Cold Weather Relevant Surface Properties

Elinor Coatings, LLC | Dante Battocchi, PhD

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Introduction

Surface engineers that are involved with water in liquid form are familiar with the terms hydrophilic, hydrophobic and superhydrophobic. As the surfaces are exposed to extreme cold the terms icephobic and snowphobic become relevant. These surface properties guide coating designs that minimize ice and snow buildup.

Hydrophilic and Hydrophobic

Law [1] provides an excellent starting point in defining surface properties related to water. Hydrophilicity is defined as an affinity to water and hydrophobicity is defined as the opposite, a lack of an affinity to water. According to Law, the scientific community defines a surface as hydrophilic when the static water contact angle θ is <90° and a hydrophobic surface is defined when the static water contact angle θ is >90° as shown below.

Figure 1: Schematic showing water/surface separation of a hydrophilic (left) and hydrophobic (right) surface. [1]

Superhydrophobic

Law proposes that “a surface is hydrophilic when its water θR is <90° and that it is hydrophobic when θR is > 90°” when considering adhesion and wetting interactions between a surface and water. A high surface tension being considered as the driving force for switching from hydrophilicity to hydrophobicity. Law further defines a superhydrophobic surface when its θA is ≥ 145 reflecting practically zero affinity with water.

The study of surface adhesion and wetting characteristics usually requires the measurement of the static contact angle θ, and the advancing and receding dynamic contact angles θA, and θR respectively. Dynamic contact angle hysteresis is equal to the difference between θA and θR. These angles and the sliding angle  are shown in Figure 2.

Figure 2: Schematics for the various contact angles: (a) static (apparent) contact angle θ, (b) sliding angle α, (c) advancing angle θA, and (d) receding angle θR. [1]

Icephobic

Hejazi et. Al. [2] cite that there are at least three different icephobic surface definitions:

1. They have a low adhesion (shear) force between ice and a solid surface. Shear strengths between 150 kPa and 500 kPa [3] [4] and even as low as 15.6 kPa [5] have been cited.

2. The surface prevents ice formation and depends on whether a droplet of supercooled water (below 32°F) freezes at the interface and it can be characterized by time delay of heterogeneous ice nucleation [6] [7] [8].

3. They repel incoming small precipitation droplets at temperatures below freezing point.

Snowphobic

The Norwegians [9] define snowphobic as a surface that prevents the adhesion and retention of snow.

Conclusion

Understanding hydrophilic, hydrophobic, and superhydrophobic characteristics is fundamental to describing initial water-surface interactions in extreme cold environments. Wettability governs whether water spreads, beads, or is shed and that sets the boundary conditions for freezing behavior. Wettability is a critical parameter combined with adhesion and surface structure that helps evaluate and characterize performance in ice and snow and guide engineering and optimization of functional coatings.

References

[1] Law, K-Y., Definitions for Hydrophilicity, Hydrophobicity, and Superhydrophobicity: Getting the Basics Right, Journal of Physical Chemistry Letters, 2014, 5, 686-688

[2] Hejazi, V., Sobolev, K. & Nosonovsky, M. From superhydrophobicity to icephobicity: forces and interaction analysis. Sci. Rep. 3, 2194; DOI:10.1038/srep02194 (2013).

[3] Meuler, A. J. et al. Relationships between water wettability and ice adhesion. ACS Appl. Mater. Interfaces. 11, 3100–3110 (2010b).

[4] Jung, S. et al. Are superhydrophobic surfaces best for icephobicity? Langmuir 27, 3059–3066 (2011).

[5] Menini, R. & Farzaneh, M. Elaboration of Al2O3/PTFE icephobic coatings for protecting aluminum surfaces. Surf. Coat. Technol. 203, 1941–1946 (2009).

[6] Kim, P. et al. Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance. ACS Nano. 6, 6569–6577 (2012).

[7] Jung, S., Tiwari, M. K., Doan, N. V. & Poulikakos, D. Mechanism of supercooled droplet freezing on surfaces. Nature Comm. 3, 615 (2012).

[8] Guo, P. et al. Icephobic/Anti-Icing Properties of Micro/Nanostructured Surfaces. Adv. Mater. 24, 2642–2648 (2012).

[9] Per-Olof A. Borrebæk, Sigrid Rønneberg, Bjørn Petter Jelle, Alex Klein-Paste, Zhiliang Zhang & Jianying He (2021) A framework for classification of snow and icephobicity, Journal of Adhesion Science and Technology, 35:10, 1087-1098, DOI: 10.1080/01694243.2020.1834286