Weekly wiki entries: Designing Superoleophobic Surfaces
Title: Designing Superoleophobic Surfaces
Reference: R. E. Cohen A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S.A. Mazzella, G.C. Rutledge, G.H. McKinley, “Science” 318, 1618 (2007).
Soft matter keywords
Biomimetic, oleophobic, surface energy, roughness, reentrant surface curvature, contact angle, hysteresis.
Abstract from the original paper
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.
Soft matter example
In this paper, the authors explore surface factors of biomimetic super-hydrophobic surfaces and introduce a new factor in the design of oleophobic surfaces. Towards the goal of the article, the authors define examples of biological super-hydrophobicity, as well as provide chemical and physical explanations of the behavior. While not the focus of the article, the authors also illuminate methods for assessing hydrophobicity, including contact angle and hysteresis measurements. After defining the inspirations as well as chemical and physical explanations for designed superhydrophobic surfaces, the authors introduce a new factor that contributes to oleophobicity of surfaces, re-entrant surface curvature. In manipulating re-entrant surface curvature, the authors demonstrate two innovative surface features that achieve this goal, micro-hoodoos and spin-coated micro-tubules.
The authors introduce re-entrant surface curvature as a third factor that governs oleophobicity. Re-entrant surface curvature, which can most easily be understood visually, pertains to surface features with recesses between the top of a surface structure and the base of the feature; such features cause liquids in contact with the surface to sag below the top of the surface without being in contact with the side walls of the surface features nor the base surface below the features, which, like roughness features, ensure air pockets to form beneath the liquid, which will increase oleophobicity.
The authors begin the article by highlighting key biological super-hydrophobic surfaces, the most notable of which is the lotus leaf, Nelumbo nucifera. Other notable biological super-hydrophobic surfaces mentioned in the article are the legs of the water legs of the water strider, troughs on the elytra of desert beetles, and geckos’ feet. The reasons for hydrophobicity, as the authors explain, are due to both chemical and physical arguments. The chemical argument is the most widely known cause to hydrophobicity and is attributed to surface tension. The second and less attributed argument is a physical one and involves roughness of the surface in that increasing roughness of a surface decreases the contact area and contact points for a liquid to adhere to the surface. Further, roughness at the micro- and nano-scale causes pockets of air to form between the upper layers of roughness features and the base, increasing hydrophobicity.
Methods mentioned in the article to measure both hydro- and oleo-phobicity are contact measurements and hysteresis experiments. These were not expanded in the article and are available for explication in other sections of the wiki.
In the article, two novel feature types are introduced that provide altered re-entrant surface curvature, micro-hoodoos and micro-tubules. Micro-hoodoos, named from natural geological rock formations, are micro-structures consisting of thin posts topped wider, flat sections, as seen in Fig. 2. The extension of the flat section forms the recess and an increase in the re-entrant surface curvature. With the flat extension, liquid in contact with the top of the hoodoo structure will form a meniscus and sag below the top surface of the structure while resisting contact with the side walls of the posts as well as the base surface. The second novel feature type introduced in the article are micro-tubules made from spin-coated polymer fibers. As seen in Fig. 3, the micro-tubules form a net-like surface that, similar to the micro-hoodoos, increase the re-entrant surface curvature and cause liquid at the surface to sag below the tubules.
By introducing a third, concomitant factor in the design of hydrophobic and now oleophobic surfaces, Cohen et al have established a new paradigm in the design of biomimetic materials while also having established avenues toward the design of omniphobic, i.e. oleophobic and hydrophobic surfaces.
 N. A. Patankar, “Langmuir”, 2008, 20, 8209-8213.  R. E. Cohen A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S.A. Mazzella, G.C. Rutledge, G.H. McKinley, “Science” 318, 1618 (2007).