For Reviewers

Liquid crystals (LCs) are a stable phase of matter characterized by anisotropic properties without the existence of a three-dimensional crystal lattice, generally lying between the solid and isotropic “liquid” phase[1,2]. It differs from liquid because its molecules possess anisotropic orientational order, which means the properties of the LC is dependent on the direction which they are measured. Owing to its unique anisotropic optical and electro-optic properties, the LC mesophase materials have attracted attention not only from optical applications. including optical switches [3,4], electrooptic filters [5], digital data storage devices [6], holography[7-9], lenses [10,11], and computer displays [12], but also from sensor applications, including temperature sensors [13] and gas sensors [14,15]. The nematic to isotropic phase transition of LC is of importance for design of LC-based electro-optic devices. Various studies on the phase transition behavior of LC have been performed; however, much has not been done to investigate the impact of the electric field on the phase transition of LC. Here, we investigate the propagation velocity of the phase boundary between the nematic and isotropic phases, as a function of the voltage applied to the LC using a polarized optical microscope (POM). We found that the propagation velocity of the phase boundary between the nematic and isotropic phases, which can be translated to thermal diffusivity, can be modulated by the electric field applied to the LC and exhibits a peak value for a specific electric field, attributed to the supercooling of the isotropic phase of the LC. Measuring the propagation velocity of the phase boundary for different electric fields offers a simple optical method to measure the electro-thermal properties and thermal anisotropy of the homogeneously aligned LC.

The LC cell was placed on the polarized optical microscope. The direction of the optical axis of the LC cell was 45° with respect to the optical axes of the polarizer and analyzer in the POM. A heater is attached to one end of the LC cell, which increases the temperature up to 55 °C in 10s. The velocity of the moving phase boundary between the nematic and isotropic phases, which can be translated into the thermal diffusivity of the LC, was measured when different voltages were applied to the LC cell. The bipolar pulse with a frequency of 100 Hz and zero DC bias was applied to the LC and exhibited a peak-to-peak voltage range of 0-8 V with an increment of 0.5 V. The changes in the structural texture of the LC and phase transition velocities for different voltages and heat were monitored using a POM and recorded by a CCD camera.

Fig. 2. 
Thermal diffusivity measurement setup, with the LC cell placed on the stage of the polarized optical microscope.

Figure. 3a depicts the POM image of the homogeneously aligned LC at the nematic phase under thermal equilibrium with the polarizer and analyzer set at 90°C to each other. Figure. 3b illustrates the crossed POM image of the phase boundary between the nematic and isotropic phases of the LC. The phase transition temperature of 5CB is 35.2 °C. The phase transition boundary is clearly observed and the moving direction of the phase boundary is from the side where the heater was placed to the opposite side. When the direction of the optical axis of the LC is aligned to 45°with respect to the optical axes of the crossed polarizers and the LC is at the isotropic phase, the light transmission through the crossed polarizer is zero, resulting in a dark color on the CCD. However, in our experiment, the isotropic phase still shows light transmission through the crossed polarizer. This effect is mainly due to the birefringence property of the PET substrates.

Fig. 3. 
POM images of 5CB during the nematic-isotropic phase transition (a) Nematic 5CB LC under thermal equilibrium, (b) 5CB LC under phase transition.

Figure 4 depicts the propagation velocity of the phase boundary when the direction of heat flow is in parallel (red circle) and perpendicular (black square) to the optical axis of the LC for different voltages. The thermal conductivity of the LC for a voltage of 1.5 V_pp exhibited a peak value. We attribute the propagation velocity peak for the voltage of 1.5 Vpp to the supercooling of the isotropic phase of the LC [16]. Additionally, the thermal conductivity of the LC along the optical axis was higher than that perpendicular to the optical axis, indicating thermal anisotropy. The speed of the phase boundary propagation along the optical axis of the LC decreased as the voltage applied to the LC increased except at 1.5 V. When subjected to an alternating electric field, the LC was polarized and gained dipole moments, resulting in the rotation of the optical axis of the LC. The torque applied to the LC aligned the optical axis of the LC along the electric field. When a high electric field is applied to the LC whose optical axis is aligned parallel to the direction of the heat flow under zero electric field, the optical axis of the LC becomes perpendicular to the direction of heat flow. This makes the heat feel the perpendicular thermal conductivity, resulting in a decrease in the propagation velocity as the electric field increases. When the high electric field is applied to the LC whose optical axis is aligned perpendicular to the direction of heat flow under zero electric field, the optical axis of the LC remains perpendicular to the direction of the heat flow, resulting in an electric field independent of thermal conductivity. However, there is a slight decrease in the propagation velocity as the electric field increases. We attribute this slight decrease to the increased nematic to isotropic phase transition temperature. The phase transition temperature is proportional to the order parameter, which in turn increases as the voltage increases. The dashed lines represent the linear fit of the propagation velocity calculated without the data point for 1.5 V_pp. The slopes of the linear fit for the parallel and perpendicular alignment are –0.007 and –0.003, respectively. For the parallel alignment, both the increased order parameter and thermal anisotropy play a role in decreasing the propagation velocity, whereas for the perpendicular alignment, the increased order parameter affects the propagation velocity for different electric fields. Therefore, the slope of the parallel alignment is steeper than that of the perpendicular alignment.

Fig. 4. 
Propagation velocity of the phase boundary between the nematic and isotropic phases as a function of voltage.

In this study, we demonstrated the propagation velocity of the phase boundary between the nematic and isotropic phases of 5CB LC for different electric fields. We found that the propagation velocity of the phase boundary exhibits a peak for a specific electric field, attributed to the supercooling of the isotropic phase of the LC. Our simple experimental setup is used to measure not only the supercooling state of the isotropic phase but also the thermal anisotropy of the LC. Furthermore, we demonstrated the electric field dependent thermal properties of the LC using an optical method.