Difference between revisions of "Alignment"

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This reorientation as depicted is called a Freedericksz transition.  It should not be confused with a phase transition because the LC remains in the same LC phase.  Simply, the orientation of the director is perturbed. The transition is smooth and is tunable by the amount of the electric field.
This reorientation as depicted is called a Freedericksz transition.  It should not be confused with a phase transition because the LC remains in the same LC phase.  Simply, the orientation of the director is perturbed. The transition is smooth and is tunable by the amount of the electric field.


=== Freedericksz Transition ===
Light that is polarized along the long axis will see anisotropic and birefringent material when the molecules are homogeneously aligned. When the molecules shift to be perpendicular to the substrate then light will see an isotropic surface.
The optical properties of the material will be different depending on if they are homogeneously aligned or homotropically aligned. This can be changed by applying an electric field. It can be tuned by changing the surface alignment layer (pretilt angle). If the molecules are completely flat then a perpendicular electric field can not induce a polarization along the long axis. However if there some angle the field will induce some polarization along the long axis which will have some component that is perpendicular to the substrate.
For an LCD application this polarization should be accomplished with a low voltage, it should occur very faster, and it should be stable over a large range of temperatures. This is not trivial because the forces working on the molecules at any one time may be unfavorable to one of these characteristics.
=== Threshold voltage ===
The threshold voltage (Vc) is the minimum voltage that will cause an LCD to switch. It depends on the square root of the on the elastic coefficient  (K33 and K22). The elastic coefficients refer to specific movements between molecules. The higher the elastic coefficients the harde it is to switch from one orientation to another.
Vc = pi sqrt K11 + ¼ (K33-2 K22)  E0 Delta E
Where:
K11 is the elastic coefficient for splaying
K22 is the elastic coefficient for twisting
K33 is the elastic coefficient for bending
Epsilon0 is the dielectric permittivity in free space
Delta Epsilon is the dielectric anisotropy of the system
A uniaxial birefringent system has a dielectric constant along the axis and dielectric constant perpendicular to the long axis. The dielectric anisotropy relates to the difference between these two. If there is a dipole moment along the long axis it will make it very easy for the molecule to couple to the electric field and align. If there is no dipole moment but there is very large polarizability along the long axis then the field will induce a dipole, it will couple and then realign. So the two contributors to the dielectric constant are the susceptibility (polarizability for a material) and the dipole moment. A material with a high dielectric anisotropy has either a large dipole moment along the long axis and/or a large susceptibility. With no dielectric anisotropy there is nothing that will make it more stable to change orientation.
It is good for the splay, twist and bend  elastic coefficient to be relatively low. If they are too low then the liquid crystal will lose order with temperature. So there must be some balance between ease of polarization and stability.
=== Anisotropic Nature of Polarizability ===
In this example a field is applied along the long axis and there is large polarization. Applying a field perpendicular results in a very small polarization. If the molecule is oriented at an angle to the electric field and there is a high degree of polarizability  along that axis and there is an oscillating electric field then the electrons will oscillate back and forth becoming a little antennae that will give off electromagnetic radiation. The polarization of the electromagnetic radiation has two components, one aligned along the axis of the applied field and a component along the perpendicular. If two polarizers are added and the molecule is located at an angle, then a dipole can be induced in the orthogonal direction which couples to the electric field and allows light to pass through. This induced polarization will be depend on the cosine of the angle.
You now have all the ingredients to make a simple display;
You understand how liquid cyrstals align
You can control molecular alignment  with surface alignment layers
Due to dielectric anisotropy we can change force the molecule to change from parallel to the surface to perpendicular to the surface
Using a cross polarizer and depending on the how the molecules align,  light can pass through or not.


[[category:liquid crystals]]
[[category:liquid crystals]]

Revision as of 16:18, 24 June 2009

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Alignment

When viewed under a microscope with crossed polarizers, one of the most striking features of liquid crystals is the appearance of texture. Normally no light will pass through cross polarized filters because first one and the other polarized waves will be cut out.

An isotropic material placed between crossed polarizers will allow no light to pass through the polarizer.

If a birefringent material is viewed between cross polarizers and the optical axis is aligned with the polarizer it will not pass light. But if the optic axis not perfectly aligned along one of the axes of the polarized light will be transmitted.

Such is the case for an LC if the director is not aligned along one of the axes of the polarizer. When the director is aligned along such an axis the image will appear dark.

Textures occur which appear as sudden change in the image, going from light to dark. In such a case there is a discontinuity in the direction of the director. Such a defect in the order is called a disclination. When one domain of aligned directors transitions to another domain with different alignment there is a sudden change in light and dark patterns. This texture from a corodene molecule is indicative of a certain kind of liquid crystalline phase.

Liquid crystals can be used to make displays because of certain properties. Liquid crystals are birefringent so depending on the orientation of optical axis of the system cross polarizer can pass or block light. Displays must have an “on” and “off” state. Liquid crystals have cohesive interactions and they have electrical properties that allow them to respond to an applied electric field. A screen has a front and back surface with liquid crystals between.

This diagram shows a smectic liquid crystal.

In principle, the director can be oriented at an angle with respect to a substrate.

The two limiting cases are called homeotropic alignment and homogeneous alignment.

Homeotropic orientation of the director relative to the surface of the plane of the substrate has the director more or less normal to the surface.

Homogenous orientation of the director relative to the surface of the plane of the substrate has the director more or less parallel to the surface. This still does not describe how things align along the third axis parallel to the surface.

The alignment of the liquid crystal is very sensitive to the polarization and polarizability of the surface in which it is in contact.

Surface Treatment and Alignment

It is therefore possible to judiciously modify the surface in such a manner as to control the orientation; this is called a surface alignment layer.

This is in part art, in part science, and in part engineering.

It has been found that if a very thin layer of a polyimide is placed onto the substrate surface and then rubbed in one direction only (not back and forth), that it is possible to favor a homogenous alignment of nematic phases. Depending on the details of the molecular surface there will be some angle between the director and the surface referred to as the pretilt angle. In the diagram the director is shown in its cone as a layered structure. The dipole moment is typically perpendicular to the director. As the director goes around the cone it is possible to change the dipole moment by 180 degrees. If there is a dipolar interaction at the surface you can unwind the molecules so their directors aligns, the dipole moment aligns, resulting a net polarization of the entire phase.

If the surface is treated with a long chain lecithin or long chain trichlorosilanes it is possible to favor a homeotropic alignment.

Recall that for materials that contain electric dipoles, such as water molecules, the dipoles themselves stretch or reorient in an applied field.

If a dipole is at an angle to an electric field it will experience a torque that will rotate it into alignment.

An electric field can also induce a polarization in a molecule (a polarizable molecule) along the long axis which will the cause it to align to the field even though it did not have a dipole to begin with.

If a liquid crystal is placed between two substrates that have been treated with polyimide so that the molecules are aligned homogeneously and in one direction.

At the surface the molecules are influenced by interaction with the substrate.

If then an electric field is applied orthogonal to this direction the liquid crystal will tend to rotate to align the director with the electric field assuming that the director and the dipole moment (more precisely the larger component of the dielectric anisotropy) are essentially colinear. The reorientation starts in the middle where there is little surface interaction.

This reorientation as depicted is called a Freedericksz transition. It should not be confused with a phase transition because the LC remains in the same LC phase. Simply, the orientation of the director is perturbed. The transition is smooth and is tunable by the amount of the electric field.

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