  We study the indentation of a free-standing lipid membrane suspended over a
nanopore on a hydrophobic substrate by means of molecular dynamics simulations.
We find that in the course of indentation, the membrane bends at the point of
contact, and the fringes of the membrane glide downward intermittently along
the pore edges and stop gliding when the fringes reach the edge bottoms. The
bending continues afterwards, and the large strain eventually induces a phase
transition in the membrane, transformed from a bilayered structure to an
interdigitated structure. The membrane is finally ruptured when the indentation
goes deep enough. Several local physical quantities in the pore regions are
calculated, which include the tilt angle of lipid molecules, the nematic order,
the included angle and the distance between neighboring lipids. The variations
of these quantities reveal many detailed, not-yet-specified local structural
transitions of lipid molecules under indentation. The force-indentation curve
is also studied and discussed. The results make connection between the
microscopic structure and the macroscopic properties, and provide deep insight
in the understanding of the stability of a lipid membrane spanning over a
nanopore.
