Cold Weather Coatings – Mitigation Strategies

6 days ago
6 min read

Elinor Coatings, LLC | Dante Battocchi, PhD

Introduction

Cold weather mitigation strategies to prevent surface ice formation that focus on superhydrophobicity consider three main approaches, not necessarily mutually exclusive. They are to a) deny ice the opportunity to nucleate from humid air and form frost, b) delay nucleation sufficiently to enable water to be shed prior to ice formation and c) to minimize the adhesion strength of ice that has nucleated and attached to a surface [1].

Icephobic Coatings

An icephobic coating is “capable of mitigating and/or suppressing all types of icing.” Therefore, an icephobic coating must be able to resist the freezing of impacting supercooled droplets either repelling or delaying freezing or lowering the freezing temperature of water droplets at continuous solid-liquid interface, or minimizing or avoiding the frost growth and, if icing occurs, to have a low ice adhesion strength [2].

Inhibiting ice formation can be achieved by preventing ice nucleation [1] and that can be accomplished via surface design. There are two fundamental types of nucleation, homogeneous and heterogeneous [3]. Homogeneous nucleation describes the process of water freezing throughout its entire volume while heterogeneous nucleation recognizes that an interface exists between the liquid and solid phases.

The surface of the energy of the substrate is very influential [1] since heterogeneous nucleation occurs at a higher temperature than homogeneous nucleation. Minimizing the substrate’s surface energy shifts nucleation towards homogeneous nucleation and therefore most strategies for attaining icephobicity therefore start with superhydrophobicity [4].

Frost can occur within some superhydrophobic surfaces when the condensing droplets are sufficiently small and penetrate the surface structure and freeze prior to superhydrophobic shedding [5].

Suppression of ice nucleation in a water droplet may be enhanced by constraining the droplet size, and with sufficient confinement, to force coalescence and shedding of any condensed precipitation before it has frozen [6], shown schematically adjacent.

Delaying nucleation maximizes the probability that any water impinging onto a surface can be shed off a surface prior to ice nucleation [6]. Superhydrophobicity is the most common surface characteristic that facilitates the timely shedding.

Surfaces that are impacted by supercooled rain at terminal velocity must be designed to prevent water droplet pinning and to promote runoff or rebounding [7]. Superhydrophobic surfaces are the best option to achieve this in most circumstances [8].

*Figure 1: Condensation expulsion due to droplet confinement as they coalesce. [1]*

Two benefits of a superhydrophobic surface are that it facilitates rapid water shedding from the surface and extends the time required for surface water to freeze [8]. The time to freeze is the resultant of the combined thermal conduction at the liquid-to-solid interface, liquid-to-air convection and radiation, with the thermal conduction at the liquid-to-solid interface being the most significant. [9]

Substrates that do not allow large vapor pressure gradients from developing under the droplet are more likely to allow the precipitation to be shed as ice [10]. It is hypothesized that solidification dynamics underpin this phenomenon and enables the droplet-substrate interface to solidify last and that thermal conductivity between droplet and substrate should be minimized.

The mechanism that enables ice shedding is where the force acting on the accumulated ice exceeds the adhesion between the ice and the surface. [11]. If only gravitational means of ice removal are considered, the applied force is a function of the accumulated ice mass.

Minimal ice adhesion is predicted when the surface consists of infinitely sharp points, high enough to minimize electrostatic forces, and with strength enough to maintain these attributes over many de-icing cycles [12]. It is possible to design a surface to induce interface crack initiation enabling dynamic propagation across the surface, releasing the ice [13].

Snowphobic Coatings

The definition of snowphobicity is limited when snow, already in a frozen state impacts a surface. Its definition is related to the reduction in shear strength adhesion at the interface between the surface and the snow [1].

An investigation into wet snow and its interaction with superhydrophobic surfaces concluded that adhesion strength is influenced by surface wettability. The more superhydrophobic a surface is the lower its interface adhesion strength [14]. Hydrodynamic surfaces were tested in-situ and no positive effect was seen from merely employing hydrophobic coatings [15].

While ice and snow are different phases of the same compound, H2O, they differ thermodynamically [17], mechanically [16] and morphologically [18]. Not surprisingly an icephobic surface does necessarily mean that it is also snowphobic.

Conversely snowphobic surfaces are likely to be somewhat icephobic. It is recommended that both water shedding and frost denial are included in snowphobic surface designs since the presence of either will negate any snow adhesion minimization efforts.

The differences between icephobic and snowphobic are shown in the following figure. A continuous propagating interface crack can facilitate the rapid removal of the surface ice. Removal of dry snow, both porous and deformable, via cleavage is naturally more arduous.

*Figure 2: Schematic of the surface and continuous solid ice interface [1].*

*Figure 3: Schematic of the surface and snow interface [1].*

Conclusion

Superhydrophobic coatings delay ice nucleation and maximize the probability that water can be shed from the surface either as runoff or via rebounding If ice is formed, it is possible to design a surface to induce interface crack initiation enabling dynamic propagation across the surface, releasing the ice.

Ice and snow differ thermodynamically, mechanically and morphologically. This means that an icephobic coating does not automatically mean that it is snowphobic.

Snowphobic surfaces are likely to exhibit some icephobic properties. It is recommended that both water shedding and frost denial are included in snowphobic surface designs since the presence of either will negate any snow adhesion minimization efforts.

References

[1] Borrebæk, P-O, A., Jelle, B. P., Zhiliang Zhang, Z., Avoiding snow and ice accretion on building integrated photovoltaics - challenges, strategies, and opportunities, Solar Energy Materials & Solar Cells 206 (2020) 110306

[2] Esmeryan, K., D., From Extremely Water-Repellent Coatings to Passive Icing Protection Principles, Limitations and Innovative Application Aspects, Coatings 2020, 10, 66]

[3] R. DeHoff, Thermodynamics in Materials Science, CRC Press, 2006, https://doi.org/10.15713/ins.mmj.3.

[4] T.M. Schutzius, S. Jung, T. Maitra, P. Eberle, C. Antonini, C. Stamatopoulos, D. Poulikakos, Physics of icing and rational design of surfaces with extraordinary icephobicity, Langmuir 31 (2015) 4807–4821, https://doi.org/10.1021/la502586a.

[5] J. Chen, J. Liu, M. He, K. Li, D. Cui, Q. Zhang, X. Zeng, Y. Zhang, J. Wang, Y. Song, Superhydrophobic surfaces cannot reduce ice adhesion, Appl. Phys. Lett. 101 (2012) 111603, https://doi.org/10.1063/1.4752436.

[6] Q. Hao, Y. Pang, Y. Zhao, J. Zhang, J. Feng, S. Yao, Mechanism of delayed frost growth on superhydrophobic surfaces with jumping condensates: more than interdrop freezing, Langmuir 30 (2014) 15416–15422, https://doi.org/10.1021/la504166x.

[7] S. Dash, M.T. Alt, S.V. Garimella, Hybrid surface design for robust superhydrophobicity, Langmuir 28 (2012) 9606–9615, https://doi.org/10.1021/la301743p.

[8] S. Jung, M. Dorrestijn, D. Raps, A. Das, C.M. Megaridis, D. Poulikakos, Are superhydrophobic surfaces best for icephobicity? Langmuir 27 (2011) 3059–3066, https://doi.org/10.1021/la104762g.

[9] A. Criscione, I.V. Roisman, S. Jakirlic, C. Tropea, Towards modelling of initial and final stages of supercooled water solidification, Int. J. Therm. Sci. 92 (2015) 150–161, https://doi.org/10.1038/ncomms1630.

[0] G. Graeber, T.M. Schutzius, H. Eghlidi, D. Poulikakos, Spontaneous self-dislodging of freezing water droplets and the role of wettability, Proc. Natl. Acad. Sci. 114 (2017) 11040–11045, https://doi.org/10.1073/pnas.1705952114.

[11] O. Parent, A. Ilinca, Anti-icing and de-icing techniques for wind turbines: critical review, Cold Reg. Sci. Technol. 65 (2011) 88–96, https://doi.org/10.1016/j.coldregions.2010.01.005.

[12] G. Fortin, J. Perron, Ice adhesion models to predict shear stress at shedding, J. Adhes. Sci. Technol. 26 (2012) 523–553, https://doi.org/10.1163/016942411X574835.

[13] Z. He, Y. Zhuo, J. He, Z. Zhang, Design and preparation of sandwich-like polydimethylsiloxane (PDMS) sponges with super-low ice adhesion, Soft Matter 14 (2018) 4846–4851, https://doi.org/10.1039/c8sm00820e.

[14] T. Kako, A. Nakajima, H. Irie, Z. Kato, K. Uematsu, T. Watanabe, K. Hashimoto, Adhesion and sliding of wet snow on a super-hydrophobic surface with hydrophilic channels, J. Mater. Sci. 39 (2004) 547–555, https://doi.org/10.1023/B:JMSC.0000011510.92644.3f.

[15] R.W. Andrews, A. Pollard, J.M. Pearce, A new method to determine the effects of hydrodynamic surface coatings on the snow shedding effectiveness of solar photovoltaic modules, Sol. Energy Mater. Sol. Cells 113 (2013) 71–78, https://doi.org/10.1016/j.solmat.2013.01.032.

[16] K.G. Libbrecht, The physics of snow crystals, Rep. Prog. Phys. 68 (2005) 855–895, https://doi.org/10.1088/0034-4885/68/4/R03.

[17] J.J. Petrovic, Mechanical Properties of Ice and Snow, 2003, https://doi.org/10.1023/A:1021134128038.

[18] Y. Furukawa, J.S. Wettlaufer, Snow and ice crystals, Phys. Today 60 (2007) 70–71, https://doi.org/10.1063/1.2825081