Liquid crystals are a class of molecules that, under some conditions, inhabit a phase in which they exhibit isotropic, fluid-like behavior – that is, with little long-range ordering – but which under other conditions inhabit one or more phases with significant anisotropic structure and long-range ordering while still having an ability to flow.

Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline molecules in the presence or absence of an electric field. In the presence of electric field, these molecules align with the electric field, altering polarization of the light in a certain way.

The ordering of liquid crystalline phases is extensive on the molecular scale but does not extend to the macroscopic scale as might be found in classical crystalline solids. The ordering in a liquid crystal might extend along one dimension, but along another dimension it might have significant disorder.

Liquid crystals are divided into two groups depending on the shape of the molecules. Calamitic liquid crystals consists of rod-like molecules and have order in the direction of the longer axes of the molecules. In contrast, discotic liquid crystals are composed of flat-shaped molecules which align in the direction of the shorter axes of the molecules.

Important types of calamitic liquid crystals include

• nematics (most nematics are uniaxial but biaxial nematics are also known)
• smectic s (smectic A , smectic C , and hexatic )

Important types of discotic liquid crystals include

• discotic nematics
• columnar phases

Biological membranes are a form of liquid crystal. Their rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface, yet the membrane is fluid and elastic. It can also host important proteins such as receptors freely "floating" inside, or partly outside, the membrane.

Lyotropic liquid crystals

A lyotropic liquid crystal is a group of liquid-crystalline assemblies that consists of two or more components and exhibits liquid-crystalline properties in certain concentration ranges. In the lyotropic phases, solvent molecules fill the space around the compounds to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases.

Even within the same phases, their self-assembled structures are tunable by the concentration: for example, in lamellar phases, the layer distances increase with the solvent volume. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals.

A compound which has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule. Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences depending on the volume balances between the hydrophilic part and hydrophobic part. These structures are formed through the micro-phase segregation of two incompatible components on a nanometer scale.

The content of water or other solvent molecules changes the self-assembled structures as follows:

• Discontinuous cubic phase (micellar phase)
• Hexagonal columnar phase (middle phase)
• Bicontinuous cubic phase
• Lamellar phase
• Bicontinuous cubic phase
• Reverse hexagonal columnar phase
• Inverse cubic phase (Inverse micellar phase)

The same characteristics can be observed in immiscible diblock copolymers.

These lyotropic liquid-crystalline nanostructures are abundant in living systems such as DNA, polypeptides, and cell membranes. Accordingly, lyotropic liquid crystals attract particular attention in the field of biomimetic chemistry.

Effect of chirality

When the molecules that form liquid crystals have asymmetric carbon atoms and when the system has not chirality but racemic modification, the orientation vector of the molecular axis of the liquid crystals changes continuously and a macroscopic spiral structure appears in the system as a result. The cycle of the spiral structure is different for each molecule, but each molecule has the property that it reflects the light corresponding to its cycle. From this property, the liquid crystals change color when the cycle of the spiral structure agrees with the visible rays of light. Some kinds of liquid crystals change the cycle of their spiral structure when the temperature changes. This principle is applied in liquid crystal thermometers.

Nematic liquid crystals, which have spiral structures, are called cholesteric liquid crystals. Cholesteric liquid crystals are not distinguished from nematic liquid crystals thermodynamically; hence cholesteric liquid crystals are sometimes called chiral nematic liquid crystals.

Although almost all chiral liquid crystals include asymmetric carbon atoms in their molecules, it has recently been discovered that macroscopic chirality appears in liquid crystals that consist of bent-core molecules which do not have asymmetric carbon atoms. However, the appearance mechanism of this macroscopic chirality is not yet clear.

References

• de Gennes, P.G. and Prost, J. The Physics of Liquid Crystals, Claredon Press (1993).
• Chandrasekhar, S. Liquid Crystals 2ND edition, Cambridge Univ Pr Published (1993).