  Park {\it et al.} have reported eight times greater atomic-scale friction in
the periodic than in the quasiperiodic direction on the two-fold face of a
decagonal Al-Ni-Co quasicrystal. We present results of molecular-dynamics
simulations intended to elucidate mechanisms behind this giant frictional
anisotropy. Simulations of a bare atomic-force-microscope tip on several model
substrates and under a variety of conditions failed to reproduce experimental
results. On the other hand, including the experimental passivation of the tip
with chains of hexadecane thiol, we reproduce qualitatively the experimental
anisotropy in friction, finding evidence for entrainment of the organic chains
in surface furrows parallel to the periodic direction.
