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Rethinking surface tension

The surface tension of liquids is well-established, says Anand Jagota, professor of chemical engineering and director of Lehigh's bioengineering program. Jagota has pondered for more than a decade the possibility that some solids, especially soft biomaterials and geometrically altered materials, might also exhibit surface tension.

Over the last two years, at the Leibniz Institute for New Materials (INM) in Saarbruecken, Germany, and then at Cornell University, Jagota and his collaborators have experimented with two classes of solids: rubber-like elastomers and a more compliant gelatin similar in stiffness to human tissue.  The researchers patterned the elastomer with ripples measuring microns in depth and then covered it with a gel and exposed it to air. Jagota viewed a cross-section of the elastomer and gel under a dark-field optical microscope, and noted that the gel faithfully replicated the surface topography of the elastomer. When the researchers removed the gel from the elastomer, however, the gel flattened almost instantaneously. It continued to match the peaks and valleys of the elastomer';s ripples, but with significantly diminished features.

The group reported their results in Physical Review E in an article titled "Surface-tension-induced flattening of a nearly plane elastic solid." The discovery, says Jagota, should motivate scientists and engineers to rethink many of their assumptions. The research was supported by the Division of Materials Science and Engineering in the Office of Basic Energy Sciences of the U.S. Department of Energy. Jagota's collaborators were Animangsu Ghatak '03 Ph.D., associate professor of chemical engineering at the Indian Institute of Technology at Kanpur, and Dadhichi Paretkar, formerly of INM and now a postdoctoral research associate at Lehigh.

Jagota and his team continue to research the ways in which the surface tension of solids interacts with the surface tension of liquids; they replicated the phenomenon of Neumann's triangle, which describes the surface tensions of three immiscible liquids -- such as an oil, water and a hydrophobic liquid that doesn't mix with oil -- that are in equilibrium. They replaced one of the three liquids with a solid, which behaved like the third liquid in Neumann's triangle.

The group published these results in the Proceedings of the National Academy of Sciences. Their article, titled "Solid surface tension measured by a liquid drop under a solid film" was coauthored by Jagota; Nichole Nadermann, former research scientist at Lehigh and now a postdoctoral fellow at the National Institute for Standards and Technology; and Chung-Yuen Hui, professor of mechanical and aerospace engineering at Cornell.

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