In atomic force microscope studies of thin films often a defined jump of the tip through the film is observed once a certain threshold force has been exceeded. In particular, on lipid bilayers this is regularly observed. In a previous paper [H.-J. Butt and V. Franz, Phys. Rev. E 66, 031601 (2002)] we presented two complementary models to describe film rupture. The aim of this study was to verify these models. Experiments were done with solid supported bilayers consisting of dioleoyloxypropyl-trimethylammonium chloride (DOTAP) and dioleoylphosphatidylserine (DOPS) in aqueous solutions and with propanol. Both models describe experimental results adequately. In particular, a narrow distribution of yield forces and an increase of the mean yield force with increasing loading rate is correctly predicted. For the lipid bilayers spreading pressures of roughly 20 mN/m (DOTAP) and 5 mN/m (DOPS) were measured. Line tensions for the edge of a lipid bilayer ranged between 3 (DOTAP) and 6 pN (DOPS).
The lateral organization of alkyl-substituted polyphenylene dendrimers on the basal plane of graphite has been investigated using atomic force microscopy. The dendrimers consist of a core of twisted, interlocked benzene rings and an external shell of dodecyl chains. Three kinds of dendrimers were used which in solution show a tetrahedral or a disklike shape. On graphite the dendrimers spontaneously form a stable, almost pinhole free monolayer. Complex two-dimensional arrangements and supramolecular ordering were observed in these monolayers prepared by spin-coating. One prominent structure observed with all three dendrimers was regions consisting of parallel rows of 6 nm spacing. In addition, pairs of dendrimers formed two-dimensional crystals on graphite. A simple model to interpret structure formation is discussed. The structure depends sensitively on the structure of the dendrimer, on the solvent, and on the concentration.
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