What Does Wettability Have to Do with Icing?
Alidad Amirfazli, York University, Canada
Icing of structures is a major hazard from aviation (e.g. inflight icing), to energy production (e.g. icing of wind turbines), to maritime (e.g. offshore platforms and patrol ships). Icing is usually the result of a droplet impacting a surface and staying long enough to freeze on the surface. Traditionally active methods such as thermal, mechanical, or chemical treatment has been used to mitigate icing or de-ice a structure. These methods are usually energy intensive, or for some applications not practical. The focus on using coatings of various kinds to mitigate icing, or the required energy for de-icing, has been intense in the past 15 years. Often wettability of coatings is used as a first indicator for its effectiveness in aiding with de-icing or mitigating the icing in the first place. This is so, as it is known that wettability of a surface influences how a droplet will remain or can be removed from a surface; also surface wettability will determine the nucleation sites for ice or frost to form, which in turn will cause freezing of the water on the surface. As such, this talk will focus on how wettability of a surface affects icing before ice is formed. Surface wettability effects on three different aspects related to icing will be discussed: (i) how a droplet will shed from a surface, (ii) delay in freezing of a droplet on a surface, and (iii) drop impact behavior. Examples from laboratory tests to field tests will be discussed.
On Physics of Icephobicity and Durable Advanced Icephobic Coatings
Hadi Ghasemi, University of Houston, USA
A fundamental understanding of solid-liquid interfaces plays a critical role in energy, water and even biological systems (e.g. freezing, condensation, evaporation, crystal growth and regenerative medicine). Knowledge on physics of these interfaces allows us to control interfacial momentum and energy transfer in multiple length and time scales and to create new surfaces with unprecedented characteristics.
Icephobic surfaces have a critical footprint on human daily lives ranging from aviation systems and infrastructures to energy systems, but creation of these surfaces for low-temperature applications remains elusive. Non-wetting, liquid-infused and hydrated surfaces have inspired routes for development of icephobic surfaces. However, high freezing temperature, high ice adhesion strength and subsequent ice accretion, low mechanical durability, and high production cost have restricted their practical applications.
Here, we provide a comprehensive definition for icephobicity through thermodynamics, heat transfer and mechanics of ice/water-material interface and elucidate physic-based routes through which nano-scale could help to achieve exceptional icephobic surfaces. We introduce two new physical concepts that are used to develop advanced interfaces. Underlying physic of these surfaces is discussed and implementation of these concepts in icephobic and anti-fouling surfaces is shown. These surfaces outperform other state-of-the-art icephobic surfaces with ice formation temperature of -34 ⁰C and ice adhesion strength of ~2 Pa (five orders of magnitude lower than state-of-the-art surfaces).
Considering Size: New Insights into the Ice Adhesion
Kevin Golovin, The University of British Columbia, Canada
Reducing the interfacial adhesion between ice and a surface could be beneficial to a wide range of commercial activities. Since the 1940s, the adhesion between ice and a surface has been defined by the force, F, required to de-bond an area of adhered ice, A, typically in shear. The shear ice adhesion strength is then defined as tice = F/A, and an increasing body of literature is available delineating the various strategies for minimizing tice. However, this definition contains an intractable scalability limit often ignored within the ice adhesion community – large areas of accreted ice will require extremely large forces to remove the ice. In this talk I discuss some of our recent work understanding materials that circumvent this issue. Such materials, which we designate as LIT (to be explained in the talk), offer the unique property that the force necessary to remove adhered ice becomes independent of the interfacial area – the force needed to remove a few square centimeters is the same as the force needed to remove a few square meters. We understand LIT materials via a cohesive zone analysis of the interfacial mechanics. LIT materials are categorically dissimilar to traditional ice-phobic systems. For example, LIT materials become more effective with decreasing thickness and increasing shear modulus (the opposite is true for ice-phobic materials). These physical parameters make LIT systems particularly attractive for aerospace applications, which durability (requiring high modulus) and added weight (requiring low thickness) are major constraints.
Transparent Materials Harvesting the Energy of Sunlight for Fogging and Icing Control
Dimos Poulikakos, ETH Zurich, Switzerland
Processes and materials involving phase change, are practically omnipresent in nature and technology. Viewed in the direction of decreasing temperature, the encountered phase transitions are condensation, freezing and de-sublimation. In this lecture, I will focus on the rational, physics-derived, nanoengineering of transparent materials, for condensation and freezing applications, relying on their interaction with sunlight and I will demonstrate sunlight-driven fogging and icing retardation and rapid defogging/deicing. Fogging and related icing, are common phenomena that can have detrimental effects on visibility and safety through otherwise transparent surfaces. Fogging and icing affect the performance of a wide range of everyday applications relying on transparency, including windshields, visors, displays, cameras, and eyeglasses. I will discuss a novel approach, based on sunlight absorbing metasurfaces, which goes well-beyond state-of-the-art anti-fogging methods, such as antifogging superhydrophilic coatings. The nanoengineering of such transparent metasurfaces is realized, by varying the concentration of embedded plasmonically enhanced light absorbing nanoparticles in an ultra-thin titania film, to achieve broadband absorption, also in the near infrared range, with tunable transparency. Such surfaces upon illumination induce significant heating at the air-substrate interface where fog and icing are most likely to form and can rapidly de-fog or de-ice or completely inhibit fog/icing nucleation altogether. I will show, for example, that such metasurfaces are able to reduce defogging time by up to four-fold compared to reference samples and markedly outperform the most widely implemented solutions, paving the way for large-scale, low-cost manufacturing of surface features, which can further be combined with other state-of-the-art functionalities to support, safety, and reduce energy-related costs related to fogging and icing of transparent materials.
Designing Durable Icephobic Surfaces
Anish Tuteja, University of Michigan, USA
Ice accretion has a negative impact on critical infrastructure, as well as a range of commercial and residential activities. Icephobic surfaces are defined by an ice adhesion strength tice < 100 kPa. However, the passive removal of ice requires much lower values of tice, such as on airplane wings or power lines (tice < 20 kPa). Such low tice values are scarcely reported, and robust coatings that maintain these low values have not been reported previously. Here we show that, irrespective of material chemistry, by tailoring the crosslink density of different elastomeric coatings, and by enabling interfacial slippage, it is possible to systematically design coatings with extremely low ice-adhesion (tice < 0.2 kPa). These newfound mechanisms allow for the rational design of icephobic coatings with virtually any desired ice adhesion strength. By utilizing these mechanisms, we fabricate extremely durable coatings that maintain tice < 10 kPa after severe mechanical abrasion, acid/base exposure, 100 icing/de-icing cycles, thermal cycling, accelerated corrosion, and exposure to Michigan wintery conditions over several months.
Bio-inspired Materials for Controlling Ice Formation
Jianjun Wang, Chinese Academy of Sciences, China
Understanding and controlling ice formation are of great importance in both fundamental research and practical applications. However, our understanding of ice formation is far from satisfactory. Nature has unique ways in regulating ice formation, for example, antifreeze proteins (AFPs) protect organisms from freezing damage by regulating ice formation via controlling the arrangement of hydroxyl groups. In this talk, I will first discuss our investigation into the fundamentals of AFPs in regulating ice nucleation via revealing the Janus effect of AFPs on ice nucleation. We found the properties of the interfacial water are essential for AFPs in controlling ice formation, i.e., the structure and the dynamics of the interfacial water. Inspired by AFPs, we have synthesized a series of materials for regulating ice formation, and applications of these materials for the cryopreservation of cells as well as anti-icing coating with ultra-low ice adhesion will be presented.