  We investigate the mechanical properties of bilayers of
1,2-dipalmitoyl-sn-glycero-3-phosphocholine in the gel phase by using peak
force tapping with quantitative nanomechanical mapping. We study both dry and
aqueous bilayers and liposomes supported on the oxidized silicon surface. We
report the observation of a marked substrate effect on the measured Young
modulus of supported bilayer stacks which decreases as the height of the stack
increases. In contrast a clear substrate effect is not observed for the top
bilayer of supported aqueous liposomes, which is however affected by the
surface curvature of the sample. Adhesion forces present quantitative
differences between dry and aqueous samples, with the former being dominated by
capillary effects and the latter by non-contact interactions between tip and
substrate. The mechanical properties of stacked bilayers reveal a threshold
between two different regimes, for the lower portion and the upper portion of
the stack, that shows a change in the plasticity of the system. The threshold
is affected by applied setpoint and aging of the sample. We propose that it
arises from the presence of thin layers of hydration water between the
headgroups of contacting phospholipid bilayers, which appears to have a major
effect on the response of the bilayer to an external mechanical stress.
