Relationship between wettability and ice removal

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2018

A new study determined that ice growth is dependent on the hydrophobicity of a surface.

KEY CONCEPTS
• Ice growth on a surface is a function of hydrophobicity with along-surface growth occurring at a contact angle below 40 degrees.
• When the contact angle is above 40 degrees, ice grows in an off-surface growth mode.
• Surface roughness lowers the contact angle when ice growth switches from the along-surface to the off-surface mode.
AS THIS COLUMN IS READ, those of us in the Northern Hemisphere are dealing with a common winter task—removing ice from windshields. This task can be difficult and time consuming.

To develop an approach for easier ice removal, further information is needed about the structure of ice when it forms on a surface. Clues about how to deal with this issue can originate from research done to repel ice’s liquid state, water, from surfaces. This leads to an examination of the concept of superhydrophobicity.

Superhydrophobic surfaces are designed to reject or repel water. This makes them attractive in lubricant systems because water contamination can create premature lubricant failure by causing problems such as corrosion. Repelling water is a popular approach to facilitating water removal from a lubricant system.

In a previous TLT article, researchers defined a superhydrophobic surface as containing microscopic ridges and posts in combination with a hydrophobic coating such as polytetrafluoroethylene (1). These characteristics enable the contact angle of water droplets impinging on a superhydrophobic surface to be greater than 120 degrees. 

About 20 years ago, modeling studies by Xiao Cheng Zeng, Chancellor’s University & Willa Cather Professor of chemistry at the University of Nebraska-Lincoln in Lincoln, Neb., determined the structure of ice on a surface. Zeng says, “We found that ice forms a two-dimensional bilayer on a surface that is similar in structure to graphene. The structure is 0.8 billionth of a meter thick and cannot be seen, but it can be measured by spectroscopic techniques. The structure of the two-dimensional ice was designated as Nebraska Ice and was verified experimentally in 2009.”

A new modeling study and experimentation has provided further insight into how ice growth is a function of the hydrophobicity of a surface. 

TWO ICE GROWTH MODES
Zeng and colleagues from several China institutions have determined the way ice grows on a surface as a function of the hydrophobicity of the surface. The work was done both through modeling and experimentation. 

The surface used in this study was prepared by depositing a thin aluminum film on a silicon wafer. Hydrophobicity was introduced through chemical vapor deposition of perfluorodecyltrimethoxysilane. The wettability of the surface was varied by adjusting the treatment time. On average, the thickness of this film is approximately 50 nanometers. 

Ice was added to this surface, and the samples were added to a cryostage kept at -15 C with humidity adjusted by adding wet air to the cryostage. Zeng says, “We found that ice growth on a surface will differ depending upon the hydrophobicity of a surface. When the contact angle for ice is below 40 degrees, then ice formation will creep along the surface in a manner known as the along-surface growth mode. As part of this process, Nebraska Ice is formed.”

Zeng feels that an appropriate term for ice growing in this manner on the surface is iceophilic. 

In the presence of a more hydrophobic surface with a contact angle greater than 40 degrees, ice grows in a different manner away from the surface. Zeng says, “We found that ice forms in an off-surface growth mode, which we term as iceophobic. Ice grows toward the sky in a manner similar to a snow flake and often forming shapes that exhibit beautiful six-fold symmetry.”

A particular significant experiment was done to demonstrate how ice grows on a surface that is 50% hydrophilic and 50% hydrophobic. As ice growth progressed from the hydrophilic to the hydrophobic side of the surface, the initial along-surface growth mode changed to off-surface growth mode. 

Surface roughness also can influence ice growth. Zeng says, “We determined that the contact angle where ice growth switches from the along-surface to off-surface growth modes drops to a smaller angle. This shift may be due to the ice not really being in contact with the rough surface as readily as a smooth surface. The hydrophilicity of the surface must be more pronounced to enable the ice to act in the along-surface growth mode.”

The difference in the two ice growth modes is shown in Figure 1. Off-surface growth improved the ability for ice to be removed as shown on the right, while along-surface growth made it difficult for ice to be removed as depicted on the left. Experiments subjecting both surfaces just to wind blowing at 5.78 meters per second led to rapid ice removal for the hydrophobic surface but no ice removal for the hydrophilic surface. 


Figure 1. Ice forming along the surface is shown on the left, while ice growing off the surface is on the right. Off-surface growth can ease ice removal from a surface. (Figure courtesy of the University of Nebraska-Lincoln.)

This work should provide guidance for the development of better ice-repellent surfaces that might make it easier to remove ice from automobiles and also to de-ice airplanes. Zeng says, “We have seen that two-dimensional ice has different types of crystal structures that can range from hexagonal to square surfaces. In the future, we are looking to better understand how these surfaces are formed.”

Additional information on this research can be found in a recent article (2) or by contacting Zeng at xzeng1@unl.edu. 

REFERENCES
1. Canter, N. (2014), “Gaining a better understanding of superhydrophobic surfaces,” TLT, 70 (8), pp. 10-11.
2. Liu, J., Zhu, C., Liu, K., Jiang, Y., Song, Y., Francisco, J., Zeng, X. and Wang, J. (2017), “Distinct ice patterns on solid surfaces with various wettabilities,” Proceedings of the National Academy of Sciences, 114 (43), pp. 11285-11290.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.