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Pathways to Intrinsically Icephobic Surfaces

Periodic Reporting for period 3 - INTICE (Pathways to Intrinsically Icephobic Surfaces)

Reporting period: 2018-11-01 to 2020-04-30

We are investigating the fundamentals of ice formation on surfaces with the goal being 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, ranging from transportation to infrastructure and having a significant economic and safety impact.

The two primary goals of this proposal were to assess the role of inherent surface properties (such as texture) and droplet size 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.
Our first major achievement was assembling our team consisting of the P.I. one postdoctoral researcher (expert in transport phenomena and thermodynamics, experimentalist), and three PhD students (mechanical and chemical engineering backgrounds). They all started on 1 November 2015, the first day of the project.

The second major achievement was creating the experimental methodology necessary to study nucleation and transport phenomena, which included assembling/fabricating optical micro-imaging systems and environmental chambers (including cryogenic) and acquiring high-speed infrared and optical cameras and sensitive optical cameras necessary for high-spatial resolution. To do this in the best possible manner, we created a brand-new laboratory space (completed August 2016) equipped with a flow-box for achieving the clean environment necessary for nucleation studies and vibration isolation tables for the imaging setups.

With these fabrication and characterization tools in-place, we were able to begin our investigations into the physical limitations for solid textured surfaces in repelling impacting supercooled water droplets. Inspired by natural examples, much work has been done on materials engineering to achieve superhydrophobicity. Normally, hydrophobicity is enhanced through the combined effects of surface roughness and chemistry, and being durable, rigid materials are the norm. However, many natural and technical materials are flexible, and its intrinsic surface effect on hydrophobicity has been ignored. We showed that substrate flexibility—tuned rationally—can work synergistically with surface micro/nanotexture to enhance superhydrophobicity as defined by impalement resistance, contact time reduction, and restitution coefficient increase. By reducing substrate areal density, the substrates have immediate responsiveness to impacting droplets (∼350 × g), mitigating the collision and passively lowering the impalement probability by ~60%. We exemplify the above findings with materials ranging from synthetic (thin steel or polymer sheets) to natural (butterfly wings). This work was all done for droplets that were at room temperature. This work was published in the peer-reviewed journals Proceedings of the National Academy of the USA (PNAS) in November 2016. Next, we investigate whether or not flexibility can affect the physical limitation for textured surfaces in repelling impacting supercooled water droplets.

Ice accumulation on surfaces impacts the performance and safety for infrastructure both on the ground and in the air. Previously, superhydrophobic surfaces have demonstrated potential for mitigating ice accretion; however, more work on material solutions to reduce impalement and contact time of impacting supercooled water droplets (high viscosity) is urgently needed. We demonstrated (Vasileiou 2017) the collaborative effect of substrate flexibility and surface microtexture and nanotexture on enhancing icephobicity as defined by impalement resistance and contact time reduction to impacting supercooled water droplets and representative viscous droplets (comparable to supercooled water). We also demonstrated the effect of flexibility on repelling even rapidly solidifying droplets. First, we investigated the influence of viscosity (range: 0.9 – 1078 mPa s using water-glycerol mixtures) on impalement resistance and droplet-substrate contact time for impacting droplets. Next, we studied the effect of droplet solidification on the rebound dynamics. Here, we simulate more challenging and realistic icing conditions by impacting supercooled water droplets (down to -15 °C) onto flexible and rigid substrates contaminated with ice nucleation promoters (AgI). We discovered a passive mechanism for shedding partially solidified droplets, which does not rely on the droplet converting surface energy into kinetic energy (classic recoil mechanism). With an energy-based model (kinetic-elastic-capillary), we identify a previou
We showed that substrate flexibility can work collaboratively with surface micro/nanotexture to enhance superhydrophobicity as defined by impalement resistance, contact time reduction, and restitution coefficient increase.

We demonstrated the synergistic effect of substrate flexibility and surface texture on boosting icephobicity as defined by impalement resistance and contact time reduction to impacting supercooled water droplets. We also demonstrated the effect of flexibility on repelling even rapidly solidifying droplets and propose design rules for materials that benefit from this newfound knowledge.

We showed sublimating coatings are able to repel many viscous and low-surface tension liquids and displays excellent omniphobic properties. The independence of the levitation principle from the underlying materials makes this technique attractive for a wide-range of repellency and droplet handling applications.

We showed that the wettability of graphene is connected to its Fermi level position, affecting the water-graphene interaction. Dopants can tune the hydrophilicity of graphene by modulating its Fermi level. By shifting graphene’s Fermi level from its Dirac point we showed enhanced hydrophilicity with experiments and first principle simulations. Enhanced vapor condensation on graphene, achieved by a shift of its Fermi level, represents applications in the area of interfacial transport phenomena.

We demonstrated that by confining solar energy to an ultrathin, rationally-designed plasmonically-enabled metasurface, one can induce significant heating to the metasurface, which can be utilized for effective removal of an ice layer already formed on the surface (deicing), and also proactively, for preventing ice formation (anti-icing). The metasurface can be partially transparent, important in numerous commercial applications, with a tunable level of transparency, without significantly compromising its performance.

We demonstrated that the accretion of frost at temperatures, well within the sublimation domain (<-45°C), can be influenced significantly by confinement. At these temperatures desublimation is the observed mode of nucleation, i.e. nucleation with no visible liquid phase beforehand. Desublimation mode nucleation is traditionally ascribed to direct nucleation of ice from vapor. We tested the effect of confinement for desublimation by comparing the frosting behavior on smooth silicon and on silicon textured with nano-bumps and pits (50 nm). We found that the desublimation of frost happens at lower supersaturation on the nanotextured surfaces. Based upon literature and our thermodynamic knowledge, we postulated that the desublimation nucleation is aided by the initial nucleation of a confined amorphous phase inside the nanopits. Further evidence for this has also been published concurrently with our publication.

We showed that surface fogging poses a significant obstruction to visibility through surfaces, not fully addressed by existing prevention techniques, such as superhydrophilic or superhydrophobic coatings. We propose partially transparent, ultrathin durable metasurfaces, working on the basis of sunlight-to-heat conversion, as an effective means of fog removal (defogging, through increased water evaporation rate) and fog formation prevention (anti-fogging), important in applications where transparency is required, such as windshields, optics and all kinds of eyewear.

We demonstrated that rational surface engineering can lead to self-removal of droplets during the droplet freezing process. We showed that this is the case when the heat removal from the droplet free surface exceeds the heat removal through the substrate. This condition can be achieved by convective cooling where a cold wind blows at the droplets resting on the surfaces, but also by exposing the droplets to a low-pressure environment where they strongly evaporate from their free surface.

We showed that there is a group dynamics in dropl