Dynamics of line defects in nematic liquid crystals.

An experiment is performed in which a topological line defect (S =-1/2) is forced to move with constant speed c under the action of an applied voltage V. We argue that the line speed is determined by a competition between the viscous damping and the free energy the system gains by displacing the line, so that c = betaV with beta=1 over 2 gamma b (epsilon(a)K over pi)(1/2) and b a dimensionless number of O(1) determined only by viscous effects close to the core of the defect.