Putting Science to Stickiness

Peel-and-stick wallpaper makes a wall look nicer and is relatively easy to apply. However, removing it from the wall is more difficult. 

This phenomenon of requiring more energy to separate soft, elastic materials than to put them together is called adhesion hysteresis.

Tevis Jacobs, associate professor of mechanical engineering and materials science at the University of Pittsburgh’s Swanson School of Engineering, is part of a team of researchers from the University of Freiburg in Germany and the University of Akron that discovered how adhesion hysteresis is caused by the roughness of the surface materials are applied to. 

“There were two prior scientific explanations for stickiness that worked well for some materials, but couldn’t explain others,” Jacobs said. “The puzzle was to find a universal mechanism that worked for all materials.”  

The team, which included researcher Antoine Sanner, Prof. Lars Pastewka, Nityanshu Kumar and Prof. Ali Dhinojwala, attacked this puzzle using a combination of computer simulations, comprehensive measurements of surface topography, and adhesion experiments with a material that defied the traditional explanations. The results proved that roughness interferes with the separation process, causing the materials to detach through a series of sudden, tiny jumps – each of which releases energy as heat. All this lost energy requires the homeowner to pull much harder to remove their wallpaper. 

The paper, “Why soft contacts are sticker when breaking than when making them,” was published in the journal Science Advances in March 2024. (doi: 10.1126/sciadv.adl1277) 

Written by Sonja Seidel, republished with permission

How surface roughness influences the adhesion of soft materials

Research team discovers universal mechanism that leads to adhesion hysteresis in soft materials

• Research team discovers universal mechanism that leads to adhesion hysteresis in soft materials.
• Through a combination of experimental observations and simulations, the team demonstrated that roughness interferes with the separation process, causing the materials to detach in minute, abrupt movements, which release parts of the adhesive bond incrementally.
• The findings will make it possible to specifically control the adhesion properties of soft materials through surface roughness and will allow new and improved applications to be developed in soft robotics or production technology in the future.

Adhesive tape or sticky notes are easy to attach to a surface, but are difficult to remove. This phenomenon, known as adhesion hysteresis, can be fundamentally observed in soft, elastic materials: Adhesive contact is formed more easily than it is broken. Researchers at the University of Freiburg, the University of Pittsburgh and the University of Akron in the US have now discovered that this adhesion hysteresis is caused by the surface roughness of the adherent soft materials. Through a combination of experimental observations and simulations, the team demonstrated that roughness interferes with the separation process, causing the materials to detach in minute, abrupt movements, which release parts of the adhesive bond incrementally. Dr. Antoine Sanner and Prof. Dr. Lars Pastewka from the Department of Microsystems Engineering and the livMatS Cluster of Excellence at the University of Freiburg, Dr. Nityanshu Kumar and Prof. Dr. Ali Dhinojwala from the University of Akron and Prof. Dr. Tevis Jacobs from the University of Pittsburgh have published their results in the prestigious journal Science Advances.

“Our findings will make it possible to specifically control the adhesion properties of soft materials through surface roughness,” says Sanner. “They will also allow new and improved applications to be developed in soft robotics or production technology in the future, for example for grippers or placement systems.”

Sudden jumping movement of the edge of the contact

Until now, researchers have hypothesized that viscoelastic energy dissipation causes adhesion hysteresis in soft solids. In other words, energy is lost to heat in the material because it deforms in the contact cycle: It is compressed when making contact and expands during release. Those energy losses counteract the movement of the contact surface, which increases the adhesive force during separation. Contact aging, i.e. the formation of chemical bonds on the contact surface, has also been suggested as a cause. Here the longer the contact exists, the greater the adhesion. “Our simulations show that the observed hysteresis can be explained without these specific energy dissipation mechanisms. The only source of energy dissipation in our numerical model is the sudden jumping movement of the edge of the contact, which is induced by the roughness,” says Sanner.

Adhesion hysteresis calculated for realistic surface roughness

This sudden jumping motion is clearly recognisable in the simulations of the Freiburg researchers and in the adhesion experiments of the University of Akron. “The abrupt change in the contact surface was already mentioned in the 1990s as a possible cause of adhesion hysteresis, but previous theoretical work on this was limited to simplified surface properties," explains Kumar. “We have succeeded for the first time in calculating the adhesion hysteresis for realistic surface roughness. This is based on the efficiency of the numerical model and an extremely detailed surface characterisation carried out by researchers at the University of Pittsburgh,” says Jacobs.

About the Cluster of Excellence livMatS

The vision of the Cluster of Excellence Living, Adaptive, and Energy-Autonomous Materials Systems (livMatS) is to combine the best of both worlds – nature and technology. livMatS develops lifelike materials systems inspired by nature. These systems adapt autonomously to their environment, harvest clean energy from their surroundings, and are insensitive to or able to recover from damage