Title: Polygons pulled from an adsorbing surface,
Abstract: We consider self-avoiding lattice polygons, in the hypercubic lattice, as a
model of a ring polymer adsorbed at a surface and either being desorbed by the
action of a force, or pushed towards the surface. We show that, when there is
no interaction with the surface, then the response of the polygon to the
applied force is identical (in the thermodynamic limit) for two ways in which
we apply the force. When the polygon is attracted to the surface then, when the
dimension is at least 3, we have a complete characterization of the critical
force--temperature curve in terms of the behaviour, (a) when there is no force,
and, (b) when there is no surface interaction. For the 2-dimensional case we
have upper and lower bounds on the free energy. We use both Monte Carlo and
exact enumeration and series analysis methods to investigate the form of the
phase diagram in two dimensions. We find evidence for the existence of a
\emph{mixed phase} where the free energy depends on the strength of the
interaction with the adsorbing line and on the applied force.