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Finding surface tension in compliant solid materials

A liquid has surface tension because, while its interior molecules are pushed and pulled in every direction by neighboring molecules, those at the surface have only half as many neighbors to interact with. The resulting density change of molecules near the surface causes a drop of water to form a sphere and enables a water strider to walk on water despite its greater density.

By contrast, says Anand Jagota, the surface tension of 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, director of Lehigh’s bioengineering program, has long wondered if some solids might exhibit surface tension. At the Leibniz Institute for New Materials (INM) in Germany and at Cornell University, he and his collaborators experimented with rubber-like elastomers and a more compliant gelatin similar in stiffness to human tissue. They patterned the elastomer with ripples measuring microns in depth, covered it with a gel and exposed it to air. Optical microscopy revealed that the gel faithfully replicated the surface topography of the elastomer.

When it was removed 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.

“We wondered if the gel would be an exact replica of the elastomer,” says Jagota. “The gel had filled all the undulations in the elastomer, but as soon as we removed it, a pent-up force acted immediately to flatten it.”

The group reported their results in Physical Review E. Jagota’s coauthors were Animangsu Ghatak ’03 Ph.D. of the Indian Institute of Technology at Kanpur, and Dadhichi Paretkar, formerly of INM and now a research associate at Lehigh. “Our results show that surface tension of soft solids drives significant deformation, and that the latter can be used to determine the former,” they wrote.

The discovery, says Jagota, should motivate scientists to rethink many assumptions. “As a basic mechanical force, surface tension in compliant solids will play a role in all mechanical phenomena involving compliant materials, especially biomaterials. How do things fracture, stick, slide, have friction, deform? What are the elastic forces that resist a cell when it spreads on a gel? How strongly do dust particles stick to the inside of a lung?

“We’re going to have to rethink many of the questions involving compliant materials.”