The physics of tearing
Researching the mundane can sometimes throw up truly useful results. An international team of researchers has recently unravelled the mystery behind why sticky tape tears in a particular way. Their findings are expected to be of considerable use in the field of micro- and nano-technologies.
The situation might sound familiar: you are wrapping a present for a friend, one hand pressed down on the wrapping paper to keep it from slipping, while the other hand attempts to cut a strip of sticky tape. And then, just when you are nearly done, that last strip of tape just tears into a sliver, leaving you trying to find the end with your fingernail while franticly attempting to keep the wrapping from unravelling.
Well believe it or not, this frustrating scenario is now the subject of some serious research by scientists from the French National Centre for Scientific Research (CNRS), the University of Santiago in Chile and Massachusetts Institute of Technology (MIT) in the US. Their work, which explains the physics behind what they call the ‘wallpaper problem’, is published Nature Materials.
'You want to redecorate your bedroom, so you yank down the wallpaper. You wish that the flap would tear all the way down to the floor, but it comes together in a triangle and you have to start all over again,' explains Pedro Reis, one of the authors of the paper and an applied mathematics instructor at MIT.
To better understand why this happens, the researchers designed a system in which to conduct controlled tear experiments. The system involves gluing adhesive film or sticky tape, in which two slits are cut to create tab, to a surface, and then steadily pulling on the tab. The researchers recorded the shapes of the resulting tears and the mechanical forces for sticky tapes that have different adhesive and mechanical properties. The physicists found that these tears are usually triangular in shape.
The team also discovered how these tears arise in the first place. As the strip of tape is pulled, energy builds up in the fold that forms where the tape is peeling from the surface. The tape can release that energy in two ways — by unpeeling from its surface or by becoming narrower.
The scientists were able to distinguish between three inherent properties of adhesive materials: their elasticity, adhesive energy (how strongly the adhesive sticks to a surface) and fracture energy (how tough it is to rip). The scientists have put together a formula which allows the characterisation of one of these properties based on the other two and on a simple measurement of the angle of the triangle.
Their results are expected to have important implications for material technology. For instance, they could help engineers calculate one of the three properties when the other two are known. This could be particularly useful for the developing ultra-thin films, which are basic elements in many micro- or nano-systems. These films, the properties of which are difficult to manipulate, are already present in our daily lives in the form of shock detectors in airbags and micro-mirrors in second-generation video projectors.