Inspired by nature, scientists have created oil-repellent coatings by mimicking the nanosized structures found on fish scales and clam shells. These coatings have found their way into anti-smudge mobile phone screens, the linings of pipes carrying crude oil, and even the surfaces of medical implants, which cannot have bacteria adhere to them lest infections ensue.
Another way to repel oil is to coat a surface with a polyzwitterionic brush—a layer of polymers bearing equal numbers of positive and negative charges. The polyzwitterionic brush is thought to repel oil by retaining a thin film of water on the coated surface, which means that the surface remains oil-proof even underwater.
“The existence of such a hydration film has been postulated, but never experimentally verified,” said Nikodem Tomczak, a Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE).
To investigate whether polyzwitterionic brush surfaces indeed create a hydration film that protects surfaces against oil staining, Tomczak’s team coated a glass surface with a polyzwitterionic material—poly(sulfobetaine methacrylate), or PSBMA for short—before immersing the material in water.
Dan Daniel, another Scientist at IMRE and a co-author of the study, developed a method called reflection interference contrast microscopy to observe the formation of a very thin water film measuring 100 nm in thickness over the PSBMA-coated surface. He also customized an instrument for quantifying the frictional forces required to move an oil droplet along the coated surface.
“The oil droplet interacted with the substrate with ultralow adhesion, allowing the droplet to slide across the surface with almost no friction, like a car tire hydroplaning over a wet road,” Daniel said.
Compared to nanostructured oil-repellent surfaces, polyzwitterionic brush surfaces that induce hydration film formation reduce the frictional force between oil droplets and the surface by about a thousand times. By further exploring how frictional force correlates with hydration film thickness, the researchers aim to further improve the performance of their polyzwitterionic brush surfaces. They will also enhance the sensitivity of the equipment used to study the hydration film.
“The aim of understanding the physics behind these materials is to formulate a design principle to create self-cleaning, super-repellent coatings. Such coatings have important uses, such as in the prevention of fouling (the accumulation of marine organisms and oil) on underwater surfaces and in water purification membranes,” said Tomczak.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).