Icephobicity (from ''ice'' and Greek φόβος ''phobos'' "fear") is the ability of a solid surface to repel ice or prevent ice formation due to a certain topographical structure of the surface.Meuler, A. J. et al. Relationships between Water Wettability and Ice Adhesion. ACS Appl. Mater. Interfaces 2010, 11, 3100–3110Zheng, L. et al. Exceptional Superhydrophobicity and Low Velocity Impact Icephobicity of Acetone-Functionalized Carbon Nanotube Films. ''Langmuir'', 2011, 27, 9936–9943Jung, S.; Dorrestijn, M.; Raps, D.; Das, A.; Megaridis, C. M.; and Poulikakos, D. Are Superhydrophobic Surfaces Best for Icephobicity?. ''Langmuir'', 2011, 27, 3059–3066 Menini, R.; Ghalmi, Z.; Farzaneh, M. Highly Resistant Icephobic Coatings on Aluminum Alloys. Cold Reg. Sci. Technol. 2011, 65, 65-69 The word “icephobic” was used for the first time at least in 1950;Chemical Industries, 1950, v. 67, p. 559 however, the progress in micropatterned surfaces resulted in growing interest towards icephobicity since the 2000s.

Icephobicity vs. hydrophobicity

The term "icephobicity" is similar to the term

hydrophobicity In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

and other “-phobicities” in physical chemistry ( oleophobicity,

lipophobicity Lipophobicity, also sometimes called lipophobia (from the Greek λιποφοβία from λίπος ''lipos'' "fat" and φόβος ''phobos'' "fear"), is a chemical property of chemical compounds which means "fat rejection", literally "fear of fat". ...

, omniphobicity, amphiphobicity, etc.). The icephobicity is different from

de-icing Deicing is the process of removing snow, ice or frost from a surface. Anti-icing is the application of chemicals that not only deice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or preve ...

and

anti-icing Deicing is the process of removing snow, ice or frost from a surface. Anti-icing is the application of chemicals that not only deice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or preve ...

in that icephobic surfaces, unlike the anti-icing surfaces, do not require special treatment or chemical coatings to prevent ice formation, Kulinich, S. A.; Farhadi, S.; Nose, K.; and Du, X. W. Superhydrophobic Surfaces: Are They Really Ice-Repellent?. ''Langmuir'', 2011, 27, 25-29Bahadur, V.; Mishchenko, L.; Hatton, B., Taylor, J. A.; Aizenberg, J.; and Krupenkin, T. Predictive Model for Ice Formation on Superhydrophobic Surfaces. ''Langmuir'', 2011, 27 , 14143–14150Cao, L. -L.; Jones, A. K.; Sikka, V. K.; Wu, J.; and Gao, D. Anti-Icing Superhydrophobic Coatings. ''Langmuir'', 2009, 25, 12444-12448 There is further parallelism between the

and icephobicity. The

is crucial for the “

hydrophobic effect The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and exclude water molecules. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar ...

” and

hydrophobic interactions The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and exclude water molecules. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar ...

. For two hydrophobic molecules (e.g., hydrocarbons) placed in water, there is an effective repulsive hydrophobic force, entropic in its origin, due to their interaction with the water medium. The hydrophobic effect is responsible for folding of

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

and other macro-molecules leading to their

fractal In mathematics, a fractal is a geometric shape containing detailed structure at arbitrarily small scales, usually having a fractal dimension strictly exceeding the topological dimension. Many fractals appear similar at various scales, as illu ...

shape. During

ice crystal Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth, na ...

(

snowflake A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. ...

) formation, the synchronization of branch growth occurs due to the interaction with the medium ( oversaturated vapor) – is somewhat similar to the

– the apparent repulsion of the hydrophobic particles due to their interaction with the medium (water). Consequently, despite the shapes of snowflakes being very diverse with “no two flakes similar to each other,” most snow crystals are symmetric with each of the six branches almost identical to other five branches. Furthermore, both hydrophobicity and icephobicity can lead to quite complex phenomena, such as

self-organized criticality Self-organized criticality (SOC) is a property of dynamical systems that have a critical point as an attractor. Their macroscopic behavior thus displays the spatial or temporal scale-invariance characteristic of the critical point of a phase ...

-driven complexity as a result of hydrophobic interactions (during wetting of rough/heterogeneous surfaces or during polypeptide chain folding and looping) or ice crystallization (fractal snowflakes). Note that thermodynamically both the hydrophobic interactions and ice formation are driven by the minimization of the surface

Gibbs energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work (physics), work that may be performed by a closed system, thermodynamically closed system a ...

, ΔG = ΔH − TΔS, where H, T, and S are the

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

, and

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

, respectively. This is because in the hydrophobic interactions large positive value of TΔS prevails over a small positive value of ΔH making spontaneous hydrophobic interaction energetically profitable. The so-called surface roughening transition governs the direction of

growth and occurs at the critical temperature, above which the entropic contribution into the Gibbs energy, TΔS, prevails over the enthalpic contribution, ΔH, thus making it more energetically profitable for the ice crystal to be rough rather than smooth. This suggests that thermodynamically both the icephobic and hydrophobic behaviors can be viewed as entropic effects. However, icephobicity is different from the

Hydrophobicity In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

is a property which is characterized by the water contact angle (CA) and interfacial energies of the solid-water, solid-vapor, and water-vapor interfaces and thus it is a thermodynamic property usually quantitatively defined as CA>90 degrees. Another difference is that the

is opposed to the

hydrophilicity A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clar ...

in a natural way. There is no such an opposition for the icephobicity, which should therefore be defined by setting a quantitative threshold. The icephobicity is much more similar to how the

superhydrophobicity Ultrahydrophobic (or superhydrophobic) surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the ...

is defined.

Quantitative characterization of icephobicity

In recent publications on the subject there are three approaches to the characterization of surface icephobicity. First, the icephobicity implies low

adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be ...

force between ice and the solid surface. In most cases, the critical

shear stress Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the ot ...

is calculated, although the

normal stress In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

can be used as well. While no explicit quantitative definition for the icephobicty has been suggested so far, the researchers characterized icephobic surfaces as those having the shear strength (maximum stress) less in the region between 150 kPa and 500 kPa and even as low as 15.6 kPa,. Second, the icephobicity implies the ability to prevent ice formation on the surface. Such ability is characterized by whether a droplet of

supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...

water (below the normal freezing temperature of 0 C) freezes at the interface. The process of freezing can be characterized by time delay of heterogeneous ice

nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...

. The mechanisms of droplet freezing are quite complex and can depend on the temperature level, on whether cooling down of the droplet is performed from the side of the solid substrate or from vapor and by other factors. Third, the icephobic surfaces should repel incoming small droplets (e.g., of

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...

) at the temperatures below the

freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

.Zheng et al., Langmuir 27:9936 (2011) These three definitions imply that icephobic surfaces should (i) prevent freezing of water condensing on the surface (ii) prevent freezing of incoming water (iii) if ice formed, it should have weak adhesion strength with the solid, so that it can be easily removed. Anti-icing properties may depend on such circumstances as whether the solid surface is colder than the air/vapor, how big is the temperature gradient, and whether a thin film of water tends to form on the solid surface due to capillary effects, disjoining pressure, etc. Mechanical properties of ice and the substrate also of great importance since ice shedding occurs as fracture, either in the Mode I (normal) or Mode II (shear) cracking, so that crack concentrators are major contributors to the reduced strength.

References

{{Reflist Water ice