Images of liquid crystal phases as seen through a polarizing microscope.

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	Smectic A* and Banana-shaped liquid crystals Prof. L.-C. Chien
	Smectic A, B and C phases and Nematic liquid crystals Dr. Mary E. Neubert
	Nematic and Smectic liquid crystals and submicron films Prof. Oleg D. Lavrentovich
	Banana-shaped liquid crystal molecules Dr. Antal Jakli

Liquid Crystal Video Clips
Courtesy of Oleg D. Lavrentovich

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Nematic liquid crystal confined between two glass plates The centers with two dark brushes emanating from them represent the ends of the defect lines - so-called disclinations. Some of the disclinations have both ends at the same plate (at different locations); they are seen as sharp lines (left hand part of the picture). Some other disclinations (right hand part of the picture) are vertical, and their ends are located one above the other, connect the two opposite plates. A gentle shear of one of the plates reveals these "hidden" defects as they elongate in the direction of shear.

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Temperature-induced nucleation of cone-like focal conic domains in a smectic A liquid crystal Nucleation of the domains is favored by the surface orientation of the molecules that prefer to be parallel to the liquid crystal surface. Such an orientation would be possible if the surface area is occupied by the circular base of a focal conic domain. However, formation of such a domain, although it reduces the surface energy, is also accompanied by an increase of the "bulk" energy of director distortions. The energy balance dictates that the focal conic domain has to be larger than some critical size to become a stable feature of the structure. The video shows how these "sufficiently large" domains appear from "nothing."

What is a liquid crystal?

Liquid Crystals are intermediate phases between crystalline solids and isotropic liquids. They are orientationally ordered fluids with anisotropic properties. A variety of physical phenomena makes them one of the most interesting subjects of modern fundamental science. Their unique properties of optical anisotropy and sensitivity to external electric fields allow numerous practical applications.

Example of a compound that shows no liquid crystal phase:

	Add Heat		More  Heat
Ice Cube		Water		Steam
Solid crystalline water 3 degrees of order		Liquid water 0 degrees of order		Gaseous water 0 degrees of order

Example of a compound that shows liquid crystal phases:

	Add Heat		More  Heat
Crystals of a solid organic compound		Nematic liquid crystal phase looks like milk		Isotropic liquid
3 degrees of order		1 degree of order		0 degrees of order
	Add Heat		More  Heat
Crystals of a solid organic compound 3 degrees of order		Smectic liquid crystal gooey material 2 degrees of order		Isotropic liquid 0 degrees of order

Liquid crystalline phases most often occur in compounds that have a shape that favors parallel packing:

Rods
Boards
Disks/cones
	Stacks of these form columns

How do we know a compound shows a liquid crystal phase?

1. Polarized Microscope
We look at it under a polarized light in a microscope fitted with a heating stage. Crystals show optical birefringence under polarized light.

Polarized Light	Polarized Light
(a)	(b)

Since the path length of (a) and (b) are different the wavelength of light passing through the crystal will be different and therefore the colors will vary. The thickness of the sample usually has the greatest effect on the colors that are seen. A liquid crystal phase acts like a single crystal when polarized light is passed through it.

When an organic compound is placed on a microscope slide with a cover slip and the slide is heated and viewed using the high magnification of the microscope, textures characteristic of each type of liquid crystal can occur. Cooling the liquid can also yield these textures when liquid crystal phases are present. If no liquid crystals are present, characteristic textures for crystals occur when the liquid crystallizes. With a great deal of practice and experience, the viewer can develop the skill to determine the type of liquid crystal phases present.

2. X-ray crystallography
Just as x-rays determine the structure of crystalline compounds they can be used to study liquid crystalline phases and establish the type of phase.

3. Differential Scanning Calorimetry (DSC)
Heat is needed to melt a crystalline solid to a liquid crystalline phase. The heat is measured using a DSC instrument. DSC cannot identify the type of phase, only that a phase transition has occurred.