Driving electroconvection in a nematic liquid crystal with an electric ac field leads to dielectric convection rolls if the driving frequency is higher than the cutoff frequency. According to the theory, these rolls have a time-dependent velocity and director field that can be described as standing waves which are phase locked to the driving field. The paper reports on measurements of the phase difference between the driving electric field and the director field by means of a stroboscopic technique. Experimental evidence of a symmetry breaking of the oscillations of the director field with respect to the polarity of the electric field is presented, and the possibility to measure flexoelectric coefficients by examining this asymmetry is discussed.

• Received 14 February 1992

DOI:https://doi.org/10.1103/PhysRevA.46.1009