Counterpropagation and amplitude modulation of rolls in a broken-symmetry electroconvection experiment

We report a liquid-crystal electroconvection pattern-formation experiment along a narrow strip in which the translational symmetry is broken with a laser-induced thermal gradient. In contrast with other broken-symmetry patterns that display a uniform drift, we observe rolls that counterpropagate into a sink point and a strong temporally periodic amplitude modulation near this point. The time dependence of the amplitude at a fixed position is periodic but not sinusoidal. On the basis of experimental spacetime contours and wavenumber profiles, along with a measurement of the change in the roll-drift frequency with applied voltage at a fixed control parameter, we propose a simple qualitative model based on (1) the enhancement of thermal transport due to advection, and (2) a hyperbolic-tangent solution to the Ginzburg-Landau equation that is relevant near the Eckhaus boundary.