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Pathways to Intrinsically Icephobic Surfaces

Periodic Reporting for period 4 - INTICE (Pathways to Intrinsically Icephobic Surfaces)

Reporting period: 2020-05-01 to 2020-10-31

We are investigating the fundamentals of ice formation on surfaces with the goal to understand the related physics and exploit it to rationally design materials that can intrinsically (without external means such as electrical heating, or continuous application of icing retardation chemical treatments) inhibit ice accumulation.

Surface icing is common in nature and technology, and its uncontrolled accumulation can have catastrophic effects on a broad palette of applications with everyday utility to society.

The primary goals of this work were to assess the role of rationally designed inherent surface properties (such as texture), environmental conditions and droplet physics on the onset of ice nucleation, as well as to determine and target the physical limits of solid textured surfaces in repelling impacting supercooled water droplets or removing ice, yielding extreme, intrinsic, anti-icing performance.
In the context of the performed research, we created a series novel experimental methodologies necessary to study nucleation and interfacial transport phenomena, including assembling/fabricating optical micro-imaging systems and environmental chambers and acquiring high-speed infrared and optical cameras for high spatial resolution. The multifaceted work of this project has been published in its entirety in very top peer refereed journals. Main findings can be summarised as follows:

-We investigated the physical limitations of solid and soft textured surfaces in repelling impacting supercooled water droplets. We showed the synergetic effect of flexibility and surface micro/nanotexture in enhancing superhydrophobicity counteracting ice formation. We then advanced our research to tackle surface icing and achieved its understanding and control, exploiting rationally collaborative thermodynamic, fluidic, structural and topographical substrate effects.

-We were able to design and realise unique nanoengineered surfaces that limit surface icing due to de-sublimation (ice deposition) effects, usually appearing in low pressure environments.

-We investigated and understood the physics of the condensation nucleation of water on 2D nanomaterials (graphene).

-Employing intrinsic ice–substrate physics, we showed how a droplet can self-dislodge from an appropriately designed surface texture (self cleaning from ice) upon freezing. What os more, we completed our understanding and are developing thorough guidelines (forthcoming publication) on how a surface texture can be designed, so that upon icing, water drops ( in a supercooled state) do not only dislodge themselves, but also self-levitate due to explosive vaporisation upon recalescent freezing, and remove themselves from the surface.

-We tackled the broad problem of fogging and icing in particular from surfaces that must be transparent and performed original research by designing unique plasmonic metasurface coatings, impressively retarding fogging and icing by harvesting sunlight. This was shown in a series of papers on novel, high-performance plasmonic metasurface coatings, tackling the phenomena of fogging/defogging and icing/deicing of transparent surfaces, the findings of which can be exploited in many applications.

-We also investigated dropwise condensation and identified surface design rules for consistent realization of the coalescence cascades.

-With reference to soft materials we investigated the influence of soft substrate viscoelasticity on droplet receding dynamics in drying situations, which can occur both at low and high temperatures. To this end, we discovered that the viscoelastic properties can have significant effects in the formation of particle deposits the droplets may contain. We found large departures from rigid materials (for example vis. the coffee stain deposit formation).

-Another serious problem we studied is bacterial colonisation, relevant to all kinds of surfaces in contact with water and of course relevant to the case of surface condensation and eventual freezing. We developed a facile, scalable, and environmentally benign strategy for inherently antibacterial ZnO nanostructured surfaces.

-We also investigated icing of road materials, ie. bitumen and related composites, very important to sealing and road applications. To this end, we demonstrated the condensation and freezing mechanisms of atmospheric water on bituminous surfaces at subzero temperatures.

-We have also investigated fundamental aspects of the remarkable Leidenfrost phenomenon, manifesting itself both at low temperatures (sublimation) and high temperatures (evaporation). We discovered a novel droplet trampolining effect emerging under certain operating conditions, which we defined and could control, enhancing or suppressing this phenomenon according to need.
We showed that substrate flexibility can work collaboratively with surface micro/nanotexture to enhance superhydrophobicity and effectively repel even rapidly solidifying droplets.

We then showed that sublimating coatings are able to repel many viscous and low-surface tension liquids and display excellent omniphobic properties.

We showed that the wettability of graphene is connected to its Fermi level position.

We demonstrated effective deicing and anti-icing by sunlight confinement with ultrathin, rationally-designed metasurfaces that can also repel fog.

We demonstrated that the accretion of frost at temperatures well within the sublimation domain can be influenced significantly by nanoscale confinement and that rationally engineered surfaces can lead to self-removal of droplets during the freezing process.

We unveiled the effect of substrate compliance on droplet evaporation and showed that above a critical receding contact line speed the substrate exhibits enhanced hydrophilicity.

We also developed environmentally sustainable surface fabrication protocols to obtain superhydrophobic surfaces with enhanced antibacterial action.

We discovered a new mechanism for the failure of superhydrophobic surfaces through condensation inside the texture.

We developed scalable plasmonic composites capable of harvesting sunlight to passively sustain both water repellency and surface transparency under challenging supersaturated environmental conditions.

Finally, we revealed the surface chemistry of bitumen in the nanometer scale at subzero temperatures and also studied the wettability of these bitumen surfaces.