Option C
liquid crystals are a phase of state of matter that lies between the solid and liquid state. In a liquid crystal the molecules tend to retrain their orientation as in a solid but they can also move to different positions as in liquid. The physical properties of liquid crystals (such as electrical conductivity, optical activity and elasticity) depend upon the orientation of the molecules relative to some fixed axin in the materal. Examples of substances which can behave as liquid crystals under certain conditions include DNA, soap solution, graphite, and cellulose together with some more specialized substances such as biphenyl nitriles.
An example of a biphenyl nitrile is 4-pentyl-4'-cyanobiphenyl (known as 5CB)
Lyotropic and Thermotropic Liquid Crystals-
Liquid crystals only show liquid properties under certain conditions. They are very sensitive to temperature and concentration. Essentialy liquid crystals can be divided into two main types.
Lyotropic liquid crystals are solutions which show the liquid crystal phase at certain concentrations. An example of lyotropic liquid crystals is soap solution. At low dilution the polar soap molecules behave randomly but at higher concentrations they group together into larger units called micelles which in the liquid crystal phase are ordered in their orientation.
Thermotropic liquid crystals are pure substances that show liquid crystals behaviorr over a range of temperature between solid and liquid states. Examples of a thermotropic liquid crystal are biphenyl nitriles used in liquid cristal displays (LCDs).
The use of thermotropic liquid crystals in a calculator screen.
Within the thermotropic liquid crystal phase the rod-shaped molecules which are typically about 2.5x10-meters in lencht exist in groups or domains. The molecules can flow and are randomly distributed as in liquid but within each domain they all point in the same direction. This is known as the nematic phase. As the temperature increases the orientation becomes increasingle more disrupted until eventually the directional order is lost and the normal liquid phase is formed.
Principles of LCD Devices-
Since liquid crystal molecules are polar their orientaion can be controlled by an applied electric field. The orientation of the molecules affects the ability of the liquid crystal molecules to transmit light. In liquid crystal displays used in digital watches calculators and laptops a small voltage is applied across a thin film of the material.
The flat screens used for computer monistors or television use liquid crystals.
This controls the areas of the display that are light and dark and hence gives the characteristic readings of the pictures or letters. The great advantage of LCDs over other types of electronic display is that they use extremely small electric currents. The disavantage is that they only work witihn a certain temperature range whhich explains why a digital watch or laptop screen may give a strange display in very hot or cold temperatures. Hence for use in an LCD a liquid crystal should be chemically stable compound, contain polar molecules, remain stable in the liquid crystal phase over a suitable range of temperature, and be able to orientate quickly (rapid switching speed).
An example of a biphenyl nitrile is 4-pentyl-4'-cyanobiphenyl (known as 5CB)
Lyotropic and Thermotropic Liquid Crystals-
Liquid crystals only show liquid properties under certain conditions. They are very sensitive to temperature and concentration. Essentialy liquid crystals can be divided into two main types.
Lyotropic liquid crystals are solutions which show the liquid crystal phase at certain concentrations. An example of lyotropic liquid crystals is soap solution. At low dilution the polar soap molecules behave randomly but at higher concentrations they group together into larger units called micelles which in the liquid crystal phase are ordered in their orientation.
Thermotropic liquid crystals are pure substances that show liquid crystals behaviorr over a range of temperature between solid and liquid states. Examples of a thermotropic liquid crystal are biphenyl nitriles used in liquid cristal displays (LCDs).
The use of thermotropic liquid crystals in a calculator screen.
Within the thermotropic liquid crystal phase the rod-shaped molecules which are typically about 2.5x10-meters in lencht exist in groups or domains. The molecules can flow and are randomly distributed as in liquid but within each domain they all point in the same direction. This is known as the nematic phase. As the temperature increases the orientation becomes increasingle more disrupted until eventually the directional order is lost and the normal liquid phase is formed.
Principles of LCD Devices-
Since liquid crystal molecules are polar their orientaion can be controlled by an applied electric field. The orientation of the molecules affects the ability of the liquid crystal molecules to transmit light. In liquid crystal displays used in digital watches calculators and laptops a small voltage is applied across a thin film of the material.
The flat screens used for computer monistors or television use liquid crystals.
This controls the areas of the display that are light and dark and hence gives the characteristic readings of the pictures or letters. The great advantage of LCDs over other types of electronic display is that they use extremely small electric currents. The disavantage is that they only work witihn a certain temperature range whhich explains why a digital watch or laptop screen may give a strange display in very hot or cold temperatures. Hence for use in an LCD a liquid crystal should be chemically stable compound, contain polar molecules, remain stable in the liquid crystal phase over a suitable range of temperature, and be able to orientate quickly (rapid switching speed).
