Application to shear force sensors by homeotropic liquid crystal (LC) orientation (non-reviewed)

Abstract

Application to sensors using liquid crystals is widely investigated by the orientation order of the liquid crystal molecules. In this paper, the capacitance by tracking for LC molecules uses the shear force sensor. The shear force sensor is applied the different direction of 0deg, 45deg and 90deg. At each significant angle, it is important that the director of liquid crystals is defined by the direction of the shear force. Homeotropic liquid crystal orientation is very sensitive to change by external shear forces. An interesting point is to use the electrical property using the capacitive sensing because of excellent tracking positions by involving digitized capacitance instead of the optical property using the visual sensing. In the paper, the shear force sensor is designed by the tracking theory of liquid crystal alignment. In addition, this paper also modifies some parameters to the capacitive transduction such as temperature, measurement frequency and measurement voltage. Recently, a liquid crystal sensor is one of the interesting areas because of low cost, low power consumption and portability. The mechanical properties between a liquid and a crystal are totally different. The orientation order of the liquid is random, but the orientation order of the crystal is one direction. Liquid crystal is the interstate between these two states. The liquid crystal is an anisotropic material which has different permittivity by the average molecule director. From this property of LC, the chemical and biological sensors are exploited because these sensors can excellently detect the chemical and biological agents by the orientation order of the liquid crystal molecules. For example, a homeotropic alignment of LC can change to homogeneous alignment by Abbott in detecting part-per- billion concentration of dimethylmethylphosphonate (DMMP). The objective of this paper is to illustrate how the shear force sensor works using the homeotropic alignment of nematic liquid crystals. As mentioned, the dielectric constants of LC are changed by the average molecule director depended on thetas and phi. The first step is to find the capacitance depended on the director n which is defined the average direction of molecule alignment. The representation of the director n is consisted by two different angles such as thetas and phi by static continuum theory of liquid crystals. The capacitance occurs between the orientation of the director n for LCs and the orientation with influence by the shear force. The electrode structure of capacitive tracking is designed by the inter- digital capacitance (IDC) for analysis in the fringing field. The shear force sensor is the investigated three different directions of phi such as 0deg, 45deg and 90deg when the director n of LCs is the homeotropic alignment that the angle thetas is 0deg. The capacitance is increased from 78pF to 83pF by the fringing field when phi is 90deg. On the other hand, the capacitance is decreased from 78pF to 71pF when phi is 0degand also decreased from 77pF to 74pF when phi is 45deg. For these experience results, we use the E4980A LCR meter from Agilent Technologies and the computer-based measurement system of Lab View 8.2. The shear force sensor with nematic liquid crystals has been investigated. The difference between the simulation data and the experimental data is that the homeotropic alignment of LCs is not a totally ordered system. For the future work, we modify the partially disordered homeotropic alignment of nematic liquid crystals. In addition, we use the pillow to prevent the flowing the LCs on the patterned surface.