Poke your finger into a jar of honey, and you feel a drag force that depends on how fast you’re moving your finger. Remove it, and some honey clings to your finger and stretches into a long fibril that eventually breaks. Those properties are characteristic of a viscous liquid. When you push against a piece of soft rubber, on the other hand, the force you feel is proportional not to speed but to displacement, and when you pull away, the rubber remains in one piece; such are the properties of an elastic solid. Typical commercial adhesives have both viscous and elastic properties—that is, they are viscoelastic.
When a film of viscous, elastic, or viscoelastic material is sandwiched between two smooth, rigid surfaces that are then pulled apart, patterns form that include “fingers” of air penetrating the gap between the surfaces. Fingering patterns in the viscous and elastic cases have both...
