What is an oleophobic coating and why do some surfaces hate oil?

Oleophobic coating on a flexible surface

Have you noticed oil stains on kitchen countertops, smudges and fingerprints on mobile screens and eye glasses due to skin oil? We all struggle with these in our daily lives. There are some surfaces, however, that readily repel oil. Nature has created the skin surface of many creatures in such a way that it is efficient in removing oil, for example, springtails and fish skins. Isn’t it fascinating? Nature surprises mankind in innumerable ways. What if we learn from the nature to create artificial surfaces with oil-repellent properties. In this article, we’ll shed light on the fascinating world of oil repellent surfaces and the science behind them.

Oleophobic surface

Surfaces that repel oil droplets are defined as oleophobic surfaces and the strength of oil repellent or oleophobicity is defined in terms of the contact angle of oil droplets on the surface, measured by a contact angle meter.

In general, a mechanical balance among the surface tensions of solid-air surface tension (ϒ_SV), liquid-air surface tension (ϒ_LV) and solid-liquid interfacial tension (ϒ_SL) generates a contact angle (θ) for a particular liquid drop.

Figure 1: contact angle (θ) of a liquid drop on a smooth solid surface. Source: Shodhganga

Depending on the contact angle of oil droplets, a surface is defined in the following ways:

Contact angle < 90°: Oleophilic

Contact angle > 90°: Oleophobic

Contact angle ~ 0° : Superoleophilic

Contact angle > 150° : Superoleophobic

The surface with love/attraction for oil is called Oleophilic (contact angle of oil drop is less than 90° in this case), while oleophobic meaning is hate/repellence for oil (contact angle of oil drop is more than 90°). The extreme cases of love and hate for oil are termed as superoleophilic and superoleophobic.

Each regime of contact angles has its own importance and application. However, today we are interested in the case where the oil contact angle is greater than 90°. Greater the contact angle, better the repulsion of oil from the surface.

So now, the question is, what makes a surface resistant or attractive to oil? The answer lies in two main properties of any surface:

1. Surface energy (the energy due to uncoordinated bonds on the surface of the materials), and

2. Texture/microstructure on the surface (the roughness on the surface)

The surface energy should be lowest for high contact angle of the oil drop. The low surface energy will allow the oil drop to move freely without any attraction to the surface.

The surface energy can be reduced by various chemical treatments, for example, treatment with perfluoro polyether siloxane, polyacrylic resin, silane coupler, etc. can reduce the surface energy by deactivating the uncoordinated charges on the surface and can help in creating an oleophobic surface.

In addition, oleophobicity can be controlled by growing various microstructures on any surface. Oil droplets behave differently on a rough surface than on a smooth surface. You can see two situations in Figure 2. In one case, the oil can fill the area with rough microstructure, while in the other it cannot.

Figure 2. Oil droplet on rough solid surfaces. Oil goes inside the grooves (left), while airpockets get trapped and don’t allow oil to penetrate (right). Source: Chem. Soc. Rev.

Airpockets occur in grooves or holes on rough surfaces. If airpockets are released during oil contact with the rough surface, the oil will occupy the grooves and fill the rough surface, as shown in Image 2.

However, if the groove size or roughness is artificially optimized to a level where airpockets inside the groove become trapped beneath the oil drop during contact, oil will simply sit on top of it and will not stick to the surface. In this case, the oil can be easily removed by simply tilting the surface.

The roughness or grove size can be controlled during nanostructure/microstructure growth on the surface by various growth techniques such as physical vapor deposition, chemical vapor deposition, hydrothermal growth, etc.

Another way to optimize the roughness is physical and/or chemical etching of the surface (removing materials from the surface).

Thus, by minimizing the surface energy and optimizing the surface roughness, a surface can be made oleophobic/superoleophobic.

A variety of materials are used for oleophobic coating such as fluoropolymers, nanostructured oxides, fluorinated oxide surfaces, and microstructured polymer fibers.

Limitations of Oleophobic Coating

The limitation of these oleophobic coatings is the lifespan of the coating. Unfortunately, oleophobic coatings are very fragile and do not last long. The surface’s ability to repel oil decreases over time due to damage to the coating. Researchers around the world are working hard to increase the lifespan of oleophobic coatings, however, there is no permanent oleophobic coating available yet.

Nowadays, new mobile glass screens (for example, iPhone 15 Pro Max) have oleophobic coating to provide a good experience to customers. Oleophobic coating helps in easy removal of fingerprints, smudges etc. from the screen.

Generally, it is advisable to clean the coated surface with a soft and lint-free cloth to minimize damage, while any abrasive cleaning will quickly wear down the coating.

The composition of the oleophobic coating on the glass screen of any electronic gadget is often considered a trade secret and therefore not shared with the public.

We hope for the developement of a permanent oleophobic coating in near future.