Overview
Principle
Light Valve
The principle could be explained in a simple way. Water valve controls amount of water flow. Each tiny liquid crystals droplets, measured at few microns, act as “light valve” controlling intensity of light passing through. A piece of PLDC film contains numerous tiny liquid crystal droplets. When all “light valves” work together at the same time, large amount of light intensity is under controlled.
Orientation & Configuration
It is also known as smart, intelligent, magic, privacy, electrochromic, light regulate, switchable or also LC liquid crystal film. PDLC controls light intensity passing through. When there is no electric current, liquid crystal droplets are randomly oriented. They do not allow light passing through, instead, they scatter light rays. The film is, therefore, opaque but light rays are dispersed. When electric current flows, liquid crystal droplets aligning parallel allowing light passing through and thus, it becomes transparent. With just simply a flip of switch, it is instantaneously changing between clear and opaque states.
Orientation
PDLCs consist of liquid crystal droplets that are dispersed in a solid polymer matrix. The resulting material is a sort of "swiss cheese" polymer with liquid crystal droplets filling in the holes. These tiny droplets are responsible for the unique behavior of the material. By changing the orientation of the liquid crystal molecules with an electric field, it is possible to vary the intensity of transmitted light.
Configuration
The configuration of the liquid crystal droplets in a polymer matrix is a focus. There are mainly three droplets configuration depending on factors such as droplet size and shape, surface anchoring and applied fields.
The radial configuration occurs when the liquid crystal molecules are anchored with their long axes perpendicular to the droplet walls.
The axial configuration of the liquid crystal droplets also occurs when the molecules are oriented perpendicular to the droplet wall, but only when there is weak surface anchoring. This configuration creates a line defect that runs around the equator of the spherical droplet. When an electric field is applied to a radial droplet, the molecules adopt the axial configuration. The radial configuration is returned when the field is removed.
The bipolar configuration is obtained by tangential anchoring of the liquid crystal molecules. This creates two point defects at the poles of the droplet and is shown in the diagram below.
Transparent & Opaque
Transmission of light through a PDLC film depends primarily on scattering which in turn depends on the difference in refractive index between liquid crystal micro-droplets and their environment. In the case of high droplet density, the environment consists mainly of other droplets, which makes the relative orientation of their directors an important factor. The droplets are anisotropic with the index of refraction parallel to the director different from that perpendicular to it.
Transparent
When power is on, each liquid crystal drop is under external electric field rank. The director of the individual droplets aligns with the electric field. There is now little difference in refractive index for neighboring droplets. It distributes in order which permits full intensity of light permeation. PDLC cell appears transparent.
Opaque
When power is off, external electric field disappears. The random array of tiny crystal droplet orientation provides significant differences in indices and the rank order is disturbed and hence, incident light ray is strongly scattered. PDLC cell becomes opaque while light disperses.
