The dependence of friction anisotropies on the molecular organization of LB films as observed by at. force microscopy

We performed lateral force microscopy on thiolipid Langmuir-Blodgett (LB) films physisorbed on mica substrates with a silicon tip of an at. force microscope. The structure of condensed domains, reflecting their sym. morphol., was obsd. The lateral (friction) forces were measured as a function of (normal) applied load, of sliding velocity and of the mol. orientation of these films. At a fixed velocity, lateral force increases with applied load in a linear fashion. Within the velocity range 0.01 to .apprx. 50 mm/s, the lateral force signal initially increases monotonically with velocity (static regime) and then stabilizes when the tip begins sliding. The friction force and the obsd. asymmetry in the quasi-static "friction-loops" (torsion of the tip during a forward/reverse scan) were dependent on the domain orientation with respect to the scan direction, while the measured adhesive force remained const. Together, friction and asymmetry reveal and map mol. packing and tilt. [on SciFinder (R)]