Part 2 of a Chromis Technologies series on using AFPs for precise surface control
Introduction
In our last article, we introduced the concept of surface engineering and its essential role as an enabler of performance in advanced technology applications ranging from biomedical devices to semiconductors. In this installment we’ll explore a fundamental aspect of surface behavior: wettability.
What Is Wettability – and Why It Matters
Wettability describes how a liquid (e.g., oil, water) interacts with a solid surface. Its essence is the contact angle – that is, the angle at which a liquid droplet meets a solid surface. A contact angle above 90o (see Figure 1) indicates low or poor wetting – water or oil beads up, minimizing contact with the surface. Such a solid is said to have “low surface energy”, meaning it is hydro- or oleo-phobic and would repel liquids. A low contact angle (below 90o) indicates high surface energy and causes the liquid to spread out, so the solid is hydro- or oleo-philic.
Figure 1: Contact Angles for 3 Phases of Wetting
This has practical implications in the physical world. In medical tubing, for example, low wettability can improve fluid flow and reduce biofouling. On the other hand, paints, inks, and glues would peel or flake if the surface is hydrophobic. Various waterproof textiles use membranes engineered so water beads on the outside (hydrophobic) but moisture from sweat wets the inside (hydrophilic) for breathability.
Amorphous Fluoropolymers (AFPs) Naturally Resist Liquids
The chemical structure of AFPs – molecules of heavily fluorinated carbon backbones – gives them extremely low surface energy. This translates directly into high hydrophobicity (water repellency) and oleophobicity (oil repellency). The strong carbon-fluorine bonds hog electrons and are non-polar, offering no “hooks” for fluid molecules to grab onto.
AFPs are also chemically inert, optically transparent, and have low dielectric constant (making them good electrical insulators). Consequently, they are widely used as coating materials to protect surfaces in the semiconductor industry, optical devices, and biomedical applications from harsh environments.
Tuning Wettability
Research has shown that AFPs have similar static surface wettability and that this property is independent of film thickness and application method (Tavana et al., 2005; Wu et al., 2018). It is possible, however, to alter AFP surface characteristics using various surface engineering technologies. Among those studied are:
- Plasma treatment – Various researchers have demonstrated increases in the surface energy of AFPs such as Teflon® AF 1600/2400 and CYTOP® after plasma treatment.
- Thermal “reflow” annealing – Shown by Wu et al. (2018) and Naderi et al. (2022) to restore the hydrophobicity of AFP films after being damaged by repeated voltage cycling in electrowetting experiments.
- Vacuum ultraviolet radiation (VUV) – According to Matienzo et al. (1994), Teflon AF 2400 showed a significant increase in wettability after exposure to VUV.
- Protein solution treatment – Ward et al. (2017) used a protein solution to modify the surface properties and improve the wettability of CYTOP.
The principal strategy that Chromis is exploring to tune surface functionality is by introducing small fractions of polar monomers into our standard CyclAFlor® formulations to increase surface energy while retaining other key properties of the materials.
Conclusion
Amorphous fluoropolymers offer a compelling combination of chemical resistance, temperature stability, and optical clarity. And while they naturally repel water and oils, their surface properties are not fixed. Advanced surface engineering – or subtle formulation tweaks – can shift AFP wettability in useful ways, opening new design possibilities in electronics, optics and photonics, biomedicine, and other high-technology applications.
Next Steps in the Series
Our next installment will explore how to make dynamic changes in AFP surface energy using electric fields, a mechanism known as electrowetting-on-dielectric (EWOD). This is the enabling technology behind digital microfluidics, which uses active control over how liquids behave on a surface for “lab-on-a-chip” devices in medical diagnostics and new drug discovery.
Have a surface challenge? Contact Chromis Technologies today to discuss how our amorphous fluoropolymers might provide the solution.
References
Tavana, H., Petong, N., Hennig, A., Grundke, K., & Neumann, A. W. (2005). Contact Angles and Coating Film Thickness. The Journal of Adhesion, 81(1), 29–39. https://doi.org/10.1080/00218460590904435
Wu, H., Hayes, R. A., Li, F., Henzen, A., Shui, L., & Zhou, G. (2018). Influence of fluoropolymer surface wettability on electrowetting display performance. Displays, 53, 47–53. https://doi.org/10.1016/j.displa.2018.02.002
Naderi, P., & Grau, G. (2022). Organic thin-film transistors with inkjet-printed electrodes on hydrophobic Teflon-AF gate dielectric with reversible surface properties. Organic Electronics, 108, 106612. https://doi.org/10.1016/j.orgel.2022.106612
Jokinen, V., Suvanto, P., Garapaty, A. R., Lyytinen, J., Koskinen, J., & Franssila, S. (2013). Durable superhydrophobicity in embossed CYTOP fluoropolymer micro and nanostructures. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 434, 207–212. https://doi.org/10.1016/j.colsurfa.2013.05.061
Ono, T., Akagi, T., & Ichiki, T. (2009). Hydrophilization of Amorphous Perfluoropolymer using Low-pressure Argon Plasma. Journal of Photopolymer Science and Technology, 22, 683-689. https://doi.org/10.2494/PHOTOPOLYMER.22.683
Xiang, Y., Fulmek, P., Sauer, M., Foelske, A., & Schmid, U. (2024). Characterization of surface modifications in oxygen plasma-treated Teflon AF1600. Langmuir, 40(9), 4779–4788. https://doi.org/10.1021/acs.langmuir.3c03639
Qiu, Y., Yang, S., & Sheng, K. (2018). Photolithographic Patterning of Cytop with Limited Contact Angle Degradation. Micromachines, 9(10), 509. https://doi.org/10.3390/mi9100509
Matienzo, L.J., Zimmerman, J.A., & Egitto, F.D. (1994). Surface modification of fluoropolymers with vacuum ultraviolet irradiation. Journal of Vacuum Science and Technology, 12, 2662-2671. https://doi.org/10.1116/1.579086
Ward, J.W., Smith, H.L., Zeidell, A.M., Diemer, P.J., Baker, S., Lee, H., Payne, M., Anthony, J.E., Guthold, M., & Jurchescu, O.D. (2017). Solution-Processed Organic and Halide Perovskite Transistors on Hydrophobic Surfaces. ACS applied materials & interfaces, 9 21, 18120-18126 . https://doi.org/10.1021/acsami.7b03232
FAQs
What is surface energy?
Surface energy refers to how much a solid surface wants to interact with a liquid. High surface energy means the liquid spreads out (“wets the surface”); low surface energy means the liquid beads up and rolls off.
What is a contact angle?
A contact angle is a measurement that describes the way in which a liquid droplet meets a solid surface (refer to Figure 1, above). A high contact angle (greater than 90o) means the surface is water-repellent. A contact angle below 90o means the surface is more wettable.
What does “hydrophobic” mean?
The literal translation is “water-fearing”. A hydrophobic surface repels water, causing it to bead up rather than spread out.
What is an amorphous fluoropolymer (AFP)?
AFPs are a type of plastic-like material consisting primarily of carbon and fluorine atoms. Due to the high strength of the carbon-fluorine bond and the absence of crystalline structure, AFPs are highly transparent, chemically resistant materials and naturally repel water.
Why would you want to control wettability?
Because wettability affects how fluids behave on surfaces, controlling it is important in many consumer and commercial applications, from waterproofing textiles to manipulating fluid samples in “lab-on-a-chip” devices for medical diagnostics and new drug discovery.
What is plasma treatment?
Plasma is ionized gas – a superheated mixture of free-moving electrons and positively-charged ions (atoms that have lost one or more electrons) – that can be used to change the surface of a material without altering its fundamental or bulk properties.
What is VUV radiation and why use it?
Vacuum ultraviolet (VUV) radiation is high-energy light that can modify the surface properties of materials such as AFPs, making them more wettable.
What is electrowetting?
Electrowetting is a technique that uses electricity to change how a liquid behaves on a surface. By selectively applying an electrical field to a PCB coated with an AFP, for example, you can raise and lower its surface energy dynamically to move fluid droplets along specific pathways. It enables technologies such as digital microfluidics
What are polar monomers and why are they added to AFPs?
Polar monomers are individual molecules with an unequal distribution of electrical charge (one part of the molecule is slightly positive and another area slightly negative). Incorporating them in the composition of an AFP can increase surface energy and make it more wettable.