SURFACE-MEDIATED DISORDER IN ALIGNED LIQUID-CRYSTAL FILMS CAUSED BY COLLISIONS WITH HE

An atomic beam of helium scattered from the surface of an aligned nema tic liquid crystal film (4'-pentyl-4-cyano biphenyl, 5CB) initiates a collision-induced transient loss of order in the film. The magnitude o f the change in order depends on the momentum direction of the atomic beam with respect to the liquid crystal director. The effect is signif icantly more pronounced when the incoming beam is perpendicular to the detector than when it is parallel. This result is consistent with a p icture of a surface-mediated disorder, in which atom-molecule collisio ns at the interface initiate a macroscopic transient disturbance of or der in the film.