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The hazards of ice accumulation include damage to infrastructure, disruption of transportation, and even major accidents. Historically, the solution has been to de-ice surfaces by chemical, mechanical or thermal means. Anti-icing coatings are a proposed alternative which passively prevent ice accumulation. The icephobicity of a coating is assessed via the ice adhesion strength and freezing delay. Elastomer coatings infused with miscible oil have shown particularly low ice adhesion. However, there has been little investigation of the effect on freezing delay. The aim of this work is to provide a comprehensive study of the icephobicity of oil-infused elastomers to better understand the effect of oil-infusion. Seven elastomer coatings were investigated: two PDMS (polydimethylsiloxane) coatings, four silicone-oil/PDMS coatings (two molecular weights of oil, at two concentrations - 25% and 50%), and a commercial coating (NuSil R-2180). Ice adhesion testing was performed via a push test method, in which ice frozen in acrylic cylinders on the coatings was displaced by a force probe. 100 repeat de-icing cycles were performed in a refrigerated laboratory at −10°C. Freezing delay was measured via visual observation of the time for a droplet of water to freeze, from deposition on the surface, and compared to a bare aluminium reference. The elastic modulus, contact angle, surface roughness and room temperature adhesion of the surfaces was characterised alongside the icephobicity testing. Despite greater degradation over time, the oil-infused coatings exhibited consistently lower ice adhesion strengths than the non-oil infused coatings, less than 50% in most of the 100 de-icing cycles. De-icing did not cause significant damage or increase in ice adhesion strength of the oil-infused coatings, but the non-oil infused coatings exceeded the equipment load limit with increasing frequency, meaning they lose icephobicity. Fitting the data to models for interfacial cavitation and interfacial slippage showed a better correlation to interfacial slippage, and shear modulus had the strongest influence on adhesion strength. This understanding can assist future coating design. There was little difference in the freezing delay of the coatings, but all provided an improvement on bare aluminium. Measurements performed before and after adhesion testing showed a small decrease in freezing delay post-adhesion. The effect of severe damage was investigated using specimens which were abraded with grit paper or cut by a scalpel. There was a slight increase in ice adhesion and small decrease in the freezing delay. Cryo-FIB/SEM showed a mixture of high and low conformation at the ice-coating interface. Recoating the surfaces restored the original performance of the coatings, which would allow for straightforward repair in practice. To better understand the mechanism by which oil-infusion lowers ice adhesion strength, a new method for investigating interfacial slippage was developed. Using a microtribometer mounted under an optical microscope, de-icing tests were performed on ice droplets. Fluorescent microparticles embedded in the top of the coatings were tracked during de-icing to monitor for polymer flow. Results were inconclusive in determining the presence of interfacial slippage; however, suggestions for further work are made. The oil-infused elastomer coatings are shown to have excellent icephobicity. Their use in anti-icing applications lowers the ice adhesion strength while maintaining freezing delay of non-oil infused coatings. Their use would reduce de-icing requirements and lessen the energy, time and resources spent on removing surface ice from, for example, bare metals. They would also reduce damage caused by accumulation and detachment of heavy ice loads. Durability of oil-infused elastomers is also shown not to be a major concern: de-icing causes minimal damage, good icephobicity is maintained even with moderate-to-severe damage, and recoating damaged surfaces is an effective method of repair.

Superhydrophobic surfaces have many potential applications from self-cleaning textiles to ice-free aircraft. Their low affinity to water stems from a heterogeneous wetting state predicated on the stability of an intervening air layer between liquid and substrate. However, this layer is fragile and irreversible transitions to a homogeneous wetting state are possible and devastating to their performance. Recent studies have examined how pressure-based transitions, known as impalement, occur for droplets impacting superhydrophobic surfaces and have proposed solutions to mitigate against this under ambient conditions. Yet to this point, little consideration has been given to how the interplay between environmental effects and the dynamics of droplet-substrate interactions affect the behaviour of the intervening layer and the different phenomena that may result; especially in relation to phase change. In thiswork, I explore how departures from the standard ambient environments influence the local thermodynamic conditions and stability of the intervening layer, elucidating the wetting state transitions that arise from these volatile conditions. In the context of droplet impact, I investigate how the pressure and gas composition of the intervening air layer critically influence the ability of droplets to rebound from a superhydrophobic surface. Through high-speed imaging and theoretical modelling, I expose a previously unknown condensation-based wetting state transition mechanism in humid conditions and explain a trend of increasing impalement with decreasing environmental pressure. When studying the freezing of sessile droplets on superhydrophobic surfaces, in a low-pressure environment I deduce the presence of a force engendered by the evaporation difference across a droplet undergoing recalescence capable of either driving it into the texture or explosively expelling it, depending on the characteristics of the substrate in question. Exploring the complementary case of ambient pressure and low-temperature, I uncover a novel condensation-based filling mechanism induced by the temperature rise during recalescence that is endemic to freezing events on superhydrophobic surfaces that require a degree of subcooling with acute repercussions for their use in ice-repellency applications. Finally, whilst developing a superhydrophobic coating robust to impinging warm water droplets using a photothermal metasurface, I establish the theoretical foundations of condensation filling in a diffusion-limited regime based on acompetition between the filling and droplet-substrate contact times. Furthermore, I rationalise the performance of a hierarchical texture by considering the resulting shift to a nucleationlimited condensation regime. The understanding of phase change-induced wetting transitions accrued here should herald advancements in the design of superhydrophobic surfaces, broadening their working envelope and extending water repellency far beyond the state-of-the-art.

Droplet freezing is important both in nature and in technology. In this thesis I investigate the fundamentals of freezing water droplets and derive design criteria for the development of intrinsically ice-repellent materials. Such icephobic surfaces could improve the performance and safety of a multitude of technical processes in energy and transport. This includes for example heat exchangers, where ice built-up reduces thermal transport, and airplane flight, where freezing of water on airfoils can result in catastrophic events. The thesis consists of three individual studies. In the first study we investigated how the environmental conditions during droplet freezing affect the freezing outcome. We found that evaporatively or convectively supercooled water droplets resting on solid substrate can self-remove during freezing. This phenomenon, which we termed self-dislodging, requires that the heat removal from the droplet’s free surface dominates the heat removal through the solid substrate. Consequently, the freezing front moves from the outside of the droplet towards the center and from the top to the bottom, resulting in a solid ice shell with an unsolidified core and an unfrozen droplet-substrate interface. We observed experimentally that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, due to the volumetric expansion associated with freezing, semi-frozen water is displaced towards the droplet-substrate interface lifting the freezing droplet away from the substrate. The combined effects of dewetting and lifting result in droplet self-removal. We found that the more the substrate is hydrophobic the more robust self-dislodging occurs. In the second study we examined how multiple water droplets interact during freezing in a low-pressure environment. Understanding droplet interactions during freezing is important as droplets do not appear in isolation, but always in groups. We found that the freezing of a supercooled droplet results in self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We suggest that as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble resulting in ice coverage of the solid surface. In the third study we introduced a controllable and upscalable method to fabricate superhydrophobic surfaces with a 3D-printed architecture for improved repellency of viscous liquids. We show a more than threefold contact time reduction of impacting viscous droplets up to a fluid viscosity of 3.7mPa s, which covers the viscosity of supercooled water down to -17 °C. Based on the combined consideration of the fluid flow within and the simultaneous droplet dynamics above the texture, we recommend future pathways to rationally architecture such surfaces that can repel supercooled water before it freezes and sticks to the surface. The three studies presented in this thesis address the topic of surface icing from three different angles, collaboratively covering a broad range of the problem. Only when taking into account the environmental conditions, freezing group dynamics and liquid solid interactions, robust icephobic surfaces can be designed in the future. With my thesis I contribute to this development process.

The processes of phase transformation continue to affect our natural and industrial worlds in a range of scales from large-scale lava flows in nature to micro-and nano- scale applications in industry. Enhancement of the knowledge framework of these processes and understanding the accompanying physical phenomena are keys to resolve challenges in scientific research and obtain rational engineering evaluations for a wide range of industrial application such as 3D printing, rapid prototyping, thermal spray coating, power plants, aerospace and turbine industries. A great deal of work has been done in the past few decades on these matters, and significant progress has been made; however, much of it remains unraveled. The main difficulty stems from the intertwined effect of heat transfer, fluid dynamics, and phase change physics with the combination of complex wetting behaviors of the contact-line. In this thesis, we focus on solidification and condensation near the contact-line. For solidification near the contact-line, we have studied the onset of solidification and its relevant physical parameters and explored ways to delay the start of solidification inspired by the unique ice-resisting abilities of penguin feathers and certain plant leaves. The dynamics of solidification and spreading is also studied and new hypothesis is introduced to explain the contact-line pinning due to solidification. Finally, in condensation near the contact-line, we have introduced a novel biphilic surface with large scale cost-effective manufacturing process and analyzed and compared its heat transfer efficiency and water droplet mobility to traditional hydrophobic and hydrophilic structures.