New research from NTNU uses macro-crack initiator (MACI) surfaces to prevent ice from sticking.
In any cold environment, ice formation can be a problem. Whether it happens on pipelines, decks or from sea spray, its presence can very quickly become a serious inhibitor to safe and efficient operation.
Indeed, in 2014 DNV GL opened a dedicated joint industry project (JIP) to develop better sea spray modelling. At the time the agency’s Arctic technology programme director Per Olav Moslet remarked: “Sea spray icing poses a threat on multiple levels, from blocking the operation of essential components to jeopardising stability and integrity and thus leading to an increased risk of capsizing.”
Techniques for de-icing tend to include applying solvents – e.g. applying a de-icing solution to aircraft wings – or heating the infrastructure itself. Other efforts to control ice build-up, especially on structures such as wind turbines, transmission lines or indeed rigs, have focused on using “superhydrophobic” materials to repel water.
These substances can be sprayed on to surfaces, or objects can be dipped into the substance. However, they often contain fluorinated chemicals and are rarely guaranteed to prevent ice formation for long periods.
Researchers at the Norwegian University of Science and Technology (NTNU) have engineered a new material which allows ice to form on a surface, but then causes it to crack off. Their recent work on the development of “super-low ice adhesion surfaces” – nattily dubbed SLICE – has just been published in Royal Society of Chemistry journal Soft Matter.
This approach – to allow ice to form but prevent it sticking – is somewhat novel within the field. “We think we have found a very interesting method to reduce ice adhesion which is unique, and a breakthrough in the anti-icing community,” commented Department of Structural Engineering professor and SLICE team lead Zhiliang Zhang.
The key to achieving such a surface is an in-depth understanding of fracture mechanics. In their abstract, the authors write: “The key to lower ice adhesion is to maximise crack-driving forces at the ice–substrate interface.” In practice, that means creating a surface chemistry that will cause cracks by weakening the atomic bonds between the ice and surface – substances referred to as nano-crack initiators (NACI).
Another approach is to cover a surface with micro-crack initiators (MICI), bumps whose roughness enables micro-cracking at the contact between the surface and the ice, also reducing the ability of ice to stick to the surface. Neither method is perfect – but research into these existing structures by Zhang and his team suggested that a combination would prove highly effective.
This new mechanism would help form large macro-cracks at the meeting between the surface and the ice – a material they dubbed MACI, or macro-crack initiator. As these cracks get larger, ice is less and less likely to stay on the surface.
To test that process, the SLICE team created subsurface layers that had microholes or pillars. They then deposited a thin film of a substance called polydimethylsiloxane (PDMS), which covered the holey, bumpy substructure layers. Different MACI structures were tested, as well as designs using multiple layers with inner holes. In practice, they reported that ice adhesion on MACI surfaces was at least 50% weaker than on pure PDMS surfaces without MACI, and produced “some of the lowest values for ice adhesion ever measured.”
“The ice adhesion strength for common outdoor steel or aluminium surfaces is around 600-1000 kPa,” Zhang said. “By introducing the novel MACI concept to the surface design, we reached the super-low ice adhesion value of 5.7 kPa.”
Zhang said the group must now develop the idea, but they believed the breakthrough with MACI might be the best solution for preventing ice build-up without incurring unwanted environmental effects. This study was backed by Statoil and the Research Council of Norway, and the team is now working to establish a dedicated NTNU Anti-icing Lab to launch further studies into ice nucleation, and the properties of ice-repelling surfaces.
“Traditional active de-icing techniques… can have major detrimental effects on structures and the environment. But passive super-low ice adhesion surfaces avoid all those detrimental effects. This is very interesting not only for the scientific community, and for Arctic applications, but for solar panels, for shipping and transmission lines. There are a lot of applications related to everyday life,” he added.