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 nematic liquid crystal film (4′-pentyl-4-cyano biphenyl, 5CB) initiates a collision-induced transient loss of order in the film. The magnitude of the change in order depends on the momentum direction of the atomic beam with respect to the liquid crystal director. The effect is significantly more pronounced when the incoming beam is perpendicular to the detector than when it is parallel. This result is consistent with a picture of a surface-mediated disorder, in which atom-molecule collisions at the interface initiate a macroscopic transient disturbance of order in the film.