Liquid crystals are long chain organic molecules that exhibit the properties of a liquid yet have the long range ordering of a solid. The liquid crystalline state constitutes an intermediate phase or mesophase between solids and liquids. There are several types of liquid crystals and the ones used for displays are called thermotropic liquid crystals. As the name implies, thermotropic liquid crystals are affected by heat. If heat is added to the crystalline phase of the liquid crystal it will transition into the smectic phase (melting point), as more heat is added the liquid crystal will go through the nematic phase (used for display applications) and finally into the isotropic phase (clearing point) see fig 1.

Twisted nematic (TN), supertwisted nematic (STN) and active matrix (AMLCD) displays use the nematic phase for their operation. This phase is selected for its physical properties and the widest temperature range. It should be noted that the fluids used for display purposes are mixtures of several different liquid crystals. By mixing several liquid crystals together, a eutectic mixture will result with the melting point depressed well below room temperature.

Liquid crystals are uniaxial. For example, if one measures the physical parameters of a water molecule, it can be shown that the data for that parameter would be the same no matter what direction the measurements were taken. Liquid crystals differ in that the value of the physical parameter is dependent on the direction it was measured relative to the molecule. We can measure the index of refraction of the molecule perpendicular to the long axis, and then parallel to the long axis, and the two values would be different, see fig 2. This is the index of refraction anisotropy of the liquid crystal or the birefringence. This would also hold true for the magnetic susceptibility, electrical conductivity, thermal conductivity, dielectric permitivity, and elastic constant.

Because liquid crystal molecules want to lie parallel to one another (orientational order) and they are uniaxial (anisotropic), we can exploit these properties to make useful devices.

As a practical matter, a given liquid crystal fluid is chosen using only a few critical parameters. The two most important are:

1) Working temperature range. - Most LCD fluids have a working temperature range of about -40c to +85c in a direct drive mode. As +85c is about +185 Fahrenheit, this temperature range covers the majority of both commercial and industrial applications. In applications where the display will be outdoor or in direct sunlight, a higher temperature range is recommended. Our #16 and #18 fluids are speciall formulated for extremely high temperature applications, and can withstand temperatures over +120c. On the low end, our #1 fluid can operate at temperatures as low as -55c.

2) Voltage Threshold - The supply voltage of your circuit determines the amount of drive voltage available to the display. In general, all of our fluids will work fine in 5V circuits in a direct drive mode. The problem comes in when you do not have 5V available, or the circuit is multiplexed.

For 3V applications, we have developed our #4 or our newer #19 fluids. These fluids have a somewhat lower threshold and will work fine at 3V either direct drive, 2:1, or 3:1 mux.

For highly multiplexed designs, the fluid selection becomes critical. The subject is somewhat complicated, but in general the higher the multiplex rate, the higher the voltage will need to be to drive a given fluid into saturation. The reason is that when you multiplex a display, each individual segment is time-division multiplexed, that is, you are only driving a segment for a portion of the total cycle. In a 3:1 mux, each pixel is only driven for one third of the time. The pixels are never really turned on all the way, and then they are left to turn off while other pixels are being driven. This results in greatly reduced contrast and viewing angle. Call our Engineering Department at 1-800-786-8710 for additional information.