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We report on the behavior of cylindrical walls formed in a substrate-free nematic film of PCH5 under the action of an in-plane ac field. In the film, with vertical molecular alignment at all the limiting surfaces, annular Brochard−Leger walls are induced well above the bend−Freedericksz threshold. They exhibit, at high field strengths, a new type of instability not encountered in sandwich, or any other, cell configuration. It manifests as a shearing of the loop-wall between the opposite free-surfaces. The shear strain is measured as a function of time, field strength, frequency, and temperature. Significantly, the strain is linear in field strength. The origin of shear and its dependence on field variables are explained through an adaptation of the Carr−Helfrich mechanism of charge separation. The sheared wall is stable against pincement up to several times the threshold field, and divides itself into two fragments under a large enough strain. With the shear distortion, linear defects appear in the opposite splay-bend regions, just as Neel lines in Bloch walls of magnetic systems. At very low frequencies, flexoelectric influence on distortion is revealed. [ABSTRACT FROM AUTHOR]

We report a novel dc field-driven propagative instability associated with the thermally induced layer undulations of the smectic C phase in a phenyl benzoate. While the undulations are two-dimensional, the drift is observed only along the wave vector qparallel to the cdirector; undulations with orthogonal qand cremain stable. The drift, which is nonhysteretic, takes place in a hopping way between equilibrium positions; it has a well-defined threshold in a given region, but the threshold varies rather widely for different regions. The average propagation velocity increases linearly from zero with the control parameter until 2 but tends to saturate thereafter. Significantly, the drift direction reverses on switching the field polarity. The mechanism responsible for the drift appears to involve a coupling between the transverse field gradients due to the conductivity anisotropy and the transverse component of the flexoelectric polarization. [ABSTRACT FROM AUTHOR]