Dynamics of laser-induced electroconvection pulses

TL;DR

This paper investigates laser-induced electroconvection pulses in nematic MBBA, revealing their formation, stability, and dynamics, and proposes a control-parameter ramp model with heat advection effects to explain observed behaviors.

Contribution

It introduces a novel laser-induced pulse formation method in nematic liquid crystals and develops a theoretical model incorporating heat advection to explain pulse dynamics.

Findings

Pulse size saturates or decreases at higher control parameters.

Pulses exhibit counterpropagating rolls at tenths of Hz.

The model accounts for heat advection and negative feedback in pulse attenuation.

Abstract

We first report that, for planar nematic MBBA, the electroconvection threshold voltage has a nonmonotonic temperature dependence, with a well-defined minimum, and a slope of about 0.12 V/degree near room temperature. Motivated by this observation, we have designed an experiment in which a weak continuous-wave absorbed laser beam with a diameter comparable to the pattern wavelength generates a locally supercritical region, or pulse, in dye-doped MBBA. Working 10-20% below the laser-free threshold voltage, we observe a steady-state pulse shaped as an ellipse with the semimajor axis oriented parallel to the nematic director, with a typical size of several wavelengths. The pulse is robust, persisting even when spatially extended rolls develop in the surrounding region, and displays rolls that counterpropagate along the director at frequencies of tenths of Hz, with the rolls on the left (right) side of the ellipse moving to the right (left). Systematic measurements of the sample-voltage dependence of the pulse amplitude, spatial extent, and frequency show a saturation or decrease when the control parameter (evaluated at the center of the pulse) approaches ~ 0.3. We propose that the model for these pulses should be based on the theory of control-parameter ramps, supplemented with new terms to account for the advection of heat away from the pulse when the surrounding state becomes linearly unstable. The advection creates a negative feedback between the pulse size and the efficiency of heat transport, which we argue is responsible for the attenuation of the pulse at larger control-parameter values.