PITTSBURGH (Dec. 2, 2019) — Tires gripping the road. Nonslip
shoes preventing falls. A hand picking up a pen. A gecko climbing a wall.
All these things depend on a soft surface adhering to and
releasing from a hard surface, a common yet incompletely understood interaction.
New research published in Proceedings of the National Academy of Sciences
(PNAS) finds the missing link between soft surface adhesion and the roughness
of the hard surface it touches. The paper, “Linking energy loss in soft
adhesion to surface roughness,” (DOI: 10.1073/pnas.1913126116) was
published in Proceedings of the National Academy of Sciences and was
coauthored by Siddhesh Dalvi, Abhijeet Gujrati, Subarna R. Khanal, Lars
Pastewka, Ali Dhinojwala, and Tevis D.B. Jacobs.
Dr. Jacobs, assistant professor of mechanical engineering
and materials science at the University of Pittsburgh’s Swanson School of
Engineering, and Dr. Dhinojwala, interim
dean and H.A. Morton Professor of Polymer Science at The University of Akron’s
College of Polymer Science and Polymer Engineering, have used in situ microscopic measurements of
contact size to unlock the fundamental physics of how roughness affects soft
“A gecko running up a vertical wall is an
excellent example of how nature has developed a solution to stick to rough
surfaces,” says Dhinojwala. “The key to achieve this adhesion on rough surface
is molecular contact. Soft material can conform to rough surfaces and create
the molecular contact necessary to stick well. We need a fundamental
understanding of the parameters that control adhesion to rough surfaces and the
There are two different parts of the process: what happens
when you load up the contact and what happens when you separate it.
Previous theories have proposed how roughness affects the
first half of the process, but offer no insight into the second half. This
problem is called “adhesion hysteresis,” meaning the soft surface contact behaves
differently as it encounters the rough surface rather than when pulled away. One
way to think about adhesion hysteresis is to think of a small rubber ball.
Pressing the ball down against a hard surface expands the area of contact;
letting go will cause the area to shrink again, but not in a predictable,
symmetrical way. This discovery marks the first model of rough adhesion that can
The key to this foundational discovery is a close look at the
rough surface itself—very, very close.
“People have been measuring roughness for a hundred years,
but conventional techniques can’t see the small detail,” says Jacobs. “We
zoomed in, combining multiple techniques, to measure roughness on top of
roughness on top of roughness. The texture goes down to the atomic scale for
The group developed a new approach using an electron
microscope to measure roughness down to below the scale of a nanometer. One of
the surfaces in this study appeared far smoother than two others when measured
using conventional techniques; however, when measured down to the atomic scale,
it proved to be the roughest of all. This small-scale roughness created a lot
more surface area for the soft material to grip. The detailed understanding of
the rough surface was the missing link that explained the predicted the
surfaces’ adhesion behavior.
“Our research answered an important question, but in another
sense, it opened up a new line of inquiry,” says Jacobs. “There are a lot of
interesting questions about what it really means for surfaces to be ‘in
contact’ and how to link what is happening at the atomic-scale to what we
observe in full-size, real-world contacts. And we’re excited to continue
Maggie Pavlick, 12/2/2019
Contact: Maggie Pavlick