MECHANICS OF ADHESION - Nanoindentation with Biological Applications - page 89

2. Adhesion and Separation: The “Snap-On” and “Pull-Off ”

Instabilities

In this section, the complete process of adhesion and separation is
considered: As before, the system consists of two bodies, initially in a
state of separated stable equilibrium, that are brought together by an
external influence such that the system passes through an unstable
state—the “snap-on” instability considered in the previous section—to a
new stable equilibrium adhered state. Here, however, we consider the
external influence to continue to bring the bodies together, reducing their
separation such that the system may even pass into a state of stable
“indentation” compression, as opposed to the continuous tension
considered in the previous section. After imposing a separation of closest
approach, the external influence then reverses direction and begins to
increase the separation of the bodies such that the system passes through
a new unstable state—the “pull off ” instability to be considered here—to
the original stable equilibrium separated configuration. The approach and
separation instabilities lead to non-equilibrium motion of the tip and do
not occur under the same conditions; as a consequence the contact cycle
is hysteretic and the energy lost is the contact “work of adhesion.”
The concepts introduced in the previous section to describe and
determine snap-on adhesion characteristics, particularly the importance
of the net force and stiffness fields in determining equilibria and stability,
will again be used to describe the complete adhesion and pull-off
separation process. In the previous section a surface interaction potential
was used that provided simple closed-form expressions for critical
parameters, thus illustrating the snap-on process. Here, a more physically
realistic interaction potential is used that, although somewhat less
analytically tractable, provides aspects absent from the previous
potential: a well-defined adhered state; the possibility of indentation; an
intrinsic length scale; and the possibility of either stability over the
complete interaction range or dual instability states corresponding to the
snap-on and pull-off processes.