The experimental technique known as probe tack is designed to test the adhesive properties of a film for very short contact times.
The principle is the following:
A flat-ended cylindrical probe is brought in contact with the adhesive film which is deposited on a rigid substrate. The probe is then maintained in contact under a controlled pressure for a certain contact time tc. The probe is then removed from the film at a constant crosshead speed and the tensile force is monitored as a function of the probe displacement.
We have developed at the PCSM laboratory an instrumented probe tack where a precise measurement of the force and displacement is coupled with the simultaneous observation of the deformation mechanisms of the adhesve film duing the debonding.
This observation in the direction parallel to the tensile axis is possible through the use of a transparent glass substrate and a 45° mirror positioned under it.
When a tensile hydrostatic pressure is applied to the adhesive film, extensive cavitation occurs above a critical value of stress. These cavities appear between the probe surface and the film. They grow and eventually can form a honeycomb structure. The shape, size and growth kinetics of these cavities depend on the nature of the adhesive film and on the experimental conditions. Shown below is an example of growth mechanism for an acrylic type adhesive:
The data shows the nominal stress on the adhesive (force divided by contact
area), as a function of the nominal strain (distance between the probe and the
adhesive normalized by the initial thickness of the adhesive).
One can
distinguish several deformation regimes:
Before the maximum force, the deformation is mostly homogeneous. Only a few cavities are nucleated before the maximum.
Immediately after the occurrence of the maximum, the force decreases sharply due to the massive nucleation of cavities. The triaxial hydrostatic stress is progressively relaxed and the structure evolves towards a situation where the walls of the cavities become very thin and are elongated in the direction of the normal stress.
Finally air penetrates from the outside walls and the filaments are progressively broken (for cohesive fracture) or debonded (for adhesive fracture).
A short movie of the formation of these cavities can be visualized here.
Warning: the movie requires the DivX:) CODEC