Nanomanufacturing of Surfaces for Energy Efficient Icing Suppression

Undesirable ice formation causes a lot of disruption - from impairing energy efficiency of household refrigerators to causing destructive accidents due to ice accumulation on infrastructure components and airplanes. The proposed research aims to address this ubiquitous problem using precise, but potentially scalable techniques to nanoengineer icephobic surfaces that can suppress ice formation, resist impact of cold drops and have minimal adhesion to ice. The proposal is motivated to provide a viable, passive and energy efficient alternative to the currently employed anti-icing techniques, which rely either on electro-thermal systems that affect the system efficiency and running costs, or make use of environmentally adverse chemicals. The surface nanoengineering to be employed will involve a precise control of both the surface texture at nanoscale and the surface hydrophobicity. The appropriate combination of these two aspects is expected to not only suppress ice formation in severely supercooled conditions (at sub-zero temperatures), but to resist impact of high speed supercooled droplets and minimize adhesion of ice on the surface - all these aspects are relevant to icing in practical applications and will be tested in the current work.

The ambition of the proposal is to make nanotextured surfaces with nanohole arrays with better than 10 nm precision (i.e. resolution). Such precise and rounded morphologies are expected to suppress ice formation according to the thermodynamic heterogeneous ice nucleation framework previously introduced by the PI and supported by atomistic modelling results in the literature. In addition, self-assembly of hydrophobic molecules on the surfaces will allow a control over the surface energy, which, in combination with the texture control, will help produce superhydrophobic surfaces that can resist impalement by high speed, cold drops, and have low ice adhesion. The drop impalement resistance can help avoid icing on aircrafts and outdoor infrastructure elements in freezing rain conditions. As a proof-of-concept for a potentially scalable, precise nanotexturing, current project will exploit electrochemical anodisation of metals through polymeric nanohole films, prepared using block-copolymers (BCP), serving as templates. The surface texturing will be limited to top ~100 nm or lower thickness of the substrate and only mild anodisation conditions will be used. The templated anodisation is well suited to aluminium and titanium - substrates prevalent in aerospace, refrigeration and automotive industry; however, similar templated etching approaches can be developed for substrates in other applications (see the PATHWAYS TO IMPACT section). PI's prior work has shown that thermally conductive substrates are better for arresting frost formation from cold drops lying on the surface, thus the metallic substrates are a very good choice. In addition, the current work, for the first time, introduces a novel means to use simple anodic surface projections to improve the resolution of BCP nanohole templates themselves to ~10 nm precision - surfaces anodised through these precise templates are expected to be ideally suited for icephobicity.

The resulting anodised substrates will be rendered hydrophobic by functionalizing with hydrophobic molecules. These precisely nanotextured hydrophobic surface are expected to suppress icing not only due to their rounded nanoscale morphology, but will also feature minimal solid-liquid contact area, thereby further suppressing the icing probability. The synthesized surfaces will be subjected to three set of tests: their ability to resist impalement by high speed, supercooled drops (target: 25 m/s); ability to delay ice formation in supercooled conditions at different humidity levels (target: 2 hours at -25 degrees Centigrade); and minimize their adhesion to frozen (ice) drops.

The primary impact of the proposed work will be to provide a passive, energy efficient solution for the undesirable icing problem. The impact of the proposed work are summarized below.

Industrial relevance: Energy efficient icing suppression is invaluable for a number of important industries. Salient, non-exhaustive examples include aerospace industry, where icing can cause uncontrolled rise in drag and seriously impair the flight lift, thereby causing fatal accidents and/or loss of resources; domestic and industrial air conditioning and refrigeration systems, where accumulation of ice leads to efficiency reduction; outdoor infrastructure and transportation components operating in freezing conditions; wind turbines operating in cold climates, which are severely affected by icing etc. The involvement of Airbus as the industrial project partner, with substantial direct and in-kind support pledge, is a testament of the strong industrial translation potential of the proposed research. The robust, nanotextured surfaces resisting high speed drop impact will also be valuable for several other applications that require stable liquid repellency such as superhydrophobic surfaces for drag reduction and promotion of dropwise condensation in steam condensers - a common component in nearly all steam power plants.

Strategic national interests: Wind is the fastest growing renewable energy sector in the UK; stable, ice-free wind turbine operation in remote locations and harsh weather zones will be a unique export technology given the current push for renewables in the EU. The icephobic surfaces are directly relevant to this end. Efficient refrigerators and air conditioning units are crucial to reduce household and building energy consumption. Robust icephobic surfaces will potentially eliminate the icing related accidents in unmanned aerial vehicles (UAVs) and facilitate efficient operation of the navy vessels involved in the explorations and the future harnessing of arctic resources, thereby offer a clear strategic and national security benefits. The emphasis on energy efficiency, as laid out in the proposal, is in synch EU's 20/20/20 Strategic Energy Technology (SET) plan, which is also adopted by the UK.

Energy efficient buildings and global sustainability: Ice as a thermal insulator affects many applications, e.g., icing on evaporators in air side heat pumps and refrigerators increases the domestic energy consumption and also in the industrial refrigeration and cold storage units - the proposed metallic icephobic surfaces will be a valuable antidote. Furthermore, the conceived templated nanotexturing approach will contribute to the precise and scalable texture control required for developing the future smart windows trapping solar radiation. Such developments will facilitate smart and energy efficient buildings with energy efficient appliances. According to the International Energy Agency (IEA), energy efficiency is expected to serve as the 'first fuel' in realizing the global ambition to steer a progressively larger fraction of the overall energy consumption to renewables. Thus, the proposed research will contribute in meeting the UK commitment to grow the renewable contribution of its overall energy consumption from 4.1% at the end of 2012 to 15% by 2020.

Economic and environmental benefits: The proposed block copolymer (BCP) templated anodisation and nanotexturing is a smart coating/surface treatment, which is projected to become a £1.5B market by 2022 according to BCC Research. Anodisation is used widely in industry; thus, the proposed approach can be integrated in the existing process chains. Furthermore, optimized surfaces should only require anodising ~100 nm surface depth. These features will improve process efficiency, minimize chemical use and, thereby, promote sustainability - the IEA also makes it clear that developing energy efficient devices will have a major impact in minimizing our global carbon footprint.