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Rethinking surface tension

Posted August 30, 2013
Rethinking surface tension

Rethinking surface tension
Dadhichi Paretkar (left) and Jagota reported the results of their studies of surface tension in solids in the journal Physical Review E. Animangsu Ghatak ’03 Ph.D. was a coauthor of their paper.

If you’ve ever watched a drop of water form into a bead or a water strider scoot across a pond, you are familiar with a property of liquids called surface tension.

The interior molecules of a liquid are pushed and pulled by neighboring molecules in every direction, but those at the surface have only half as many neighbors to interact with. The resulting density change of molecules arranged near the surface causes a drop of water to form a sphere and prevents the insect from sinking even though its density is greater than the water’s.

The surface tension of liquids is well-established, says Anand Jagota, but the same property in solid materials has seemed a moot point. Technically, it exists, but its force is usually too weak to deform a solid by more than an angstrom.

Jagota, professor of chemical engineering and director of Lehigh’s bioengineering program, 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.

A faithful, but attenuated replica

The researchers patterned the elastomer with ripples measuring microns in depth and then covered it with a gel and exposed it to air. Dark-field optical microscopy of a cross-section of the elastomer and gel revealed that the gel faithfully replicated the surface topography of the elastomer.

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