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

We performed lateral force microscopy on thiolipid Langmuir-Blodgett (LB) films physisorbed on mica substrates with asilicon tip of an atomic force microscope. The structure ofcondensed domains, reflecting their symmetric morphology, wasobserved. The lateral (friction) forces were measured as a function of (normal) applied load, of sliding velocity and of themolecular orientation of these films. We found that at a fixedvelocity, lateral force increases with applied load in a linearfashion. Within the velocity range 0.01 to~50m/s, the lateral force signal initiallyincreases monotonically with velocity (static regime) and thenstabilises when the tip begins sliding. The friction force andthe observed asymmetry in the quasi-static ``friction-loops'(torsion of the tip during a forward/reverse scan) were foundto be dependent on the domain orientation with respect to the scan direction, while the measured adhesive force remainedconstant. Together, friction and asymmetry reveal and mapmolecular packing and tilt.