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Cholesteric liquid crystal
Cholesteric liquid crystal
Cholesteric liquid crystal
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Cholesteric liquid crystals (ChLCs), also known as chiral nematic liquid crystals, are a supramolecular assembly and a subclass of liquid crystal characterized by their chirality. Contrary to achiral liquid crystals, the common orientational direction of ChLCs (known as the director) is arranged in a helix whose axis of rotation is perpendicular to the director in each layer. ChLCs can be thermotropic and lyotropic. ChLCs are formed from a variety of anisotropic molecules, including chiral small molecules and polymers. ChLCs can be also formed by introducing a chiral dopant at low concentrations into achiral liquid crystalline phases.[1]

Examples of ChLCs range from scarab beetle shells to liquid crystal displays.[2] Many natural molecules and polymers spontaneously form the cholesteric phase. ChLCs have been used to manufacture products ranging from smart paints to textiles to and sensors. Scientists often employ biomimicry to develop ChLC-based materials inspired by natural examples.[3]

History

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Cholesteryl benzoate, the first liquid crystal. Due to its chirality, cholesteryl benzoate forms the cholesteric phase.

Cholesteric liquid crystals (ChLCs) have a history dating back nearly 150 years. In 1888, the first liquid crystal — cholesteryl benzoate, a thermotropic ChLC [4]  — was discovered by Austrian botanist and chemist Friedrich Reinitzer.[5] Although he initially believed that cholesteryl benzoate consisted of aggregates of tiny, flowing crystals, he was confounded by the presence of two melting points. The first transition (around 145-146 °C) corresponded to a phase transition to a liquid state that possessed vibrant colors, and the second high-temperature melting point (178-180 °C) changed this cloudy liquid to a clear melt. He also discovered that this process was fully reversible.[6] These discoveries Reinitzer reported in what is recognized as the first paper on liquid crystals.[1]

Reinitzer was a close collaborator with Otto Lehmann, along with whom Reinitzer is considered the "father" of liquid crystals.[1] Lehmann was the inventor of the first hot stage microscope capable of studying the thermal properties of thermotropic ChLCs, which he created in 1876.[5] One of the key features of his microscope was crossed-polarizers. Polarized light microscopy remains highly important in the study of liquid crystals, including ChLCs.[1] While Reinitzer quickly lost interest in the substances, Lehmann continued studying his apparently "flowing crystals" on his hot stage microscope, realizing they exhibited orientationally-dependent, vibrant colors under crossed polarizers.[1][7] Lehmann was the first to coin the term liquid crystal.[8] Studies in liquid crystals soon blossomed, and in 1922 Georges Friedel created the classification system of liquid crystals still used today. In this system, he named the chiral variety of liquid crystals cholesteric, as they were discovered from a cholesterol derivative.[5][9]

Liquid crystals emerged from the status of a curiosity necessitating high temperatures to function in the 1960s, with the advent of liquid crystal display technology.[1] Although liquid crystal displays (LCDs) are typically made of nematic liquid crystals, ChLCs have been utilized in display technology. Examples include a thermal sensor range meter created by James Fergason using ChLCs in 1959, an invention which was patented in 1960.[10] Another example is a stress-sensing card, which when applied to skin — ordinarily black — becomes blue when the wearer is relaxed and red when stressed. The technology relies on body temperature differences between relaxation and stress.[11]

Theory

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Structure

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Cholesteric liquid crystals, here depicted as a half-rotation of the cholesteric helix. The liquid crystalline director is along the backbone of the long axis of the mesogen. In the cholesteric phase, the director rotates layer by layer, ultimately creating a twisted supramolecular structure.

Due to their properties intermediate between pure liquids and crystalline solids, liquid crystals are known as mesogens, a name deriving from Greek for mésos, or "intermediate". The property underpinning all liquid crystals is anisotropy (directional nonuniformity, typically manifested by an elongated rodlike shape), which under appropriate conditions (ex. high temperatures and concentrations) allows for local order around a preferred axis, named the director.[1] Cholesteric liquid crystals are no exception. Like nematic liquid crystals, ChLCs exhibit a medium-range director along which the long axis of the liquid crystals are arranged. Unlike nematics, along a twist axis perpendicular to the director, ChLCs are arranged in layers that rotate with helical pitch p, typically defined as twice the periodicity along the twist axis.[12]

There exist two classes of liquid crystals based on the conditions under which they form: thermotropic and lyotropic. Thermotropic liquid crystals undergo phase transitions based on temperature, whereas lyotropic liquid crystalline phases transition based on concentration within a solvent, most commonly water.[13] For example, 5CB — a classic example of an achiral nematic thermotropic LC — undergoes an isotropic-nematic transition at 308K and a nematic-crystalline transition at 252K.[14] Similarly, poly(n-hexyl isocyanate), a lyotropic liquid crystal, undergoes the analogous isotropic-nematic transition at weight fractions ranging from 0.225 to 0.438 in toluene, depending on molecular weight of the polymer.[15] Cholesteric liquid crystals comprise both classes. Both small molecules and polymers can form cholesteric liquid crystals. In nature, examples include DNA, chitin, cellulose, and collagen, among others.[2]

The local ordering in both nematic and ChLCs can be characterized according to the local nematic order parameter S. This parameter is formulated according to the following equation, a rank-2 tensor:[16]

Here, for an anisotropic molecule, the nematic order tensor is a function of number of molecules N, the outer product of the unit vector along the long axis v(i), and a traceless correction term, the Kronecker delta δ.[16] The largest eigenvalue of the resulting tensor is the local nematic order. For an isotropic sample, the nematic order will be calculated as 0. For a fully-ordered sample, the nematic order approaches 1. Typically, the cholesteric pitch ranges in values from as small as 100nm to several micrometers.[17] The orientation of the nematic director at a certain distance along the director twist axis (usually defined as the z-axis in Cartesian coordinates) is:

Here, q is defined as the helicity of a ChLC, . The helicity is positive for a right-handed cholesteric helix, and negative for left-handed helices.[18] The origin of the helical pitch can be described with Frank-Oseen elastic free energy density:

Where div is the divergence for a vector field n (representing the individual molecular long-axis vectors) and curl is the curl of the same vector field. In 3D space, with unit vectors i, j, and k along each coordinate axis. The constants K are known as Frank elastic constants, and are empirical.[12] By minimizing the free energy, we obtain an expression for the cholesteric helicity q.

Inducement of chirality

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Polymeric liquid crystals; left: PHIC, right: PBLG. Since PBLG is synthesized from homochiral monomers, it forms a strong α-helix via hydrogen bonding and readily forms cholesteric phases, whereas PHIC can helix-flip and requires a dopant to form the cholesteric phase

ChLCs can form either from intrinsically chiral molecules or polymers or can be formed via chiral-dopant mediated processes. An example of a chiral-dopant mediated process is poly(n-hexyl isocyanate) (PHIC). PHIC, which is a helical polymer that is typically racemic and exhibits a nematic liquid crystalline phase due to the presence of dynamic helix-flips (ie. where the helix flips its handedness),[19] becomes cholesteric when exposed to a small amount of chiral dopant. The mechanism by which this transition occurs is via the slight displacement of the racemic mixture to a small enantiomeric excess, which then drives the formation of cholesteric helices.[20]

Some dopants; from left to right: cholesterol, cholesteryl chloride, bridged biaryls

Different chiral dopants may be quantitatively compared using their empirical helical twisting power:

Where C is the mole fraction of the dopant, corrected for enantiomeric purity.[21] Dopants also induce chirality on small molecules by biasing a specific chiral spatial configuration, which has an amplifying effect that ultimately leads to the formation of a chiral phase from a small enantiomeric biasing.[22]

An example of inherently chiral ChLCs is poly-γ-benzyl-l-glutamate (PBLG), a lyotropic liquid crystal that forms cholesteric phases without dopant.[23] This is attributed to the strong α-helix formed between individual peptides, whose handedness arises from homochiral monomers during synthesis.[24] Examples of small-molecule ChLCs include cholesterol-doped 5CB and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine.[25] The pitch of thermotropic ChLCs is temperature-dependent.[26]

Optical Textures

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Due to their anisotropy, liquid crystals are birefringent. Formally, this means that the index of refraction is directionally dependent, with characteristic indices defined along perpendicular optical axes. Upon incident light, these different indices break up the waves into multiple with different wavelengths.[1]

When observed under a polarized optical microscope, ChLCs can create a number of textures due to a combination of their inherent birefringence and their relative alignment with the incident light. To relate the pitch with the observed textures, the Bragg equation is used:

Where n is the average refractive index of the birefringent material and Φ is the observation angle. Therefore, the pitch of the ChLC influences the observed texture.[27]

The classic fingerprint texture of a cholesteric liquid crystal. The distance between dark fringes is half the cholesteric pitch.

Among the most common textures is the oily streak texture, which was the first texture experimentally observed in cholesteryl benzoate.[6] This texture arises when the director helix axis is parallel to the incident light, and manifests as small streaks of birefringence against a dark background under crossed polarizers. If the pitch is very short, this orientation can also give rise to the Grandjean texture.[17] This texture appears monochromic under crossed-polarizers.[28]

Another texture is the classical fingerprint texture, where the director helix axis is perpendicular to incident light. Here, the cholesteric helix can be easily observed and measured, as the pitch is calculated as the distance between two dark fringes.[29] This information can be used to measure helical twisting power of the liquid crystal or monitor changes to the physical structure of the cholesteric helix in applications such as optical sensing.[21][30] Particularly long pitches arranged this way give rise to the focal-conic texture.[31]

The textures can be tuned with external stimuli. Pijper and coworkers invented a ChLCs whose pitch can be dynamically controlled via light irradiation. A chiral, photoswitchable chromophore was functionalized onto the ends of PHIC polymers, whose enantiomeric excess could be tuned with irradiation time. Upon irradiation with characteristic wavelengths of light, the texture changed from fingerprint to nematic to the opposite-handed fingerprint.[30]

Characterization

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By nature, cholesteric liquid crystals share a significant number of characterization methods with achiral liquid crystalline phases. Some are highlighted:

Differential Scanning Calorimetry

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Differential scanning calorimetry (DSC) is a technique that measures heat flow differences between a sample and a reference.[32] In the case of thermotropic liquid crystals, DSC can determine the presence of phase transitions between isotropic and cholesteric, and cholesteric and other phases. This double melting point is characteristic of the liquid crystalline phase.

Nuclear Magnetic Resonance Spectroscopy

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Like in nematic liquid crystals, nuclear magnetic resonance (NMR) spectroscopy can be used to probe the supramolecular structure of ChLCs.[14] Due to the relative orientation of quadrupole moments in individual molecules, NMR can observe characteristic peak splitting that evolves with temperature and phase changes. For example, NMR studies of cholesteryl alkanoates found that cooling the isotropic phase to the smectic phase (a phase where molecules are arranged in discrete planes) via the cholesteric phase showed a singlet peak become increasingly split with decreasing temperature and increasing local order.[33]

Polarized Light Microscopy

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Chemical structure of octadecyltrichlorosilane
Pictographic representation of the mechanism of liquid crystal alignment via an octadecyltrichlorosilane self-assembled monolayer

Due to the birefringence of liquid crystalline samples, liquid crystals display vivid characteristic textures under cross-polarizers (note that isotropic samples will be completely dark under crossed polarizers).[1] These textures are typically characteristic of the studied phase, meaning that ChLCs can be qualitatively identified by simple microscopy. On top of allowing for qualitative phase assignment, the cholesteric pitch can be quantitatively determined by simple measurement of the fringe displacement in the fingerprint texture.[17]

Polarized light microscopy is widely used in studies of liquid crystals.[34] However, textures are reliant on the surfaces by which they are confined — that is, their relative alignment with incident light. In order to provide control for alignment, adsorbents such as octadecyltrichlorosilane (OTS) have been proposed to create self-assembled monolayers on the glass surface. These monolayers then act as aligners for the bulk liquid crystalline mesophase.[35]

Circular Dichroism Spectroscopy

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Circular dichroism (CD) spectroscopy characterizes chiral materials by differential absorption of left and right handed circularly-polarized light.[36] In the context of ChLCs, CD spectroscopy can distinguish between different helical senses — for example, a cholesteric helix that primarily transmits left-handed circularly polarized light is considered left-handed. The CD spectrum is also dependent on other quantities associated with the supramolecular helix, such as pitch and orientation/texture with respect to incident light.[37]

Examples and Applications

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Helical Templates

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The helical structure of ChLCs can serve as templates to induce supramolecular helical structures in otherwise-structureless dopants. When the dopant is introduced into a liquid crystalline matrix, it self-assembles into the empty spaces between helices, creating helical structures. For example, Li and coworkers templated latex nanoparticles in a matrix of cholesteric cellulose nanocrystals (CNCs). These nanoparticles arranged into the cholesteric defects, creating helicoidal nanoparticle assemblies.[38]

Natural Mimics

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ChLCs are widely present in nature. Many biological polymers such as DNA, chitin, cellulose, collagen, and certain viruses (for example, filamentous bacteriophages)[39] can exhibit cholesteric phases naturally, or via dopant-mediated processes.[2]

Natural selection has favored the natural development of ChLCs in some beetle shells.[40] The classic metallic sheen of scarab beetle genera such as plusiotis arises from ChLCs — mostly chitin — in their shells.[3] The appealing aesthetics of these insects have led scientists to pursue nature-inspired design of ChLC-based products ranging from mood rings to nail polish, mimicking scarab beetle sheen.[41]

Color-Changing Films

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Kizhakidathazhath and coworkers invented a color-changing film based on cholesteric liquid crystal elastomers (ChLCE). Formed from lyotropic ChLCs, dry films with a frozen cholesteric helix were created by rapid solvent evaporation followed by photocrosslinking of the resulting gel. This thin film is mechanochromically responsive, changing colors with stress and bending.[42]

Smart Textiles

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Kao and coworkers incorporated ChLC microspheres into a polyvinyl alcohol matrix. The composite was found to have superior mechanical properties compared to raw polyvinyl alcohol, and remained color stable even under extreme conditions, such as high electric field. The composite was able to be spun into thin fibers.[43]

Similarly, Geng and coworkers created ChLCE-based spinnable fibers that exhibit mechanochromic responses, changing colors from red to blue with increasing strain.[44]

Smart Paints

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Ko and coworkers invented a smart color-changing paint whose color is both tunable and subsequently freezable using ultraviolet (UV) light. Starting with an achiral photopolymerizable monomer precursor, chiral dopants were added to tune the pitch (according to the aforementioned helical twisting power equation), allowing for colorimetric tuning. Upon irradiation with UV light, the monomers polymerized, creating freestanding colored films with frozen molecular arrangement. The authors were able to create red, green, and blue films using this method.[45]

References

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Cholesteric liquid crystal

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Cholesteric liquid crystals (ChLCs), also known as chiral nematic liquid crystals, are a type of thermotropic phase distinguished by a spontaneous helical supramolecular assembly formed from chiral molecules or induced in a nematic host. This structure arises when rod-like molecules align parallel to planes that successively rotate around an axis perpendicular to these planes, creating a periodic helical twist with a pitch length typically on the order of visible wavelengths (0.1 to several micrometers). The helical pitch, which determines the selective Bragg reflection of circularly polarized , is highly sensitive to molecular composition, temperature, and external fields, often resulting in vivid, tunable color effects. First observed in the late through cholesterol derivatives, ChLCs represent one of the earliest known mesophases and exhibit orientational order parameters similar to nematics (ranging from 0.2 to 0.8). The unique optical properties of ChLCs stem from their one-dimensional photonic bandgap, where the helical superstructure acts as a Bragg reflector for of wavelength λ ≈ nP cosθ (with n as the average , P as the pitch, and θ as the incidence ). This selective reflection typically spans a narrow bandwidth (<200 nm for single-pitch systems) due to the inherent birefringence (Δn < 0.4) of the material, but can be broadened through polymer stabilization or doping to cover broadband visible or infrared spectra. Thermally responsive ChLCs change pitch with temperature, enabling iridescent color shifts, while their textures—such as planar (Grandjean), focal conic, or fingerprint—reveal the helical organization under microscopy. These materials can be monomeric (e.g., cholesterol esters) or polymeric, with lyotropic variants forming in aqueous solutions of chiral amphiphiles. ChLCs find diverse applications leveraging their responsive optics and self-assembly. In displays, cholesteric liquid crystal displays (ChLCDs) enable electronic paper with key features including reflective operation without backlight, bistability for zero static power consumption, over 16 million colors via 3-layer RGB structures, continuous gradation, fast updates in 1-2 seconds, high reflectivity (>30% and up to 70%), suitability for sunlight viewing, low blue light emission, and durability in wide temperatures (-20 to 70°C); they also support sensors, temperature-indicating devices, and tunable lasers due to their photonic bandgap properties. Broadband variants are used in energy-efficient smart windows, anti-counterfeiting labels, and infrared-shielding films for solar cells and military camouflage. Biologically inspired uses extend to mimicking structures in DNA, collagen, and silk, highlighting their role in soft matter physics and materials science. Ongoing research focuses on enhancing pitch control and stability for next-generation photonics and optoelectronics.

Fundamentals

Definition and Characteristics

Cholesteric liquid crystals, also known as chiral nematic liquid crystals, represent a distinct mesophase in which rod-like molecules exhibit long-range orientational order similar to that of nematic phases but are arranged in a helical superstructure. The local director, which defines the average molecular orientation, remains parallel to itself within each pseudo-layer of the structure, but successive layers rotate progressively around an axis perpendicular to the director, forming a continuous helix with a characteristic pitch length. This chirality arises from the inherent asymmetry in the molecular composition, distinguishing cholesterics from achiral nematics. Key characteristics of cholesteric liquid crystals include the helical pitch, typically on the order of hundreds of nanometers to a few micrometers, which determines the periodicity of the twist and can be tuned by factors such as temperature, concentration of chiral dopants, or external fields. In the Grandjean texture, a planar alignment where the helical axis is perpendicular to the bounding surfaces, equidistant disclination lines known as Grandjean planes become visible under polarized light, separating regions differing by half a pitch and highlighting the uniform helical stacking. Optically, these materials exhibit selective Bragg reflection of circularly polarized light matching the handedness of the helix, within a bandwidth given by Δλ=Δn⋅p\Delta \lambda = \Delta n \cdot p, where Δn\Delta n is the birefringence and pp is the pitch; this results in iridescent colors when the pitch falls in the visible spectrum. Phase behavior in cholesteric liquid crystals involves reversible transitions influenced by temperature. Upon heating, the cholesteric phase clears to an isotropic liquid at the clearing temperature TcT_c, often preceded by narrow blue phases in highly chiral systems. On cooling, the reverse transition occurs, and further lowering the temperature may induce a shift to smectic or crystalline phases, depending on the molecular structure. In comparison to other liquid crystal phases, cholesterics differ from nematics by the presence of the spontaneous helical twist, which imparts chirality and optical activity absent in the uniformly aligned nematic director field. Unlike smectics, which feature positional order in layered arrangements, cholesterics maintain translational isotropy with only orientational order modulated by the helix, though chiral variants like smectic C* can exhibit similar twisting within layers.

Historical Development

The discovery of cholesteric liquid crystals occurred in 1888 when Austrian botanist Friedrich Reinitzer examined cholesteryl benzoate and observed two distinct melting points: the first at 145.5°C yielding a cloudy, iridescent phase, and the second at 178.5°C producing a clear isotropic liquid. Reinitzer shared his findings with physicist Otto Lehmann, who confirmed the intermediate phase's anisotropic properties and introduced the term "liquid crystals" to describe such materials. This observation marked the initial recognition of the cholesteric phase, distinguished by its selective light reflection and color changes with temperature. In the early 20th century, research progressed through systematic synthesis efforts. Daniel Vorländer, often regarded as the father of liquid crystal chemistry, began synthesizing hundreds of mesogenic compounds in the 1900s and, by the 1920s, had developed synthetic analogs of cholesterol derivatives that exhibited cholesteric phases, enabling broader study beyond natural sterols. These efforts established key structure-property relationships, such as the role of rigid rod-like cores in promoting mesophases. Vorländer's work in 1929 specifically highlighted cholesteric behavior in ester derivatives, paving the way for non-cholesterol-based systems. A major theoretical breakthrough arrived in 1951 with Hendrik de Vries' proposal of a helical molecular arrangement in cholesteric phases, where director orientations twist uniformly to explain the observed circular birefringence and selective reflection of circularly polarized light. This model, building on earlier ideas by Oseen, provided a foundation for understanding the phase's chirality and optical effects. In the 1960s, experimental techniques advanced with the detailed observation of the Grandjean texture—a planar alignment where helical axes are perpendicular to the surface, producing vivid, interference-like colors—which facilitated direct visualization and pitch measurements in thin films. The 1970s saw refined theoretical models for cholesteric elasticity and pitch. Pierre-Gilles de Gennes extended continuum theories to describe cholesteric deformations, linking pitch to chiral interactions and elastic constants in his seminal 1974 monograph, which unified nematic and cholesteric behaviors under variational principles. Concurrently, W.J.A. Goossens developed models for pitch variation in binary cholesteric mixtures, relating composition to helical twisting power and enabling predictions of phase stability. These contributions solidified the theoretical framework for cholesteric dynamics. By the 1980s, cholesteric liquid crystals transitioned toward practical applications, particularly in thermography. The commercialization of non-steroidal cholesteric mixtures in 1982 enabled temperature-sensitive films for medical imaging, stress analysis, and consumer products like mood rings and fever thermometers, leveraging their reversible color shifts over narrow temperature ranges. Post-2000 developments focused on polymer-stabilized cholesteric liquid crystals (PSCLCs), where UV-polymerized networks lock the helical structure, enhancing mechanical stability and electro-optic switching for bistable displays and smart windows. These advances, including low-power reflective devices reported since 2003, have expanded applications in flexible electronics while preserving the phase's inherent chirality.

Theoretical Framework

Molecular Structure

Cholesteric liquid crystals consist primarily of rod-like mesogenic molecules, characterized by a rigid central core—often composed of conjugated aromatic units such as or tolane—and flexible alkyl chain tails that enhance molecular mobility and phase stability. These mesogens typically exhibit calamitic (rod-shaped) architecture, enabling parallel alignment within local domains while allowing rotational freedom around their long axes. To induce the characteristic chirality, optically active dopants are incorporated into an achiral nematic host; these dopants feature asymmetric carbon centers, commonly derived from natural chiral molecules like sugars (e.g., isosorbide derivatives) or peptides, which impart a preferred handedness to the molecular assembly. The supramolecular organization of cholesteric liquid crystals arises from the twisting of the local molecular director n(r)\mathbf{n}(\mathbf{r}) along a helical axis, typically denoted as the zz-direction. In this structure, molecules within successive pseudo-layers remain parallel to each other but rotate progressively relative to adjacent layers, forming a helical superstructure. The director's orientation is described by the angle θ(z)=qz\theta(z) = q z, where qq is the wavevector representing the twist rate. The helical pitch pp, defined as the distance over which the director completes a full 2π2\pi rotation, is given by p=2π/qp = 2\pi / q. In Cartesian coordinates, assuming the helix axis aligns with zz, the director vector takes the form n(z)=(cos⁡(qz),sin⁡(qz),0)\mathbf{n}(z) = (\cos(qz), \sin(qz), 0), illustrating the continuous spatial variation of molecular alignment. Several factors govern the helical pitch pp. The concentration of the chiral dopant cc inversely affects the pitch, approximately following the relation p≈1/(HTP⋅c)p \approx 1 / (HTP \cdot c), where HTPHTP is the helical twisting power of the dopant—a measure of its ability to induce twist per unit concentration. Temperature also plays a critical role, with pp generally increasing as temperature rises due to thermal disruption of intermolecular interactions that stabilize the helix; in certain materials, this can manifest as a pitch gradient along the helical axis. Blue phases serve as intermediate chiral structures in the phase sequence between the isotropic liquid and the canonical cholesteric phase, occurring in highly chiral systems over narrow temperature ranges near the isotropic transition. The cubic blue phases (BPI and BPII) feature a three-dimensional arrangement of double-twist cylinders packed into defect-laden lattices with crystallographic symmetries, such as body-centered cubic or simple cubic, bridging the one-dimensional helicity of cholesterics with full isotropy.

Chirality Induction

Chirality in cholesteric liquid crystals arises primarily through two mechanisms: intrinsic molecular asymmetry and extrinsic induction via doping. Intrinsic chirality occurs in molecules possessing inherent structural handedness, such as those with asymmetric carbon atoms. Classic examples include cholesteryl esters, which form cholesteric phases due to their chiral steroid backbone, enabling self-assembly into helical structures without external additives. Similarly, helicene derivatives, characterized by their helically twisted polycyclic aromatic scaffolds, exhibit intrinsic chirality that promotes cholesteric organization through steric crowding and π-π interactions. Extrinsic chirality induction involves incorporating small amounts of chiral additives into achiral nematic hosts to impose a helical twist. Typically, chiral dopants are added at concentrations of 1-10 mol% to nematic liquid crystals, sufficient to generate a cholesteric phase with a defined pitch. The efficiency of this induction is quantified by the helical twisting power (HTP), defined as HTP=1c⋅p,\text{HTP} = \frac{1}{c \cdot p}, where cc is the dopant concentration (in mole fraction) and pp is the helical pitch length. Higher HTP values indicate stronger twisting ability, often exceeding 100 μm⁻¹ for optimized dopants like binaphthyl derivatives. At the molecular level, chirality induction stems from intermolecular interactions that favor twist deformations over splay or bend in the nematic matrix. Steric effects from the asymmetric shape of chiral molecules create local distortions, while dipole-dipole couplings between polar groups enhance orientational asymmetry, preferentially stabilizing helical arrangements. These interactions contribute to the twist term in the Frank elastic free energy density, given by Ftwist=K22(n⋅∇×n)2,F_{\text{twist}} = \frac{K_2}{2} (\mathbf{n} \cdot \nabla \times \mathbf{n})^2, where K2K_2 is the twist elastic constant and n\mathbf{n} is the director field; minimization of this energy drives the formation of a uniform helix with pitch inversely proportional to the chiral strength. Reversible chirality manipulation allows dynamic control of the helical structure. Photo-induction using azobenzene-based chiral dopants enables handedness inversion through trans-cis isomerization under UV/visible light, altering the HTP and thus the helix direction without permanent chemical change. Electric fields can also invert helix handedness in certain doped systems by coupling with dielectric anisotropy, inducing selective unwinding and reconfiguration of the director field to the opposite twist sense.

Optical Properties

Cholesteric liquid crystals exhibit distinctive optical properties stemming from their helical superstructure, which acts as a one-dimensional photonic crystal capable of manipulating light through selective reflection and polarization-dependent interactions. This helical arrangement, with a pitch typically on the order of visible wavelengths, enables Bragg-like diffraction that imparts vivid structural colors and circular birefringence to the material. The hallmark optical phenomenon is selective Bragg reflection, where the cholesteric phase reflects circularly polarized light of a specific handedness (matching the helix) within a narrow wavelength band centered at λ=navg⋅p⋅cos⁡θ\lambda = n_\text{avg} \cdot p \cdot \cos\theta, with navgn_\text{avg} as the average refractive index, pp the helical pitch, and θ\theta the angle of incidence relative to the helical axis. Light of the opposite circular polarization is largely transmitted, resulting in up to 50% reflectance for unpolarized incident light due to the material's circular dichroism. This selective reflection arises from the periodic modulation of the dielectric tensor along the helix, mimicking a stack of anisotropic layers that constructively interfere for the resonant wavelength. The width of the reflection band, Δλ=Δn⋅p\Delta\lambda = \Delta n \cdot p, where Δn=ne−no\Delta n = n_e - n_o is the birefringence, determines the color palette; for typical Δn≈0.15−0.2\Delta n \approx 0.15-0.2 and visible-range pitches (p≈300−700p \approx 300-700 nm), Δλ\Delta\lambda spans 50-100 nm, producing saturated hues like blue or green. Iridescent color shifts occur due to temperature-induced changes in pitch and refractive index, governed by dλdT≈pdndT+ndpdT\frac{d\lambda}{dT} \approx p \frac{dn}{dT} + n \frac{dp}{dT}, where dpdT\frac{dp}{dT} often dominates in chiral nematic mixtures, leading to a redshift or blueshift of several nm per °C. In well-aligned samples, the planar (Grandjean) texture orients the helical axis perpendicular to the surface, maximizing selective reflection and yielding brilliant, mirror-like colors observable under normal incidence. In contrast, the focal conic texture features tilted or randomized helical axes across domains, promoting light scattering and a matte, opaque appearance with minimal selective reflection. Defects within these textures often manifest as oily streaks, elongated regions where the helix is compressed or elongated, disrupting uniform reflection and introducing local color variations. The strong birefringence of cholesteric liquid crystals (Δn>0.1\Delta n > 0.1) facilitates the Mauguin regime when light propagates nearly parallel to the helical axis, provided the condition pΔn/λ≫1p \Delta n / \lambda \gg 1 holds; in this adiabatic approximation, the polarization of the guided mode follows the twisting director without significant coupling to other modes, preserving through the structure and enabling applications in polarization-maintaining s.

Characterization Techniques

Thermal Analysis

Thermal analysis techniques are essential for characterizing phase transitions and thermal stability in cholesteric liquid crystals, providing quantitative insights into the energetic and structural changes during heating and cooling cycles. (DSC) is widely employed to measure heat flow associated with phase transitions, revealing endothermic peaks corresponding to the cholesteric-to-isotropic transition. These peaks typically exhibit changes (ΔH) in the range of 1-10 kJ/mol, reflecting the relatively weak intermolecular forces in the ordered mesophase compared to crystalline solids. For instance, in various cholesteryl esters, ΔH values for the cholesteric-to-isotropic transition are often around 0.5-1.5 kJ/mol, though higher values up to 13 kJ/mol have been reported for certain cholesterol-based derivatives. DSC also enables precise determination of the clearing temperature (Tc), the point at which the cholesteric phase transitions to the isotropic liquid, and assessment of phase purity by identifying sharp, single peaks indicative of homogeneous samples. Thermogravimetric analysis (TGA) complements DSC by evaluating the thermal stability of cholesteric mesogens up to their temperatures. TGA traces show minimal mass loss below 200°C, with typically onsetting between 200-300°C under inert atmospheres, depending on the molecular structure and substituents. For example, in cellulose-based cholesteric systems like derivatives, thermal begins above 200°C, while cholesterol-functionalized mesogens often exhibit onset temperatures around 300°C. This range highlights the robustness of cholesteric materials for applications requiring moderate thermal endurance, as the mesophase persists without significant degradation up to these limits. The helical pitch in cholesteric liquid crystals exhibits a pronounced dependence, influencing selective reflection and enabling thermochromic effects. As increases toward Tc, the pitch often expands linearly or nonlinearly, shifting the reflected and thus the color from shorter () to longer () visible wavelengths. Molecular-statistical theories predict that the reciprocal pitch (1/p) varies nearly linearly with , a confirmed experimentally in various cholesteric systems and exploited to tune reflection colors for temperature-responsive devices. This expansion arises from thermal disruption of chiral interactions, with the pitch-lengthening rate depending on the chiral concentration and molecular asymmetry. Enthalpy changes from DSC directly inform entropy variations during order-disorder transitions in cholesteric liquid crystals. For first-order transitions like cholesteric-to-isotropic, the entropy change (ΔS) is calculated as ΔS = ΔH / Tc, where Tc is the transition temperature in , yielding typical values of 5-30 J/mol·K that quantify the increased molecular disorder in the isotropic phase. This relation, rooted in the equality of change (ΔG = 0) at equilibrium, underscores the entropic driving force behind mesophase destabilization with rising temperature. In compilations of thermochemistry, such ΔS values for cholesteric systems align closely with those of nematic phases, emphasizing the subtle orientational ordering in chiral nematics.

Spectroscopic Methods

Spectroscopic methods provide essential insights into the molecular orientation, , and helical structure of cholesteric liquid crystals by analyzing spectral signatures arising from their anisotropic and twisted arrangements. These techniques probe interactions between and the material at molecular and mesogenic scales, revealing parameters such as order parameters, helical pitch, and selective absorption related to the cholesteric . Nuclear Magnetic Resonance (NMR) spectroscopy, particularly , is widely used to quantify molecular order and helical properties in cholesteric liquid crystals. In , the quadrupolar splitting of spectral lines allows determination of the orientational order parameter S=⟨(3cos⁡2θ−1)/2⟩S = \langle (3\cos^2\theta - 1)/2 \rangle, where θ\theta is the angle between the deuterated bond and the local director, averaged over molecular orientations; this parameter reflects the degree of alignment along the twisting director. The helical pitch is inferred from spectral splitting patterns influenced by director diffusion and helix distortion, with line shapes varying based on the pitch length and alignment of the helical axis relative to the . Circular Dichroism (CD) measures the differential absorption of left- and right-circularly polarized light, providing a direct probe of the helical structure in cholesteric liquid crystals. The CD signal arises from the material's inherent and the helical arrangement, with intensity quantified by the dissymmetry factor g=Δϵ/ϵg = \Delta \epsilon / \epsilon, where Δϵ\Delta \epsilon is the difference in molar absorptivities for circular polarizations and ϵ\epsilon is the average absorptivity; this factor highlights the selective interaction with circular light. CD spectra exhibit peaks at the edges of the selective reflection band, where the helical pitch matches the , enabling characterization of the pitch and through the sign and magnitude of the effects. Raman and infrared (IR) spectroscopy exploit vibrational modes to investigate director alignment and helical twist in cholesteric liquid crystals. In , polarized measurements of specific vibrational bands, such as C=O stretches around 1700 cm⁻¹, reveal variations in director orientation across the helical structure, with intensity ratios mapping the distribution of chiral and achiral components to quantify local twist and pitch gradients. Similarly, IR spectroscopy analyzes absorbance changes in dipole transitions (e.g., 800–1600 cm⁻¹ bands) under polarized light, showing shifts and intensity variations that indicate director tilting and twist propagation along the , particularly in phases with helical distortion. Ultraviolet-visible (UV-Vis) is employed to monitor the photonic bandgap in polymer-stabilized cholesteric liquid crystals, where the selective reflection band appears as a transmission minimum. The bandgap position and width, determined from absorption edges in the visible range, reflect the helical pitch stabilization by the network, with shifts observed under thermal or electrical stimuli that alter the without disrupting the .

Microscopic Observations

Polarized light microscopy (PLM) is a primary technique for visualizing the helical structure of cholesteric liquid crystals, revealing characteristic textures that indicate the pitch and orientation of the director field. In wedge-shaped cells, such as Grandjean-Cano configurations, PLM observes Grandjean steps as a series of parallel stripes corresponding to integer multiples of the helical pitch, where each step appears as a uniformly birefringent plane with distinct interference colors due to the selective reflection of circularly polarized light. These steps form because the helical axis aligns perpendicular to the cell surfaces, creating a staircase-like modulation of the pitch across the wedge thickness. In thin films or planar-aligned samples, PLM displays fingerprint textures as periodic lines arising from the projection of the helical structure onto the plane of observation, with the spacing between lines directly proportional to the helical pitch. These textures, often seen in homeotropic or slightly tilted alignments, highlight the one-dimensional periodicity of the cholesteric phase and are particularly useful for estimating pitch values in unconfined or weakly anchored systems. For assessing uniaxial alignment, conoscopic figures in PLM—obtained by inserting a Bertrand lens—reveal isogyres and maltese crosses that confirm the optic axis orientation, indicating a uniformly twisted structure without significant defects when the figure shows symmetric four-lobed patterns. Advanced imaging with enables three-dimensional visualization of the helical superstructure in cholesteric liquid crystals, particularly in polymer-stabilized networks where fluorescence labeling highlights director distortions. By scanning along the helical axis, confocal techniques resolve the periodic rotation of the director, revealing the full 3D twist in samples with pitches on the order of micrometers, and are especially effective for tracking texture transitions under external stimuli. Scanning electron microscopy (SEM) complements this by providing high-resolution images of fractured or polymer networks that preserve the cholesteric , showing layered structures that mimic the helical periodicity and allowing direct measurement of pitch in solidified forms. SEM is particularly valuable for defect analysis, such as disclinations, where line defects appear as topological singularities in the network morphology, often visualized as breaks or loops in the layered planes after selective to remove the liquid crystal component. In reflection mode, or dedicated reflectance microscopy maps color variations across cholesteric samples to visualize pitch gradients, where shifts in reflected produce a continuum of hues corresponding to local changes in helical periodicity. This technique is applied to samples with spatially varying , such as those induced by concentration gradients of chiral dopants, allowing quantitative correlation between color bands and pitch profiles through calibration with known reflection spectra. Dynamic textures in cholesteric liquid crystals are observed under during responses to external fields, where shear or induce unwinding of the , transitioning from colorful planar textures to darker, homeotropic alignments. In real-time PLM imaging, low cause progressive suppression of selective reflection, manifesting as fading colors and increased transmission, while shear flows align the axis parallel to the flow direction, producing transient fingerprint-like patterns that evolve with . These observations highlight the responsiveness of the cholesteric phase, with unwinding thresholds depending on field strength and pitch length.

Preparation Methods

Synthesis of Materials

Cholesteric liquid crystals often rely on cholesterol-derived mesogens, which are synthesized through esterification reactions. A common route involves the reaction of with acid chlorides, such as nonanoyl chloride, in the presence of a base like to form cholesteryl nonanoate, a prototypical cholesteric material exhibiting a Grandjean texture and selective reflection in the . This esterification proceeds via nucleophilic acyl substitution, where the hydroxyl group of attacks the carbonyl carbon of the acid chloride, yielding the ester linkage essential for mesophase stability; typical conditions include reflux in dry for 1-2 hours, followed by recrystallization from acetone to isolate the product with melting points indicating cholesteric (around 80°C) and isotropic (around 93°C) transitions. Variations extend to other derivatives, where esterification methods such as using EDC/DMAP with substituted benzoic acids are employed to tune the clearing and pitch length, as demonstrated in series of cholesteryl 4-alkoxybenzoates that maintain cholesteric phases over broad thermal ranges. Non-steroidal cholesteric mesogens, designed to mimic the helical organization without steroidal bulk, are typically constructed from rod-like cores appended with chiral tails. derivatives, such as those derived from 4-(4-alkoxybenzylideneamino) esters, are prepared by condensing p-alkoxyanilines with 4-formyls or their esters, followed by esterification with chiral alcohols like (S)-2-methylbutanol under Steglich conditions using dicyclohexylcarbodiimide (DCC) and (DMAP) in . These yield mesogens with induced cholesteric phases upon heating, featuring linkages that promote planarity and π-π interactions for nematic alignment twisted by the chiral center. Tolane-based mesogens, incorporating diphenylacetylene units for enhanced rigidity, are synthesized via of iodoarenes with terminal alkynes bearing chiral tails, such as (S)-2-methylbutyl groups attached through Williamson etherification; for instance, 4-ethynylphenyl (S)-2-methylbutyl is coupled to 4-iodobenzoate derivatives, resulting in compounds with helical twisting powers suitable for short-pitch cholesterics. These synthetic strategies allow fine-tuning of the dihedral angles and to achieve room-temperature cholesteric phases without the issues of steroidal analogs. Chiral dopants are crucial for inducing helicity in nematic hosts, with binaphthyl derivatives prepared by axial chirality exploitation of (S)- or (R)-2,2'-dihydroxy-1,1'-binaphthyl. The synthesis involves selective O-alkylation of the binaphthol hydroxyls using dibromoalkanes or dichloroalkynes in the presence of in refluxing or acetone, often with phase-transfer catalysis like 18-crown-6 to enhance yield; for example, 1,4-dibromobutane bridges the naphthols to form cyclic dopants with high . These exhibit helical twisting powers (HTP) exceeding 20 μm⁻¹, such as +65.8 μm⁻¹ for alkynyl-bridged variants in cyanobiphenyl hosts, attributed to the rigid atropisomeric core amplifying local twisting. Peptide-based dopants, leveraging helical secondary structures, are assembled via solution-phase coupling starting from protected like or , followed by attachment of mesogenic tails (e.g., cyanobiphenyl esters) to the N- or using coupling agents like or ; N-methylation improves , yielding dopants with HTP values from 13.5 to 24.6 μm⁻¹ in standard nematics. Optimization for high HTP (>20 μm⁻¹) focuses on short sequences (di- or tripeptides) with rigid linkers to minimize conformational flexibility while maximizing dopant-host compatibility. Purification of these materials is essential to eliminate impurities that disrupt mesophase purity and optical selectivity. Recrystallization from solvents like , acetone, or / mixtures is routinely applied to cholesterol esters and Schiff bases, leveraging differences in solubility to achieve >99% purity and sharp phase transitions, as confirmed by . on , using eluents such as / gradients, is preferred for binaphthyl and tolane derivatives to separate stereoisomers or reaction byproducts, often followed by (HPLC) for enantiopure dopants ensuring consistent HTP. These techniques collectively ensure high phase purity, critical for reproducible helical pitch and selective reflection in cholesteric formulations. Recent advances as of 2025 include the development of green synthesis routes, such as enzyme-catalyzed esterifications for derivatives and microfluidic systems for precise chiral mixing, improving and in cholesteric material production.

Fabrication Techniques

Fabrication techniques for cholesteric liquid crystals (CLCs) involve aligning and stabilizing the helical structure to achieve desired , such as selective reflection, while enabling practical applications in devices and films. These methods focus on inducing uniform textures, fixing the cholesteric phase against or mechanical perturbations, and engineering pitch variations for enhanced functionality. Common approaches include surface treatments for molecular orientation, networks for structural locking, encapsulation for flexibility, and formation for effects. Alignment methods are essential for directing CLC molecules into planar or homeotropic textures. Surface rubbing entails mechanically abrading a polyimide-coated substrate with a cloth to generate nanogrooves, which create anisotropic that orients LC directors parallel to the grooves for planar alignment. This technique is widely used for uniform textures in displays but can introduce defects in highly viscous phases. Photoalignment, a non-contact alternative, employs linearly polarized UV on photosensitive layers like cinnamate-based polymers to induce uniaxial anchoring via photodimerization, enabling precise patterning for complex CLC geometries. For homeotropic alignment, where directors are perpendicular to the substrate, are applied across thin films (typically <100 μm), leveraging to reorient molecules; this is often combined with surfactants for stability and is key for tunable optical responses in sensors. Polymer stabilization fixes the CLC helix through in situ photopolymerization, incorporating reactive monomers into the mixture prior to curing. Diacrylate monomers, such as RM82, are mixed at concentrations of 5-20 wt.% with the CLC host and a like Irgacure 369, then exposed to UV light (e.g., 365 nm at 100 mW/cm² for several minutes) to form a three-dimensional network. This anchors LC molecules, preventing helix unwinding or defects under external stimuli while preserving selective reflection; lower concentrations (~5-8 wt.%) suffice for minimal , whereas higher levels (~20 wt.%) enhance robustness for electro-optic switching. The resulting polymer-stabilized CLCs (PSCLCs) exhibit improved thermal and mechanical stability compared to pure CLCs. Microencapsulation encapsulates CLCs in shells via techniques to create robust, flexible structures. Emulsification disperses CLC droplets (1-100 μm in diameter) in a continuous aqueous phase using , followed by interfacial to form solid shells around the droplets, often with monomers like or precursors. This process, including bulk or microfluidic emulsification, yields monodisperse capsules that maintain the cholesteric order and color tunability while protecting against leakage and enabling integration into bendable films or textiles. Such microcapsules are particularly valuable for wearable and anti-counterfeiting applications due to their processability and . Broadband reflectors are fabricated by a pitch across the CLC film to extend the reflection bandwidth beyond the typical narrow range (<200 nm). Chiral induces this : for instance, stacking layers with varying concentrations and applying allows dopants to migrate, creating a continuous pitch variation fixed by , achieving reflections over 1000-2400 nm in the visible to near-infrared. Dual twisting combines cholesteric phases with handedness or twist grain boundary structures, often via temperature/light during , to superimpose multiple bands and yield ultrabroadband mirrors (e.g., 750-2500 nm). These methods, typically in PSCLCs, enable applications in energy-efficient coatings and .

Applications

Display and Sensing Devices

Cholesteric liquid crystals (ChLCs) are widely utilized in reflective displays owing to their bistable properties, which enable low-power operation suitable for electronic paper (e-paper) applications. In these devices, ChLCs exhibit two stable states: the planar texture, which selectively reflects light at a wavelength determined by the helical pitch, producing a bright image, and the focal conic texture, which scatters light minimally and appears dark. Switching between states is achieved via applied voltage pulses—a low-voltage pulse stabilizes the focal conic state for dark pixels, while a high-voltage pulse followed by a low-voltage stabilization yields the planar state for bright pixels—allowing static images to be maintained without continuous power consumption. This bistability, combined with high reflectivity (greater than 30%, up to 70% in advanced formulations) without backlighting, results in contrast ratios approximately 2.5 times higher than conventional ChLC displays, with pixel response times as fast as 10 ms and full-color page updates in 1-2 seconds. These displays are suitable for sunlight viewing and emit low blue light, enhancing eye comfort for prolonged use. They also demonstrate durability across a wide temperature range of -20 to 70°C. The materials for these displays typically involve polymer-stabilized ChLC mixtures with chiral dopants to control the pitch, ensuring operation in ambient for portable e-readers and . Drive schemes optimize switching to achieve wide viewing angles (up to 160°) through surface treatments and networks that prevent texture degradation over time, supporting multiyear stability without power. These features make ChLC-based e-paper energy-efficient, with no need for polarizers or color filters, and capable of full-color rendition by stacking three layers with varying pitches for red, green, and blue, enabling over 16 million colors and continuous gradation without dithering or color reduction. In sensing applications, ChLCs serve as thermochromic sensors by leveraging the temperature-dependent shift in helical pitch, which alters the selective reflection (dλ/dT). This property enables color changes across visible spectra, with sensitivities ranging from 18.9 to 71.2 nm/°C depending on the formulation and range, allowing real-time visual detection without external power. For instance, in medical , ChLC films embedded in arrays monitor by mapping distributions (34–38°C) with response times around 0.25 s, mimicking techniques for non-invasive assessment in remote healthcare settings. Strain sensors based on ChLCs detect mechanical deformation through pitch compression under stress, shifting the reflection band and producing observable color changes for real-time monitoring. A typical configuration involves a crosslinked ChLC network coated on a deformable substrate like ; uniaxial strain reduces the pitch via Poisson contraction, yielding wavelength shifts of about 40 nm for 13% strain, enabling high-resolution tracking of viscoplastic behavior in wearables. These sensors offer passive, optical readout without , ideal for in textiles or composites.

Smart Materials

Cholesteric liquid crystals (ChLCs) have emerged as key components in due to their ability to exhibit dynamic structural colors through selective reflection, which can respond to environmental stimuli such as , , mechanical strain, and structural deformation. These properties arise from the helical organization of the molecules, allowing the pitch of the to modulate in response to external changes, thereby shifting the reflected and producing visible color alterations. In applications like responsive films, ChLCs enable non-invasive monitoring without power sources, making them ideal for everyday smart integrations. Color-changing films based on ChLCs are particularly valuable for humidity and pH-responsive indicators in . For instance, polymerizable ChLC films demonstrate sensitive color shifts with increasing relative from 30% to 100%, where the helical pitch expands due to absorption, red-shifting the reflection band from to near-infrared. Similarly, films incorporating cellulose nanocrystals (CNCs), which self-assemble into cholesteric phases, respond to pH variations by altering the pitch through electrostatic repulsion changes on the nanocrystal surfaces; at low pH (e.g., 2-4), leads to aggregation and shifts, while higher pH (e.g., 10-12) causes expansion and red shifts, enabling visual detection of spoilage or in . These films offer irreversible or reversible responses depending on the formulation, providing low-cost, eco-friendly alternatives to chemical dyes. In smart textiles, woven ChLC fibers facilitate adaptive and strain sensing through mechanochromic effects. Robust ChLC fibers, when stretched up to 200% strain, exhibit reversible color changes from to ( shift of 155 nm) as the helical pitch compresses under tension, allowing integration into fabrics for dynamic patterning that mimics environmental surroundings. For adaptive , electrochromic ChLC-clad fibers woven into textiles enable rapid color tuning via low-voltage stimuli, achieving broadband across visible s without external wiring, suitable for or outdoor applications. These fibers maintain mechanical integrity during , supporting scalable production of responsive garments. Smart paints utilizing ChLC coatings provide optical feedback for by detecting cracks and deformations through reflection changes. ChLC (CLCE) coatings applied to surfaces like insulation panels visually indicate crack initiation and propagation; as cracks form, local strain compresses the helical , shifting the reflected color from to blue, enabling non-destructive assessment with high . Spray-deposited CLCE paints exhibit brilliant, tunable structural colors and mechanochromic responses under uniaxial stretching, with pitch-dependent shifts observable over the , ideal for infrastructure like bridges or where early damage detection prevents failures. ChLCs also inspire natural mimics, particularly in replicating the iridescent beetle shell structures for . The bouligand architecture of cuticles, featuring twisted plywood layers akin to cholesteric helices, produces angle-dependent circularly polarized colors; ChLC templates emulate this by self-organizing into helicoidal patterns during , transferring the structure to polymers or nanocomposites for durable, photonic coatings. These biomimetic templates achieve tailored with pitch gradients matching beetle shells (e.g., 200-400 nm for green-to-red shifts), enhancing applications in anti-counterfeiting or aesthetic surfaces while preserving the natural efficiency of manipulation.

Photonic and Emerging Uses

Cholesteric liquid crystals (ChLCs) serve as distributed feedback resonators in lasing applications due to their helical structure, which provides polarization-selective feedback and enables low-threshold operation around 1 kW/cm². These lasers exhibit tunability through external stimuli such as or mechanical strain, allowing shifts that leverage the pitch modulation of the for applications in optical communications and biomedical diagnostics. Post-2020 advances have included developments in terahertz and up-conversion lasing, enhancing their utility in high-resolution and by exploiting collective effects. In applications, ChLCs form one-dimensional structures with three-dimensional helical arrangements that generate photonic bandgaps, selectively reflecting circularly polarized light according to , where the central wavelength λ equals the average n times the helical pitch p times cos(θ), with θ as the incidence angle. These bandgaps, tunable from to wavelengths via pitch adjustments, enable light manipulation in filters and sensors. As actuators, ChLCs bend light through helix deformation induced by stimuli like strain or electric fields, producing color shifts—for instance, a shift from 695 nm to 494 nm under mechanical stretching—facilitating dynamic photonic devices such as adaptive and structural color displays. ChLCs act as helical templates for assembling nanoparticles into chiral metamaterials, guiding plasmonic gold nanoparticles or luminescent quantum dots via to transfer molecular . This process yields enhanced chiroptical properties, including with dissymmetry factors up to 0.12 for nanoparticles and circularly polarized with g-factors around 2 for CdSe/ZnS quantum dots, supporting applications in and chiral sensing. Emerging uses of ChLC elastomers in involve actuation through thermal, photothermal, or electrical stimuli, achieving strains up to 400% and response times as low as 0.1 seconds via in conductive fibers. These materials mimic for robotic grippers and shape-morphing structures, with 2024 developments focusing on hybrid composites for multi-directional motion and biomimetic systems. Additionally, ChLC microdroplets function as microsensors for biomedical imaging by detecting amphiphiles like fatty acids at concentrations from 10 nM, altering helical orientation to produce observable color changes for applications such as monitoring in models.

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