MECHANICS OF ADHESION - Nanoindentation with Biological Applications - page 100

Figure 4-12. The variations in the net force (solid line) with tip position for combined
Mie (M) and harmonic (S) interactions (dotted lines) between a probe tip and a surface as
an actuator moves the probe away from the surface. The probe spring stiffness is sub-
critical such that stable equilibrium positions of the tip are possible only for limited
domains of imposed probe position. The domains are separated by a non-equilibrium
“pull-off ” configuration. The values are normalized using the characteristic values ( Eqs.
4-22 a nd 4-25 ) .

remains in stable equilibrium at almost invariant tip position of
z ∗ / z 0 ≈ 1 . Further increases in s lead to a critical condition at

2.693 for which the tip is once again in a condition of incipient
instability. However, in this case, thermal perturbation or any further
incremental increase in s lead to the tip moving via a non-equilibrium
( F ≠

s ∗ / z 0 =

0 ) path to “pull off” the surface to a new stable equilibrium
position considerably displaced (away from the surface) from the
instability point. Further increases in s lead to stable incremental motion
of the tip away from the surface until at s / z 0 = 3.5, the system has