Nematic order describes a unique state of matter that exists between a conventional solid and a liquid. In this intermediate phase, molecules exhibit a distinct level of organization, yet retain some fluidity. This arrangement, characterized by molecular alignment without fixed positions, forms the basis for various natural phenomena and technological advancements. This unique state combines properties of both rigidity and flow.
Understanding Nematic Order
Nematic order is a state where elongated, rod-like molecules align along a common direction, called the “director.” This alignment provides orientational order, meaning molecules point in roughly the same direction, like pencils in a box. However, unlike a solid, these molecules do not arrange into a fixed, repeating lattice, allowing them to move and flow past each other like a liquid, without fixed positional order.
The degree of this alignment is quantified by a nematic order parameter, a scalar value ranging from 0 to 1. A value near 0 indicates a disordered, isotropic liquid, while a value of 1 signifies perfect alignment of all molecules along the director. This orientational preference gives nematic materials anisotropic properties, meaning their optical, electrical, and magnetic characteristics vary with the direction of measurement.
Most nematics are uniaxial, with rotational symmetry around this director axis. While the molecules align along a preferred direction, they do not have a specific orientation in the plane perpendicular to that direction.
Nematic Order in Everyday Technology
The most widespread application of nematic order is found in Liquid Crystal Displays (LCDs), which are found in televisions, computer monitors, smartphones, and other electronic devices. LCDs exploit the ability of nematic liquid crystal molecules to reorient when an electric field is applied.
In a typical LCD, a thin layer of nematic liquid crystals is sandwiched between two glass plates with transparent electrodes and polarizing filters. When no electric field is present, the liquid crystal molecules are arranged to rotate polarized light, allowing it to pass through a second polarizer, which makes the pixel appear bright. Applying an electric field causes these elongated molecules to reorient themselves, altering how they interact with the polarized light. This change can block the light from passing through, making the pixel appear dark.
Nematic Order in Biological Systems
Beyond technology, nematic order manifests in various biological systems, influencing cellular functions and tissue formation. Biological molecules, such as actin filaments and microtubules within the cytoskeleton, can exhibit nematic-like alignment. This organization is particularly evident in processes like cell division, where microtubules form a “nematic” network of aligned structures at the spindle center.
The actin cytoskeleton, a dynamic network of protein filaments, often organizes into nematic bundles, which are highly aligned and densely packed actin filaments. These bundles, such as contractile rings and stress fibers, play a part in cell shape changes and movement. Active nematics, for example, apply liquid crystal theories to understand the collective movements and tissue morphogenesis of cytoskeletal filaments, bacteria, and eukaryotic cells.
Distinguishing Nematic Order from Other Phases
Nematic order is distinct from other states of matter and other liquid crystal phases. Unlike a true solid, which possesses both long-range positional and orientational order, nematics lack positional order, allowing them to flow. Conversely, a true liquid has neither positional nor orientational order, with molecules randomly distributed and oriented.
Other liquid crystal phases also differ from nematic order in their molecular arrangement. Smectic liquid crystals, for example, exhibit a higher degree of order than nematics; their molecules are not only aligned in parallel but also arranged in distinct layers. These layers can slide past each other, giving smectics a soap-like consistency. Cholesteric liquid crystals, sometimes called twisted nematics, are characterized by a helical twisting of their molecular alignment. This helical arrangement, where each successive layer is rotated slightly, gives cholesterics unique optical properties, such as selective light reflection.