Light-induced rotation of dye-doped liquid crystal droplets

TL;DR

This study combines theoretical and experimental approaches to examine the rotational behavior of dye-doped liquid crystal droplets under laser illumination, finding no enhancement in rotation due to dye absorption and confirming the existence of a photoinduced stress tensor.

Contribution

It provides the first direct experimental evidence of a photoinduced stress tensor in dye-doped liquid crystals and clarifies the role of dye absorption in optical torque enhancement.

Findings

No enhancement in droplet rotation with dye absorption.

Confirmation of a photoinduced stress tensor.

Predicted dynamical effects in non-rigidly locked droplets.

Abstract

We investigate both theoretically and experimentally the rotational dynamics of micrometric droplets of dye-doped and pure liquid crystal induced by circularly and elliptically polarized laser light. The droplets are dispersed in water and trapped in the focus of the laser beam. Since the optical torque acting on the molecular director is known to be strongly enhanced in light-absorbing dye-doped materials, the question arises whether a similar enhancement takes place also for the overall optical torque acting on the whole droplets. We searched for such enhancement by measuring and comparing the rotation speed of dye-doped droplets induced by a laser beam having a wavelength either inside or outside the dye absorption band, and also comparing it with the rotation of pure liquid crystal droplets. No enhancement was found, confirming that photoinduced dye effects are only associated with an internal exchange of angular momentum between orientational and translational degrees of freedom of matter. Our result provides also the first direct experimental proof of the existence of a photoinduced stress tensor in the illuminated dye-doped liquid crystal. Finally, peculiar photoinduced dynamical effects are predicted to occur in droplets in which the molecular director is not rigidly locked to the flow, but so far they could not be observed.