Liquid Crystals: Structure, Properties and Applications Jeremiah C. Millare

Introduction to Liquid Crystals Discovery: began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points.

Introduction to Liquid Crystals Sturcture: Crystalline materials demonstrate long range periodic order in three dimensions. By definition, an isotropic liquid has no orientational order. Substances that aren't as ordered as a solid, yet have some degree of alignment are properly called liquid crystals.

Introduction to Liquid Crystals Characterizing Liquid Crystals: Positional order refers to the extent to which an average molecule or group of molecules shows translational symmetry (as crystalline material shows). Orientational order represents a measure of the tendency of the molecules to align along the director on a long-range basis. Bond Orientational Order describes a line joining the centers of nearest-neighbor molecules without requiring a regular spacing along that line.

Introduction to Liquid Crystals Transformation: Most liquid crystal compounds exhibit polymorphism, or a condition where more than one phase is observed in the liquid crystalline state. The term mesophase is used to describe the "subphases" of liquid crystal materials. Mesophases are formed by changing the amount of order in the sample, either by imposing order in only one or two dimensions, or by allowing the molecules to have a degree of translational motion.

Liquid Crystal Phases Nematic Phases Molecules have no positional order but tend to point in the same direction (along the director).

Liquid Crystal Phases Cholesteric Phases The cholesteric (or chiral nematic) liquid crystal phase is typically composed of nematic mesogenic molecules containing a chiral center which produces intermolecular forces that favor alignment between molecules at a slight angle to one another.

Liquid Crystal Phases Smectic Phases The word "smectic" is derived from the Greek word for soap. Molecules in this phase show a degree of translational order not present in the nematic.

Liquid Crystal Phases Smectic C* The director makes a tilt angle with respect to the smectic layer. The difference is that this angle rotates from layer to layer forming a helix.

Liquid Crystal Phases Columnar Phases Columnar liquid crystals are different from the previous types because they are shaped like disks instead of long rods. This mesophase is characterized by stacked columns of molecules. The arrangement of the molecules within the columns and the arrangement of the columns themselves leads to new mesophases.

External Influences on Liquid Crystals Surface Preparations For example, when a thin polymer coating (usually a polyimide) is spread on a glass substrate and rubbed in a single direction with a cloth, it is observed that liquid crystal molecules in contact with that surface align with the rubbing direction. Freedericksz Transition Deformation occurs where the director changes its orientation from one molecule to the next because of an electric or magnetic field.

Light and Polarization Light can be thought of as a wave, that vibrates back and forth as it moves. Individual light waves each have their own wavelength, as well as direction of vibration. Ordinary visible light is a mixture of many kinds of waves. The light that enters your eye is composed of waves of a multitude of different wavelengths.

Example of Light Polarization We perceive light waves that vibrate horizontally as 'glare'. As light hits a particular portion of the truck's surface and returns to our eye, the waves get polarized. This means that a higher than normal number of them are vibrating horizontally. When this light enters our eye, the large number of horizontally vibrating waves seem to overpower the other waves, and everything near that part of the truck is hidden by the 'glare' caused by the polarized waves. Whenever we change a beam of light waves so that many of the waves are vibrating in the same direction, we have 'polarized' the light beam.

How can a beam of light be polarized? In order to polarize light, we just need to pass it through a barrier that contains many very narrow slits. What comes out the other side is called polarized light.

Birefringence in Liquid Crystals When light enters a birefringent material, such as a nematic liquid crystal sample, the process is modeled in terms of the light being broken up into the fast (called the ordinary ray) and slow (called the extraordinary ray) components. Because the two components travel at different velocities, the waves get out of phase. When the rays are recombined as they exit the birefringent material, the polarization state has changed because of this phase difference.

See Virtual Textbook

Applications of Liquid Crystals Liquid Crystal Displays Liquid Crystal Thermometers Optical Imaging

LCDs You probably use items containing an LCD (liquid crystal display) every day. They are all around us -- in laptop computers, digital clocks and watches, microwave ovens, CD players and many other electronic devices. LCDs are common because they offer some real advantages over other display technologies. laptop computer display may act funny in cold weather or during a hot day at the beach.

How LCDs Work? As light strikes the first filter, it is polarized. As the light passes through the liquid crystal layers, the molecules also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If we apply an electric charge to liquid crystal molecules, they untwist Consequently, no light can pass through that area of the LCD, which makes that area darker than the surrounding areas.

Next Attraction Colored LCDs LEDs OLEDs