RESISTANCE TO ROLLING IN THE ADHESIVE CONTACT OF 2 ELASTIC SPHERES

For the stability of agglomerates of micron sized particles it is of c onsiderable importance to study the effects of tangential forces on th e contact of two particles. If the particles can slide or roll easily over each other, fractal structures of these agglomerates will not be stable. We use the description of contact forces by Johnson, Kendall a nd Roberts, along with arguments based on the atomic structure of the surfaces in contact, in order to calculate the resistance to rolling i n such a contact. II is shown that the contact reacts elastically to t orque forces up to a critical bending angle. Beyond that, irreversible rolling occurs. In the elastic regime, the moment opposing the attemp t to roll is proportional to the bending angle and to the pull-off for ce P-c. Young's modulus of the involved materials has hardly any influ ence on the results. We show that agglomerates of sub-micron sized par ticles will in general be quite rigid and even long chains of particle s cannot be bent easily. For very small particles, the contact will ra ther break than allow for rolling. We further discuss dynamic properti es such as the possibility of vibrations in this degree of freedom and the typical amount of rolling during a collision of two particles.