Creating Superhydrophobic Surfaces with Plasma Treatment

Plasma treatment can be of benefit when creating superhydrophobic surfaces. It promotes good adhesion of the hydrophobic coating, roughening of the surface, and plasma polymerization.

Image Credit: Esti Budi/Shutterstock.com

Applications of Superhydrophobicity

Hydrophobic surfaces show low surface energy, so water droplets tend to repel against the surface and create beads that form a spherical droplet with a high water contact angle (>90°).

Surfaces can be called superhydrophobic when the water droplet maintains the spherical shape with a water contact angle of >150° and the water droplets typically roll off. The superhydrophobic surface is, by definition, a challenge to wet.^1-3

Superhydrophobic surfaces are useful in several practical applications that demand surfaces to remain pristine and do not have water-soluble coatings. Some examples are food safety applications, self-cleaning, stain resistance, and anti-fouling needs.^1-4

Plasma treatment can produce superhydrophobic surfaces by cleaning surfaces to encourage strong adhesion of hydrophobic coatings, creating nanoscale surface roughness, and depositing hydrophobic layers with plasma polymerization.

This article explores examples of applied plasma treatment that produced superhydrophobic surfaces through various techniques.

Hydrophobic Coatings

Plasma treatment can introduce functional groups containing oxygen and increase surface hydrophilicity. This encourages strong adhesion and consistent surface spread of the hydrophobic coating.

Figure 1. Plasma activation of substrate and subsequent coating with a hydrophobic silane layer.

Image Credit: Harrick Plasma

Matin et al. created a superhydrophobic and self-cleaning glass surface by plasma-treating glass to eliminate organic contamination and present surface hydroxyl groups. Following this, immersion in water or alcohol further forms a hydroxylated substrate, which is then coated with octadecyltrichlorosilane (ODTS). The ODTS-coated glass exhibited surface roughness of 40-60 nm rms and water contact angles of 155-180°.

In Su et al.’s study, a substrate with polymethyl methacrylate (PMMA) micropillars was plasma-treated to introduce hydroxyl groups before deposition of 1H,1H,2H,2H-perfluoroctyltriethoxysilane (PFOTS). The combined fluorinated hydrophobic coating on a surface and microscale roughness lead to a superhydrophobic surface with water contact angles of 148-155°.

Nanoscale Surface Roughening

It is possible to achieve superhydrophobicity by altering the surface's physical structure (morphology) and developing nanoscale physical features. With large surface asperities, such as surface roughness and surface projections, air is caught between textured grooves, which creates air pockets between liquid and surface and drastically reducing the fraction of surface-liquid contact (Cassie-Baxter state).⁵ The water droplets are prone to sit above the surface asperities and are able to readily roll off the surface, contributing to superhydrophobicity.

Silica (SiO₂) nanoparticles (NPs) often add nanoscale texture to flat surfaces. As an alternative, NPs are introduced to surfaces with microscale characteristics, creating a hierarchical, dual-scale structure with microscale and nanoscale roughness (Figure 2).

Figure 2. Dual-scale structure with microscale and nanoscale roughness. 

Image Credit: Harrick Plasma

Research by Zhao et al., Eriksson et al., and Oh et al. utilized plasma treatment to render surfaces hydrophilic before coating with NPs to encourage strong adhesion and even coating NPs to produce a textured surface with nanoscale roughness.

NP-coated substrates were plasma-treated to increase hydrophilicity before a hydrophobic coating, such as a hydrofluoroether (HFE), fluorinated silane (PFTS, THFS), or perfluoroalkyl copolymer (FluoroPEL), was applied. The increased nanoscale roughness from the NP layer combined with the hydrophobic fluoropolymer coating resulted in a textured surface with superhydrophobic features.

Plasma Polymerization

Plasma polymerization is an additional method for depositing a polymeric, hydrophobic coating, which utilizes a fluorinated or hydrocarbon gas plasma. Fluorinated (-CF_x) and methylated (-CH₃) functional surfaces possess a low surface energy and are not energetically ideal for interactions with water, resulting in a hydrophobic surface with high water contact angles.¹

Commonly, a two-step process is used. First, the surface is plasma-treated with air, O₂, or N₂ plasma to eliminate unstable organic contamination and activate the surface with oxygen-containing functional groups. This is followed by fluorinated or methylated plasma polymerization to create a hydrophobic hydrocarbon or fluoropolymer coating.

Psarski et al. created a nanocomposite consisting of epoxy resin combined with Al₂O₃ NPs and glass microbeads, which resulted in a surface with hierarchical topography.

The epoxy nanocomposite was air plasma-treated to activate the surface, then plasma polymerized to introduce a fluorocarbon coating using three unique perfluorinated monomers. Additional purging with the fluorocarbon vapor completely functionalized and polymerized residual active sites and unsaturated monomers.

As a result, the surface showed nanoscale roughness from the Al₂O₃ NPs and a layer of low-surface-energy fluoropolymer coating and demonstrated superhydrophobicity with a water contact angle >150° for each perfluorinated monomer used.

Through the use of the High Power Expanded Plasma Cleaner, Vijayan et al. applied a methyl methacrylate-oxygen MMA-O₂ plasma treatment to fabricate superhydrophobic poly(tetrafluoroethylene) (PTFE) surfaces. The water contact angle saw an increase with plasma duration and arrived superhydrophobicity with water contact angle of ~154°.

It has been discovered that the combination of MMA vapors and O₂ plasma in a single process imparted higher hydrophobicity to PTFE than treatment with MMA vapor or O₂ plasma on their own.

References and Further Reading

Hydrophobic Coatings: Articles by Harrick Plasma Users

• Matin A and Merah N. “Glass substrate with superhydrophobic self-cleaning surface”, US Patent US10493489B2 (2019).
• Su J, Esmaeilzadeh H, Wang P, Ji S, Inalpolat M, Charmchi M and Sun H.”Effect of wetting states on frequency response of a micropillar-based quartz crystal microbalance”, Sens. Actuators A (2019) 286: 115-122. 10.1016/j.sna.2018.12.012

Nanoscale Surface Roughening: Articles by Harrick Plasma Users

• Zhao X, Park D, Choi J, Park S, Soper S and Murphy M. “Robust, Transparent, Superhydrophobic Coatings Using Novel Hydrophobic/Hydrophilic Dual-sized Silica Particles”, J. Colloid Interface Sci. (2020) 574: 347-354. 10.1016/j.jcis.2020.04.065
• Eriksson M, Claesson P, Jarn M, Tuominen M, Wallqvist V, Schoelkopf J, Gane P and Swerin A. “Wetting Transition on Liquid-Repellent Surfaces Probed by Surface Force Measurements and Confocal Imaging”. Langmuir (2019) 35: 13275-13285. 10.1021/acs.langmuir.9b02368
• Oh J, Liu S, Jones M, Yegin Y, Hao L, Tolen T, Nagabandia N, Scholar E, Castillo A, Taylor T, Zevallos LC and Akbulut M. “Modification of aluminum surfaces with superhydrophobic nanotextures for enhanced food safety and hygiene”, Food Control (2019) 96: 463-469. 10.1016/j.foodcont.2018.10.005

Plasma Polymerization: Articles by Harrick Plasma Users

• Vijayan V, Tucker B, Baker P, Vohra Y and Thomas V. “Non-equilibrium hybrid organic plasma processing for superhydrophobic PTFE surface towards potential bio-interface applications”, Colloids Surf. B (2019) 183: 110463. 10.1016/j.colsurfb.2019.110463
• Psarski M, Pawlak D, Grobelny J and Celichowski G.  “Hydrophobic and superhydrophobic surfaces fabricated by plasma polymerization of perfluorohexane, perfluoro (2-methylpent-2-ene), and perfluoro (4-methylpent-2-ene)”.  J. Adhes. Sci. Technol. (2015) 29(19): 2035-2048. 10.1080/01694243.2015.1048131

Supplemental References (Do not Report Using Harrick Plasma Instruments)

1. Li X-M, Reinhoudt D and Crego-Calama M. “What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces.” Chem. Soc. Rev. (2007) 36: 1350-1368. 10.1039/B602486F
2. Celia E, Darmanin T, Taffin de Givenchy E, Amigoni S and Guittard F. “Recent advances in designing superhydrophobic surfaces.” J. Colloid Interface Sci. (2013) 402: 1-18. 10.1016/j.jcis.2013.03.041
3. Law K-Y. “Definitions for Hydrophilicity, Hydrophobicity, and Superhydrophobicity: Getting the Basics Right” J. Phys. Chem. Lett. (2014) 5(4): 686-688. 10.1021/jz402762h
4. Wang D, Sun Q, Hokkanen MJ, Zhang C, Lin F-Y, Liu Q, Zhu S-P, Zhou T, Chang Q, He B, Zhou Q, Chen L, Wang Z, Ras RHA and Deng X. “Design of robust superhydrophobic surfaces.” Nature (2020) 582: 55–59. 10.1038/s41586-020-2331-8]
5. Cassie ABD and Baxter S. “Wettability of porous surfaces.” T. Faraday Soc. (1944) 40: 546-551. 10.1039/TF9444000546

This information has been sourced, reviewed and adapted from materials provided by Harrick Plasma.

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