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By Jana Wiegand
  • Biological and Environmental Engineering
From stretchable electronics to reversible adhesives, developing targeted materials depends on an intimate understanding of how the new materials will react when they come into contact with other substances.

This is especially challenging when the materials become deformed along their points of contact, potentially changing their basic surface properties.

Engineers can probe these properties for solid materials by simply depositing a liquid drop on their surface and measuring the degree to which the material absorbs or deflects the liquid. This methodology is part of the field of wetting. And while these principles are relatively straightforward when describing how Teflon-coated pans repel oil, for example, they become much more complex when the underlying solid is soft and squishy.

A new paper, “Singular Nature of the Elastocapillary Ridge,“ describes unprecedented numerical simulations that demonstrate how exceptionally soft polymeric solids possess an intricate surface elasticity — proving that the surface tension of these materials can be modified by stretching or squeezing. The research published on Sept. 25 in the journal Physical Review X.

“This ability to functionalize a soft surface on demand could be crucial for the development of futuristic technologies, such as wearable electronics, self-cleaning surfaces and soft robotics,” said lead author Anupam Pandey, a postdoctoral researcher in the Department of Biological and Environmental Engineering at Cornell’s College of Agriculture and Life Sciences.

 Pandey started this project when he was in the University of Twente’s Physics of Fluids Group, in the Netherlands. He said the work used highly detailed mathematical modeling to resolve contradictory observations from the wetting technique.

“The debate was whether or not the experimental observations were a manifestation of surface elasticity,” he said.

What made the problem particularly challenging was a 200-year-old paradox in the field of wetting. On rigid surfaces, the contact angle between the droplet and the surface is governed by the balance of horizontal forces, described by a classic equation known as Young’s Law. However, on soft solids, the droplet deforms the underlying substrate by pulling the surface into a ridge-like structure.

This posed a problem because researchers typically measure the contact angle between the droplet and the surface, but now the ridge obscured the angle they needed to quantify. Since the ridge’s appearance implied that vertical elastic forces might also be acting on the soft solid, the researchers needed to understand exactly which forces were at play, as well as how much the deformation could be changing the inherent surface properties .

On small scales, Pandey said these soft and flexible materials — like gels and rubbers — have highly deformable, liquid-like structures. But they still behave like solids at large scales thanks to cross-linking between molecular chains.

To see what properties might be affected by the soft solid’s deformation, the team analyzed all the forces acting on the wetting ridge. They discovered that the exact shape of the ridge depended on how much the surface had been stretched or compressed, which then shaped the balance of forces and overall geometry.

“Our results demonstrate that existing experimental observations are in fact a consequence of surface elasticity,” Pandey said. “This resolves the paradox of the missing vertical force and opens up new avenues for designer polymeric surfaces.”

Coauthors include Jacco Snoeijer at the University of Twente, Bruno Andreotti at École Normale Supérieure, Stefan Karpitschka at the Max Plank Institute for Dynamics and Self-Organization, and Harald van Brummelen at Eindhoven University of Technology.

Header image: Water droplets. Photo by Nithya Ramanujam/Unsplash.

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