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NEW APPROACH TO STICKY PROBLEM

November 04, 1999

"Over the past 20 years people have been trying to develop techniques for studying structures of polymer surfaces to understand how these determine their adhesive performance," says Dr Leggett of the Manchester research team. "However, such surfaces are extremely difficult to study directly. They degrade very quickly under beams of charged particles, so subjecting them to something like electron microscopy effectively fries the material unless elaborate preparatory measures are used."

Alternatively, scientists have used spectroscopic techniques to study surface structure, but these generally involve relatively large areas and do not yield information at the nanometre scale.

Dr Leggett has used a technique based on atomic force microscopy. Here an ultra-sharp tip is scanned across the surface under examination. "It is a bit like a record player, with the stylus dragged across the surface following the bumps and grooves," he says.

The tip is attached to a cantilever on which is directed a laser beam. As the tip traces the topography of the surface the deflection of the cantilever can be measured. From this information an image of the surface can be constructed. The extent of the cantilever's deflection can also give information about the strength of the forces between the tip and the surface.

To measure adhesion in the perpendicular plane the tip is first coated with an appropriate chemical layer. It is then brought slowly down to the surface until it makes contact, pressed downwards into the surface then gradually withdrawn.

"When the tip is a long distance from the surface there is zero force between them," says Dr Leggett. "As the tip comes close to the surface it experiences a small attractive interaction and snaps into contact. As you push it in you get a repulsive force and when you withdraw it the tip remains in contact with the surface because of the adhesion force. You can measure the force required to break the contact - the 'pull-off' force or adhesion force. It is possible to quantify the force of adhesion at that point."

By scanning a defined area of the surface in this way it is possible to construct a map of the adhesion forces.

The system can also be used to measure lateral forces of adhesion. By dragging the tip across the surface the frictional forces between the tip and the surface can be measured. "This is an important surface property which is also related to the strength of the adhesion," says Dr Leggett.

Using this system the UMIST team has examined complex polymer surfaces which until now have not been amenable to such intricate investigation.

By using a combination of these techniques and studies of model materials, the scientists have been able to distinguish those adhesive forces which are attributable to the surface chemistry and those that are due to the mechanical characteristics of the surface.

"In the phenomenon of adhesion the relationship between chemical and mechanical effects is important, so the ability to disentangle the two is a significant advance," says Dr Leggett. "I think that atomic force microscopy is a very powerful technique for studying adhesion and could find a lot of use in industry research and development laboratories - it is probably under-used by commercial laboratories at the moment."

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Engineering and Physical Sciences Research Council (EPSRC)