Cohesive Zone - Adhesive Van der Waals potential

Hi,

The problem I want to solve is: instabilities that arise due to
molecular potential (VDW interactions) when a rigid solid block
(contactor) approaches an elastic film bonded to a support (no
interactions or slip whatsoever between the film and the support). These
interactions come into play when the air gap between the elastic film
and contactor becomes less than some critical distance. I want to do
this in Abaqus Explicit.
a) I have tried using cohesive zone model traction-separation law (on
single element blocks) in VUMAT to model the air gap between the film
and the contactor (giving tractions calculated as the derivatives of VDW
potential wrt the displacements). But the film is getting compressed
instead of getting attracted to the contactor (because van der waals
forces increase as the distance or teh air gap decreases). Is this
expected or is it because of the stiffness of cohesive zone (since
traction separation law need stiffnesses to be specified to calculate
stresses and these stiffnesses are derivatives of tractions and are
finite. So the cohesive zone model has some finite stiffness to it), and
is producing compressive stresses in the film when the contactor is
brought closer to the film. Is this behaviour anticipated? I can give
you the .inp and .f files if you want. please help... am still using the
3 element model to check this and stuck in this problem since months.
thanks!!

regards,
praveena