How surface roughness influences the adhesion of soft materials

"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 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 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.