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.[1][2][3][4][5] The word “icephobic” was used for the first time at least in 1950;[6] however, the progress in micropatterned surfaces resulted in growing interest towards icephobicity since the 2000s.
The term "icephobicity" is similar to the term hydrophobicity and other “-phobicities” in physical chemistry (oleophobicity, lipophobicity, omniphobicity, amphiphobicity, etc.). The icephobicity is different from de-icing and anti-icing in that icephobic surfaces, unlike the anti-icing surfaces, do not require special treatment or chemical coatings to prevent ice formation,[7][8][9][10][11]
There is further parallelism between the hydrophobicity and icephobicity. The hydrophobicity is crucial for the “hydrophobic effect” and hydrophobic interactions. 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 and other macro-molecules leading to their fractal shape. During ice crystal (snowflake) formation, the synchronization of branch growth occurs due to the interaction with the medium (oversaturated vapor) – is somewhat similar to the hydrophobic effect – 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-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).[7]
Tenga en cuenta que termodinámicamente tanto las interacciones hidrofóbicas como la formación de hielo están impulsadas por la minimización de la energía de Gibbs superficial , ΔG = ΔH − TΔS, donde H, T y S son la entalpía , la temperatura y la entropía , respectivamente. Esto se debe a que en las interacciones hidrofóbicas un gran valor positivo de TΔS prevalece sobre un pequeño valor positivo de ΔH, lo que hace que la interacción hidrofóbica espontánea sea energéticamente rentable. La llamada transición de rugosidad de la superficie gobierna la dirección del crecimiento de los cristales de hielo y ocurre a la temperatura crítica, por encima de la cual la contribución entrópica a la energía de Gibbs, TΔS, prevalece sobre la contribución entálpica, ΔH, lo que la hace más rentable energéticamente para el hielo. cristal sea áspero en lugar de liso. Esto sugiere que termodinámicamente tanto el comportamiento hielofóbico como el hidrofóbico pueden verse como efectos entrópicos. [7]
Sin embargo, la icefobicidad es diferente de la hidrofobicidad . La hidrofobicidad es una propiedad que se caracteriza por el ángulo de contacto del agua (CA) y las energías interfaciales de las interfaces sólido-agua, sólido-vapor y vapor de agua y, por lo tanto, es una propiedad termodinámica generalmente definida cuantitativamente como CA>90 grados. Otra diferencia es que la hidrofobicidad se opone a la hidrofilicidad de forma natural. No existe tal oposición a la icefobicidad, que por tanto debería definirse fijando un umbral cuantitativo. La icefobicidad es mucho más similar a cómo se define la superhidrofobicidad . [7]
En publicaciones recientes sobre el tema hay tres enfoques para la caracterización de la icefobicidad superficial. [7] En primer lugar, la icefobicidad implica una baja fuerza de adhesión entre el hielo y la superficie sólida. En la mayoría de los casos, se calcula el esfuerzo cortante crítico, aunque también se puede utilizar el esfuerzo normal . Si bien hasta el momento no se ha sugerido una definición cuantitativa explícita para la fobia a la hielo, los investigadores caracterizaron las superficies icefóbicas como aquellas que tienen una resistencia al corte (tensión máxima) menor en la región entre 150 kPa y 500 kPa e incluso tan baja como 15,6 kPa. [1] [7]
Second, the icephobicity implies the ability to prevent ice formation on the surface. Such ability is characterized by whether a droplet of supercooled 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. 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 or fog) at the temperatures below the freezing point.[12]
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.[4][7]