Ms. Jennifer L. Sanders Profile

Jennifer L. Sanders

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Proceedings Article | 19 February 2014 Paper

Twisting and tweezing liquid crystals with lasers

Helen Gleeson, Mark Dickinson, Jennifer Sanders, Yiming Yang

Proceedings Volume 9004, 900406 (2014) https://doi.org/10.1117/12.2044247

KEYWORDS: Liquid crystals, Particles, Optical tweezers, Crystals, Liquids, Laser crystals, Refractive index, Polarization, Liquid crystal lasers, Nonlinear crystals

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Exciting new directions for liquid crystals (LCs) are emerging on the length scale of the wavelength of light. Two complementary micron-sized systems are formed by LC droplets and by dispersions of colloidal particles in LCs. The dimensions of each of these systems are ideal for laser tweezer manipulation, allowing a new range of photon-addressed LC systems to be envisaged. Trapping and moving micron-sized particles in LCs is a beautiful approach that can build novel colloidal photonic materials. However, it is also a unique way of studying fundamental LC properties, particularly anisotropic viscosity coefficients in the low Ericksen regime, which can be accessed by laser trapping. Rather few nematic materials have been studied using laser traps; we describe two different approaches to deduce the viscosity coefficients of nematic mixtures. Micron-sized LC droplets are emerging as intriguing photonic systems in their own right. Angular momentum can be transferred from laser traps to droplets, with specific polarization properties and droplet geometries resulting in a variety of novel photon-driven effects. Fast optical switches, rotating at speeds >1kHz, can be produced from nematic droplets in circularly polarized beams. Both droplet geometry and beam polarization influence the droplet rotation, allowing control of the phenomenon. Surprisingly, a chiral nematic droplet can sometimes undergo continuous rotation in a linearly polarized trap, a phenomenon caused by optically-induced changes in chirality. We describe this remarkable effect which demonstrates how the control of chirality through polarization can result in an optically driven transducer.

Proceedings Article | 28 August 2010 Paper

Laser tweezers for determining anisotropic viscosity coefficients of nematic liquid crystals

Jennifer Sanders, Mark Dickinson, Helen Gleeson

Proceedings Volume 7762, 776221 (2010) https://doi.org/10.1117/12.862853

KEYWORDS: Liquid crystals, Optical tweezers, Particles, Calibration, CMOS cameras, Dielectrics, Microfluidics, Microscopes, Particle systems, Refractive index

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The dynamic response characteristics of a liquid crystal (LC) device are dependent upon its viscosity coefficients. Local shear viscosity coefficients, or Miesowicz viscosity coefficients, η_i, are of particular importance for backflow effects and their optimisation allows for faster LC device response times. With such a wide range of LC materials available, information regarding their viscous properties is often incomplete. Micromanipulation with laser tweezers offers an alternative method for determining shear viscosity coefficients. Micron sized dielectric particles are dispersed in homeotropically and planarly aligned nematic LC, sandwiched between two coverslips. The microfluidic behaviour of the LC is investigated using a computer controlled laser tweezer system where particle tracking is performed using a high speed CMOS camera to record bead displacement for power spectral density analysis. We investigate the effective viscosity coefficients parallel and perpendicular to the director n, η^II_eff and η⊥_eff respectively. These are directly related to the Miesowicz viscosity coefficients for homeotropic alignment η1, and homogenous alignment η2 and η3. The results infer practically pertinent details about the viscoelastic properties of liquid crystals, and particles in LC systems.

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