New oil-repelling material from MIT
MIT researchers have developed flexible surface coatings that repel oils. According to the research team, this is the first material able to do it. They say that these findings could have applications in aviation, space travel and hazardous waste cleanup. Their oil-repelling, or 'oleophobic' material, is using specially prepared microfibers, which are a blend of a specially synthesized molecule called fluoroPOSS (short for 'fluorinated polyhedral oligomeric silsesquioxanes') and a common polymer. The U.S. Air Force, which developed the fluoroPOSS molecules, wants to use this new material to protect components of airplanes and rockets from jet fuel.
You can see on the left "a steel grid (square pores with 1 mm spacing) coated with electrospun fibers containing 9.1 wt% fluorodecyl POSS used for oil-water separation. Octane droplets (colored with oil red O dye) easily pass through the membrane whereas water droplets (dyed with methylene blue) bead up on the surface." (Credit: Anish Tuteja and Wonjae Choi, MIT) Here is a link to a larger version of this picture. You'll find another related picture on this EurekAlert! page. And you'll find additional images and a link to a short video on this MIT page
This project has been led by Anish Tuteja, a postdoctoral associate at the MIT. The focus of his work is to use nanoparticles to modify the surface properties of polymer composites. He worked under the supervision of Robert Cohen, professor of chemical engineering, who provided the help from his research group. He also was mentored by Gareth McKinley, professor of teaching innovation in the Department of Mechanical Engineering and who manages the Non-Newtonian Fluid Dynamics research group.
So how did the team discover this new material? "The tendency of oils and other hydrocarbons to spread out over surfaces is due to their very low surface tension (a measure of the attraction between molecules of the same substance). Water, on the other hand, has a very high surface tension and tends to form droplets. [...] That difference in surface tension also explains why water will roll off the feathers of a duck, but a duck coated in oil must be washed with soap to remove it. The MIT team overcame the surface-tension problem by designing a material composed of specially prepared microfibers that essentially cushion droplets of liquid, allowing them to sit, intact, just above the material's surface."
You can see above how such a fabric looks like, but here are more details. "When oil droplets land on the material, which resembles a thin fabric or tissue paper, they rest atop the fibers and pockets of air trapped between the fibers. The large contact angle between the droplet and the fibers prevents the liquid from touching the bottom of the surface and wetting it. The microfibers are a blend of a specially synthesized molecule called fluoroPOSS, which has an extremely low surface energy, and a common polymer. They can be readily deposited onto many types of surfaces, including metal, glass, plastic and even biological surfaces such as plant leaves, using a process known as electrospinning."
For more information, this research work has been published in Science under the title "Designing Superoleophobic Surfaces" (Volume 318, Number 5856, Pages 1618-1622, December 7, 2007). Here is the abstract. "Understanding the complementary roles of surface energy and roughness on natural nonwetting surfaces has led to the development of a number of biomimetic superhydrophobic surfaces, which exhibit apparent contact angles with water greater than 150 degrees and low contact angle hysteresis. However, superoleophobic surfaces -- those that display contact angles greater than 150 degrees with organic liquids having appreciably lower surface tensions than that of water -- are extremely rare. Calculations suggest that creating such a surface would require a surface energy lower than that of any known material. We show how a third factor, re-entrant surface curvature, in conjunction with chemical composition and roughened texture, can be used to design surfaces that display extreme resistance to wetting from a number of liquids with low surface tension, including alkanes such as decane and octane."
For more technical details, you might want to read this page about "Effects of Nanoparticle Addition on the Bulk, Surface and Interfacial Properties of Polymers" at CONFEX, the Conference Exchange. Here is a short excerpt about producing super-oleophobic surfaces. "Theoretical calculations suggest that a super-oleophobic surface would need to have a surface energy < 5 mN/m, whereas the lowest solid surface energies reported to date are in the range of ~6 mN/m. In this work, we explain how a third factor, surface curvature (apart from surface chemistry and roughness), can be used to significantly enhance liquid repellency, by studying electrospun polymer fibers containing very low surface energy perfluorinated nanoparticles (FluoroPOSS). Increasing the POSS concentration in the elecrospun fibers allows us to systematically transcend from super-hydrophilic to super-hydrophobic and finally to the first ever super-oleophobic surfaces (exhibiting low hysteresis and contact angles with decane and octane greater than 150°)."
Finally, you might want to read some comments from Chemistry World, "Giving oil the slip" (Tom Westgate, December 6, 2007).
Sources: Anne Trafton, MIT News Office, December 6, 2007; and various websites
You'll find related stories by following the links below.