Superhydrophobic Surfaces

Harrick Plasma → Applications → Alter Surface Chemisty → Superhydrophobic Surfaces

Plasma treatment can be a beneficial step for creating superhydrophobic surfaces through promoting strong adhesion of a hydrophobic coating, surface roughening, and plasma polymerization.

Applications of Superhydrophobicity

Hydrophobic surfaces have low surface energy such that water droplets tend to repel the surface and bead up, forming a spherical droplet with high water contact angle (>90°).

Surfaces are defined as superhydrophobic when the water droplet retains its spherical shape with water contact angle of  >150° and water droplets tend to roll off. Thus, the superhydrophobic surface is difficult to wet [1-3].

Superhydrophobic surfaces are important for many practical applications where surfaces must remain pristine and free of water soluble coatings, such as for food safety, self-cleaning, stain resistance, and anti-fouling purposes [1-4].

Plasma treatment can be utilized to produce superhydrophobic surfaces, such as by cleaning surfaces to promote strong adhesion of hydrophobic coatings, generating nanoscale surface roughness, and depositing hydrophobic layers through plasma polymerization.

In this note, we explore examples of how researchers applied plasma treatment to produce superhydrophobic surfaces by these various techniques.

hydrophobic coatings

Plasma treatment can be applied to introduce oxygen containing functional groups and increase surface hydrophilicity. This, in turn, promotes strong adhesion and uniform spreading of a hydrophobic coating on the surface, such as illustrated in Figure 1.

Matin et al. developed a superhydrophobic and self-cleaning glass surface by plasma treating glass to remove organic contamination and introduce surface hydroxyl groups. This was followed by immersion in water or an alcohol to further form a hydroxylated substrate and subsequent coating 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 prior to deposition of 1H,1H,2H,2H-perfluoroctyltriethoxysilane (PFOTS). The combination of a fluorinated hydrophobic coating on a surface with microscale roughness resulted in a superhydrophobic surface with water contact angles of 148-155°.

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

nanoscale surface roughening

Superhydrophobicity can also be achieved by changing the physical structure (morphology) of the surface and creating nanoscale physical features. With large surface asperities (surface roughness, surface projections), air is trapped between textured grooves, creating air pockets between liquid and surface and dramatically reducing the fraction of surface-liquid contact (Cassie-Baxter state) [5]. As a result, water droplets tend to sit on top of the surface asperities and can easily roll off the surface, contributing to superhydrophobicity.

Silica (SiO₂) NPs are commonly used to add nanoscale texture to flat surfaces. Alternatively, NPs are added to surfaces with microscale features, creating a hierarchical, dual-scale structure with microscale and nanoscale roughness (Figure 2).

In studies by Zhao et al., Eriksson et al., and Oh et al., plasma treatment was utilized to render surfaces hydrophilic prior to coating with nanoparticles (NPs) to promote strong adhesion and uniform coating of NPs, thus producing a textured surface with nanoscale roughness.

In addition, NP-coated substrates were subsequently plasma-treated to increase hydrophilicity prior to applying a hydrophobic coating, such as a fluorinated silane (PFTS, THFS), hydrofluoroether (HFE), or perfluoroalkyl copolymer (FluoroPEL). The combination of increased nanoscale roughness from the NP layer and hydrophobic fluoropolymer coating resulted in a textured surface with superhydrophobic characteristics.

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

plasma polymerization

Plasma polymerization is another method to deposit a polymeric, hydrophobic coating, utilizing a fluorinated or hydrocarbon gas plasma. Fluorinated (-CF_x) and methylated (-CH₃) functional surfaces have low surface energy and are not energetically favorable for water to interact with, leading to a hydrophobic surface with high water contact angles [1].

Typically, a two-step process is applied, where (1) the surface is plasma-treated with an air, O₂, or N₂ plasma to remove volatile organic contamination and activate the surface with oxygen-containing functional groups, followed by (2) fluorinated or methylated plasma polymerization to generate a hydrophobic fluoropolymer or hydrocarbon coating.

Psarski et al. developed a nanocomposite consisting of epoxy resin mixed with Al₂O₃ NPs and glass microbeads to create a surface with hierarchical topography. The epoxy nanocomposite was air plasma-treated to activate the surface, followed by plasma polymerization to deposit a fluorocarbon coating, using three different perfluorinated monomers. Further purging with the fluorocarbon vapor helped to complete functionalization and polymerization of residual active sites and unsaturated monomers.

The resulting surface exhibited nanoscale roughness from the Al₂O₃ NPs along with a layer of low surface energy fluoropolymer coating and displayed superhydrophobicity with water contact angle >150° for each perfluorinated monomer used.

Using 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 increased with plasma duration and reached superhydrophobicity with water contact angle of ~154°.

It was found that combining MMA vapors and O₂ plasma in one process imparted greater hydrophobicity to PTFE than treatment with MMA vapor or O₂ plasma alone.