Usage of liquid crystals in optical sensors of mechanical forces and motion

ABSTRACT Physical backgrounds for highly sensitive optical sensors of mechanical perturbations based on flow phenomena in liquid crystals are presented. It is shown that linear declinations of the optical axis of a nematic liquid crystal induced by a pressure gradient from the initial homeotropic orientation whic h are registered via polarized light can be considered as the basic mechanooptical effect for sensor applications. The ways of optimization of technical characteristics of liquid crystal sensors including usage of electric fields are discussed. The examples of sensors of acceleration, vibration and inclination based on the same principals are considered. It is shown that usage of liquid crystals provides an extremely high threshold sensitivity and electric control of the main technical parameters of optical sensors Keywords: liquid crystals, sensors of mechanical forces and motion 1. INTRODUCTION A lot of optical sensors of pressure, strain, flow, vibration etc. based on different physical phenomena are used in modern industry [1,2]. At present the elaboration of new types of opti cal sensors (fiber optic sensors [3], for example) is under a growing interest. Nematic liquid crystals (NLC) can be considered as very perspective materials for sensor applications as they show a very high sensitivity to the action of flows a nd electric fields. In this paper we will describe the physical backgrounds and particular technical decisions which make possible to propose liquid crystal sensors of mechanical forces, position and motion based on the physical phenomena in flows of liquid crystals [4]. Such devices have to be referred to optical sensors as they initially convert the mechanical stimulus in to the optical response. The principal difference of nematic liquid crystals from isotropic liquids is the existence of an long range orientational order described in terms of the unit vector - director n referred to the mean direction of long molecular axes and the scalar order parameter S, which characterizes the averaged declination of individual molecules from the mean direction [5]. Optical properties of NLC correspond to the unia xial media with a local optical axis parallel to n . The latter can be stabilized in relatively thin (1..300  m) layers by a proper surface treatment. A number of well elaborated methods of such treatment [6] make possible to get the given initial structure of NLC layer. In particular, it is possible to obtain monodomain samples with homeotropic (n – normal to the layer plane) or planar (n – in the layer plane) initial configuration. The non – homogeneous layer structures, like twisted or homeoplanar ones also can be realized via surface treatment. The global display applications of liquid crystals arose from the possibility of usage of low electric voltage to change the initial orientation and optical properties of LC layers. The sensor application described below is based on the fundamental property of liquid crystals - the intrinsic connection between the velocity gradients arising in shear flows and the local orientation of an optical axis [4,5,7]. In the case of nematic liquid crystals (NLC) this property provides a very high sensitivity of sensors due to zero values of a static shear elastic module.