The dependence of friction anisotropies on the molecular organisation of LB films as observed by AFM

We performed lateral force microscopy on thiolipid Langmuir-Blodgett (LB) films physisorbed on mica substrates with a silicon tip of an atomic force microscope. The structure of condensed domains, reflecting their symmetric morphology, was observed. The lateral (friction) forces were measured as a function of (normal) applied load, of sliding velocity and of the molecular orientation of these films. We found that at a fixed velocity, lateral force increases with applied load in a linear fashion. Within the velocity range 0.01 to similar to 50 mu m/s, the lateral force signal initially increases monotonically with velocity (static regime) and then stabilises when the tip begins sliding. The friction force and the observed asymmetry in the quasi-static "friction-loops" (torsion of the tip during a forward/reverse scan) were found to be dependent on the domain orientation with respect to the scan direction, while the measured adhesive force remained constant. Together, friction and asymmetry reveal and map molecular packing and tilt.