Breaking the ice

Each spring, under the effect of waves, the edges of the northern hemisphere’s ice floes fracture into millions of fragments. Scientists from the PMMH physics and mechanics of heterogeneous environments laboratory (CNRS / ESPCI Paris-PSL / Sorbonne Université / Université Paris Cité) are trying to understand the parameters and conditions that cause the dislocation of ice floes in what are known as marginal ice zones, which stretch hundreds of kilometres. As it happens, these zones, located on the border between the sea and the pack ice, are expanding due to intensified seasonal melting, making the ice field more fragile.

One of the PMMH’s teams, which specialises in wave propagation in fluids and thin solids (such as the sea ice), devised a laboratory experiment involving a model ice floe. Their objective is to reproduce, on a small scale, the fracturing of the ice floe under the effect of waves.

This experiment requires a pool of water with an oscillating wave generator that also reproduces miniature swells. While the approach may seem basic, the use of such instruments enables scientists to control both wave velocity and height.

The most complicated aspect of this model was to find a way to reproduce an ice floe in the laboratory. This primarily involved identifying a material that is as fragile as ice, but softer. Unable to use ice consisting of seawater, the researchers had to find a substitute. They closely examined twenty different products in an effort to recreate an imitation ice floe in the lab. They ultimately chose a varnish-glue used in the arts. When sprayed on the water’s surface, the varnish forms a uniform layer measuring a few hundred microns that floats on the pool’s surface. This crust is both brittle and soft!

Close inspection reveals the appearance of large cracks in the “white ice floe”, as well as the deformation of the laser’s route under the action of waves. The broader the generator’s movements, the more the pool’s surface – and that of the varnish – is deformed. The scientists are trying to determine the fracture threshold based on the size and wavelength of the waves. The more frequent and powerful (high) they are, the more the ice floe breaks.

As in the Far North, as the frequency and height of waves increase, the first clean breaks give way to the ice pack breaking up into increasingly small blocks. The scientists are trying to determine what fragment sizes, typically ranging between 1 and 100 metres in length, will ultimately remain.

While basic, this research helps provide a better grasp of sea ice behaviour. Climate change reduces the ice floe’s thickness, especially in the summer, thereby making it even more susceptible to breaking under the effect of waves.