Nov 28, 2010
Why does your beloved smart phone’s touch screen get contaminated with smudges and fingerprints? Has your flight ever been delayed because the plane had to be de-iced using nasty chemicals? Have you spoilt your favorite shirt or jacket by spilling wine or ketchup on it? I am sure that all of us have asked at least one of these questions before. All of these questions can be answered by understanding how liquids interact with solid surfaces. Understanding how liquids interact with solids can drive the development of innovative new products that solve these problems.
Research groups led by Prof. Robert E. Cohen, St. Laurent Professor of Chemical Engineering and Prof. Gareth H. McKinley, SoE Professor of Teaching Innovation and Associate Head for Research for Mechanical Engineering at MIT have investigated various aspects of liquid wettability in detail.[1-9] Research interest in this area is not new. Almost 70 years ago, Cassie and Baxter (1944) were interested in the wetting of wool surfaces with water. They reported that the wettability of liquid droplets can be decreased by trapping air pockets between the asperities of a textured surface. As the contact area between the solid and the liquid decreases, the tendency of that liquid to wet correspondingly decreases. Tuteja et. al. quantified the wettability of a surface-liquid combination in terms of a breakthrough pressure at which the trapped air pockets cannot be sustained and the liquid completely wets the textured surface. [5-7]
Designing oleophobic (oil-repellent) textile surfaces for domestic, industrial and defense applications is one of the major contributions of this research effort. Liquids like gasoline or alcohols (referred to collectively as oils) have low surface tensions, and therefore these liquids have a tendency to spread on any smooth surface (even Teflon). Researchers at Edwards Air Force Base in California have developed a set of new molecules that are even less wettable than Teflon and show promise for use in oleophobic coatings. Commercially available textile fabrics have inherently rough surfaces because of their periodic texture. If air pockets can be stabilized within these topographical features, then the wettability of liquid droplets on the surface can be drastically reduced. To aid in the rational design of oleophobic surfaces, I developed a set of design charts that reliably predict the wetting tendency of a liquid droplet on a given textured surface.
A wide range of surfaces like fabrics (Figure 1(a)), wire meshes (Figure 1(b)), bird feathers, or plant leaves can be made oleophobic by conformally coating them with low solid surface energy materials. Moreover, the wettability of these surfaces can be tuned and switched using the design charts as a guiding principle. Choi et al. reported that biaxial stretching of polyester fabrics leads to tunable and repeatedly switchable wettability. We also found that as the biaxial strain increases, liquids with progressively lower surface tensions will wet the fabric. Such a fabric could therefore be used as a selective filter to separate oil and water or two immiscible oils. Filters made of oleophobic fabrics can be manufactured at a large scale using conventional textile manufacturing and require no energy input to achieve liquid-liquid separation. Moreover, these filters are inexpensive, reusable and biodegradable.
Chemical and biological warfare agents present significant threats to the life and well-being of a soldier. Consequently, scientists at the US Army Research Center in Natick, MA are extremely interested in using this technology to develop water- and oil-repellent uniforms for soldiers. Currently, joint research efforts are focused on rendering current army uniform fabrics oleophobic and on developing a new fabric weave that is optimized for robust oleophobicity. Apart from the Army and the Air Force, this research effort has attracted the attention of companies in telecom, energy/environment, biotech, and the copiers/printers/photographyindustries.
Apart from their work on oleophobic textiles, researchers in the Cohen and McKinley groups are developing surfaces with reduced adhesion to ice and clathrate-hydrates, which are ice-like solids that form in oil and gas pipelines and obstruct the flow of oil. The formation and accretion of ice on exposed surfaces may hinder the operational performance of aircraft, helicopters, ships, offshore oil platforms, power lines, wind turbines, locks and dams, and telecommunications equipment. Oleophobic coatings have demonstrated a four-fold reduction in ice adhesion compared to bare steel surfaces and therefore have tremendous potential to ameliorate the abovementioned problems. 
Researchers in the Cohen and McKinley groups are also developing a simple and inexpensive spraying technique to coat large-area substrates and render them oleophobic as well as a lithography-based technique to produce transparent oleophobic surfaces for touch screens. Each of these new directions has the potential to radically change the sorts of products we use every day.
Figure 1. Water (colored blue) and rapeseed oil (a common vegetable oil, colored red) droplets bead up on (a) a polyester fabric and (b) on a wire mesh surface, when coated with the oleophobic coating.
Acknowledgements: I thank Prof. Robert E. Cohen, Prof. Gareth H. McKinley, and Prof. Michael F. Rubner, all from MIT; Mr. Quoc Truong, and Dr. Eugene Wilusz from US Army NSRDEC, Natick, MA; Dr. Joseph Mabry from Edwards AFB, CA, and my colleagues from the Cohen, Rubner and McKinley research groups at MIT for their support.
1. Chhatre, S. S.; Choi, W.; Tuteja, A.; Park, K.-C.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Scale Dependence of Omniphobic Mesh Surfaces. Langmuir 2010, 26, (6), 4027-4035.
2. Chhatre, S. S.; Tuteja, A.; Choi, W.; Revaux, A. l.; Smith, D.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Thermal Annealing Treatment to Achieve Switchable and Reversible Oleophobicity on Fabrics. Langmuir 2009, 25, (23), 13625-13632.
3. Choi, W.; Tuteja, A.; Chhatre, S.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H., Fabrics with tunable oleophobicity. Advanced Materials 2009, 21, (21), 2190-2195.
4. Choi, W.; Tuteja, A.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H., A Modified Cassie-Baxter Relationship to Explain Contact Angle Hysteresis and Anisotropy on Non-Wetting Textured Surfaces. Journal of Colloid and Interface Science 2009, 339, (1), 208-216.
5. Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E., Designing Superoleophobic Surfaces. Science 2007, 318, (5856), 1618-1622.
6. Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Robust omniphobic surfaces. Proceedings of the National Academy of Sciences, USA 2008, 18200-18205.
7. Tuteja, A.; Choi, W.; McKinley, G. H.; Cohen, R. E.; Rubner, M. F., Design parameters for superhydrophobicity and superoleophobicity. MRS Bulletin 2008, 33, (8), 752-758.
8. Chhatre, S. S.; Guardado, J. O.; Moore, B. M.; Haddad, T. S.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Fluoroalkylated Silicon-Containing Surfaces – Estimation of Solid-Surface Energy. ACS Applied Materials & Interfaces Article ASAP.
9. Meuler, A. J.; Smith, J. D.; Varanasi, K. K.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Relationships between Water Wettability and Ice Adhesion. ACS Applied Materials & Interfaces Article ASAP.