Velocity-Dependent Forces in Atomic Force Microscopy Imaging of Lipid Films

We have imaged adsorbed fluid lipid bilayers by atomic force
microscopy. The patches were formed by rupture of phospholipid
vesicles onto magnesium fluoride. We show that the membrane patches
are fluid but can be stably imaged at scan rates higher than 6 p d
s . At lower scan rates the tip penetrates through the layer. The
penetrating tip does not destroy the fluid patches, and the
previous image can be restored after increasing the scanning
velocity. The dynamic forces that possibly explain the effect are
discussed.