Adhesive tape or sticky notes are easy to connect to a surface, but could be difficult to remove. This phenomenon, often known as adhesion hysteresis, could be fundamentally observed in soft, elastic materials: Adhesive contact is formed more easily than it’s broken. Researchers on the University of Freiburg, the University of Pittsburgh and the University of Akron within the US have now discovered that this adhesion hysteresis is brought on by the surface roughness of the adherent soft materials. Through a mixture 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 on 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 leads to the journal Science Advances.
“Our findings will make it possible to specifically control the adhesion properties of soppy materials through surface roughness,” says Sanner. “They can even allow recent and improved applications to be developed in soft robotics or production technology in the long run, for instance for grippers or placement systems.”
Sudden jumping movement of the sting 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 fabric since it deforms within the contact cycle: It’s 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 ageing, 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 could be explained without these specific energy dissipation mechanisms. The one source of energy dissipation in our numerical model is the sudden jumping movement of the sting 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 within the simulations of the Freiburg researchers and within the adhesion experiments of the University of Akron. “The abrupt change within the contact surface was already mentioned within the Nineteen Nineties as a possible reason behind adhesion hysteresis, but previous theoretical work on this was limited to simplified surface properties,” explains Kumar. “Now we have succeeded for the primary time in calculating the adhesion hysteresis for realistic surface roughness. This is predicated on the efficiency of the numerical model and an especially detailed surface characterisation carried out by researchers on the University of Pittsburgh,” says Jacobs.
