The effect of preload on the pull-off force in indentation tests of microfibre arrays

We determined how preload and work of adhesion control the force required to pull a circular cylindrical indenter off a microfibre array. Five regimes, with different contact behaviours, are identified for the unloading phase of indentation. These regimes are governed by two dimensionless parameters. Above a critical preload, the pull-off force and the pull-off stress reach a plateau value. The critical preload, as well as the plateau pull-off force (stress), is found to depend on a single dimensionless parameter q, which can be interpreted as a normalized work of adhesion.