Interior Atomic Structure Helps Promote Adhesion
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The force with which two surfaces adhere to one another can depend on the composition of the material below the surface, as discovered by physicists working with Karin Jacobs and Peter Loskill at Saarland University in Germany and by researchers in Kellar Autumn’s team at Lewis & Clark College in Portland (Oregon, USA), who carried out systematic measurements of adhesive forces.
Geckoes are well known to be the largest animals that can walk across a ceiling. To enable them to do this, the reptiles have under their toes millions of fine hairs, each of which has around one hundred tiny spatulate thickenings at the tips. These spatulate endings come into very close contact with the surface that the gecko is walking on and are attracted by the molecular forces of the surface. The teams of researchers from Saarbrücken and Portland, who previously studied the adhesive forces of bacteria and proteins on surfaces, were able to demonstrate that even an animal as large as a gecko can detect the composition of the material below the surface.
For their experiments into depth sensitivity, the researchers carefully removed the hairs from the toes of a tokay gecko. (The hairs regrow when the gecko next sheds its skin.) They bunched the hairs together and bonded them to the tip of a highly sensitive dynamometer. This was then pulled across the surface of a silicon disk which had a silicon dioxide coating that varied in thickness. The resulting friction and attraction forces were measured with a high level of precision.
New description of adhesive forces
On the basis of these findings, the researchers have developed a new description of the adhesive forces of surfaces, which takes into consideration for the first time the structure of the material under the surface. “Until now, the adhesive forces have always been derived from the surface energy. This is a property of the outermost atomic layers which are close to the surface and reach down to a depth of around one nanometre,” says Karin Jacobs. “But our new description also relates to the molecular van der Waals force from deeper layers.”
The experiments with the hairs from the gecko’s toes have demonstrated that, as a result of the van der Waals force, the atomic structure in the interior of the material also has an effect on the surface in the form of macroscopically detectable differences in the adhesive forces. The scientists in Saarbrücken and Portland have therefore coined the new term “subsurface energy” to describe how the material below the surface contributes to these forces.
The studies have been published in the Journal of the Royal Society Interface.
For more information, please contact: Professor Karin Jacobs, k.jacobs@ physik.uni-saarland.de