Contributed by Anderson (張家福)
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As a result of the rodlike or disklike nature, the distribution of the orientation of LC molecules plays an essential role in the optical properties of liquid crystal in display Applications. Uniform or well prescribed orientation of the LC molecules is required In most LCDs. The role of the surface is to ensure a single domain. Without special treatments of the surfaces, the LC will generally have many domains and many domains and many disinclinations, or discontinuities.

Where the LC director is uniformly parallel to the surface, one must provide a preferred orientation to the alignment by physically or chemically treating the surface. Rubbing the surface has been a simple and effective way of achieving a preferred orientation. Typical rubbing materials include linen cloth and lens paper. Although the Technique of rubbing to achieve parallel homogeneous alignment is still very much an Art,it is generally believed that physically rubbing the surface produces a uniform and unidirectional tilt of the dangling bonds or side chains on the surface. The unidirectional tilt of these surface molecules may lead to the parallel homogeneous alignment of the LC molecules.Note that the LC direction usually tilts up in the direction of rubbing

In the nematic phase, the molecules are rodlike with their long axes aligned approximately parallel to one another. Thus at any point in the medium, we can define A vector n to represent the preferred orientation in the immediate neighborhood of the Point. This vector is known as the direcor. In a homoge neous nematic LC, the director is a constant throughout the medium. In an inhomogeneous nematic liquid crystal, the Director n can change from point to point and is, in general, a function of space. If we define a unit vector to represent the long axis of each molecule, then the director n is the statistical average of the unit vectors over a small volume element around the point. The order parameter S of a LC is defined as.

Boundary Conditions:

Consequences :
(1)tilt/voltage-dependent twist
(2) can remove and from system

=The pretilt angle also plays an important role in twisted nematic liquid crystal Cells,we consider a liquid crystal cell with a pair of orthogonal rubbing direct ions.The rubbing direction are important to prepare a twisted nematic liquid crystal cellWith a total twist angle of 90.Having no polarity and no chirality,the rodlike molecules can twist in either direction in the absenxe of a small pretilt angles.Both Right-handed and left-handed twist of 90 are possible to match the boundary conditions imposed by the rubbing.When a small pretilt angle exists,only the right-handed twist is possible in the cell described in Fig.In the cell shown in Fig,the Upper parts of LC molecular axes tilt toward the right side

Free Energy

Local Stability of Homeotropic Expand

A smectic LC is closest in structure to solid crystals.It is interesting to note that inSubstances that from both a nematic phase and a smectic phase,thesquence of phase Changes on risinrg temperture is as follow:

Like most liquids and solids,LCs exhibit curvature elasticity.The elastic constants of LC determine the restoring torques that arise when the system is very weak compared With those of solids.In LCDs,an electric field is often applied yo cause a reorientation of the molecules.It is the balance between the electric torque and the elastic restoring torque that determines the LC ststic combination pattern.Any static deformation of LCs can be divided into a combination of three basic deformations.These are splay, Twist,and bend,as illustrated

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Related reading on laynetworks.com: Measurement Method for Cell Gap and Twist Angle of Reflective LCD

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