michael Zaiser, Chair of Materials Simulation at FAU. ‘But this explanation is wrong in most cases.’

In their experiment Professor Zaiser and his colleagues from the University of Edinburgh demonstrated that people and objects also suddenly sink into homogeneous laboratory-created snow which does not exhibit a crust on its surface or any other differences in firmness. A high-speed camera which enabled the researchers to look at the snow’s microstructure revealed the cause. ‘We can see from the high-resolution images that the microstructure collapses once a critical load is reached, causing a sudden drop in firmness,’ explained Michael Zaiser. ‘This collapse continues downwards and the load bearing down from above sinks.’

Sinter bridges break in multiple places

In general snow has lots of space for compression, with settled snow that has already lain for several weeks still comprising up to 70 percent air. That the snow does not immediately collapse is the result of the sinter necks formed between the ice crystals, which keep them apart. If the snow is compressed, some of these bridges break and the remaining bridges are left to bear the weight. Once a critical point is reached, the remaining ice bridges also cave in and a partial structural collapse occurs. The ice crystals are compressed in this process, which increases their resistance and normally prevents the load from sinking all the way through the layer of snow.

The researchers were interested to discover that this process could be repeated in a single sample: a collapsed layer can collapse a second or even multiple times when loaded, provided that enough free volume remains. This is caused by the rapid development of new sinter bridges, which can form within seconds and break again. ‘This is also something that you can often observe when walking outside in winter: you follow in the footsteps of someone walking in front of you, choosing a path where the snow has already been packed down in the hope that you won’t constantly be breaking through the snow, but it still happens sometimes, even if you aren’t much heavier than the person whose footsteps you’re walking in,’ said Michael Zaiser.

Possible contribution to avalanche research

Following the systematic analysis of the collapse phenomena which could also be confirmed in natural snow, the materials scientists developed a computer model which describes the processes with great precision and can reproduce them in a computer simulation. ‘The initial programming and first simulation of the model was done by Gerhard Weinländer in his Bachelor’s thesis project,’ said Michael Zaiser. ‘He is now, as a student, a co-author in the most renowned journal of physics in the world, which is incredibly unusual.’

The researchers may also be able to contribute to avalanche research with their findings: ‘The same process of microstructural collapse and the associated destabilisation which we can observe in the laboratory or under our boots may also occur at the formation of a slab avalanche,’ explained Michael Zaiser. ‘The Scottish Avalanche Information Service, for example, uses ‘foot penetration’ as an indicator when determining avalanche risk. More research will be needed to identify the connections with our findings.’