Real-time sensing of the thermal diffusivity for dynamic control of anisotropic heat conduction of liquid crystals

Molecular orientational order can be used to characterize the anisotropic behavior in mechanical, optical, and thermophysical properties. The creation of appropriate molecular orientation has the potential for producing a novel material or thermal switching device, which can control anisotropic heat conduction. Liquid crystals, which are widely used in display elements, have anisotropy not only in their optical, but also in their thermophysical properties, under given molecular orientational alignment conditions; this material can be a variable device with anisotropic heat conduction by controlling the molecular alignment. In the present study, a real-time sensing system for thermal diffusivity using the forced Rayleigh scattering (FRS) method was developed to investigate the transient behavior in the thermal anisotropy of nematic liquid crystals. This technique can be used to measure the in-plane thermal diffusivity perpendicular to the transient thermal grating created by interfering pulsed laser beams, and the thermal anisotropy of the sample can be determined using this non-contact method. The present FRS system can provide continuous measurements of the thermal diffusivity with subsecond time resolution, allowing evaluation of the dynamic behavior of anisotropy in the thermal diffusivity even during a transient process. In this article, the anisotropy of the in-plane thermal diffusivity of 4-4'-pentyl-4- biphenylcarbonitrile (5CB) with molecular alignment induced by either a rubbed substrate or an electric field has been measured. Also, the time evolution of the anisotropic thermal diffusivity in real-time under a dynamically controlled external electric field has been measured. The experimental results demonstrate the capability of dynamic anisotropic control of heat conduction by molecular alignment variations.