#cholesteric

Geometric phase-encoded liquid crystal optical sensing

Sensing technology, integral to environmental monitoring, data acquisition, and precision data processing, is evolving rapidly. Researchers are at the forefront of developing swift, accessible, and cost-effective sensors. Among these innovations, cholesteric liquid crystals (CLCs) in stimulus-responsive photonic crystals exhibit exceptional promise. Their unique helical structure and photonic properties enable the production of vivid, power-independent structural colors, paving the way for advanced visual analysis tools. However, a significant challenge hinders CLC's broader application in optical sensing: Although they visibly alter color in response to stimuli, accurately gauging these changes necessitates costly spectroscopic equipment, constraining their practical deployment. Responding to the growing need for compact and planar optical elements, researchers have investigated Pancharatnam-Berry geometric phases, derived from light's spin-orbit interactions. Recent developments include integrating the geometric phase into reflected light via CLC helical superstructures, leading to novel photonic applications.

Optical indicators based on cholesteric liquid crystal polymers

Optical indicators based colored polymers are autonomous responsive labels that provide an optical signal to represent a specific exposure over time. Polymers that possess a high degree of order can establish structural color, which originates from the interaction of light with a periodic nanostructure, causing a specific wavelength to be reflected. These optical properties can be used for the fabrication of battery-free indicators that show color changes upon exposure to a stimulus.

For his Ph.D. research, Ir. Yari Foelen, explored novel features and response modes for chemical indicators and time-temperature integrators based on cholesteric liquid crystal polymers. These optical indicators can serve as detection systems for unsafe conditions or as an alternative for static expiration dates for food and pharmaceutical products.

Speaking in colors

Is there any signal that is easy to interpret than color? Our eyes intuitively perceive optical signals and subconsciously associate specific feelings or interpretations to different colors. Traffic lights demonstrate how universally effective the use of color as a signal is. The next generation of trackers and indicators are non-electronic and therefore made of materials that deliver an autonomous response to a stimulus. One of these materials are cholesteric liquid crystal polymers, which offer an unparalleled versatility to design a responsive indicator.

Liquid crystals are not solid, but some of their physical properties are directional -- like in a crystal. This is because their molecules can arrange themselves into certain patterns. The best-known applications include flat screens and digital displays. They are based on pixels of liquid crystals whose optical properties can be switched by electric fields.

Some liquid crystals form the so-called cholesteric phases: the molecules self-assemble into helical structures, which are characterised by pitch and rotate either to the right or to the left. "The pitch of the cholesteric spirals determines how quickly they react to an applied electric field," explains Dr. Alevtina Smekhova, physicist at HZB and first author of the study, which has now been published in Soft Matter.

Simple molecular chain

In this work, she and partners from the Academies of Sciences in Prague, Moscow and Chernogolovka investigated a liquid crystalline cholesteric compound called EZL10/10, developed in Prague. "Such cholesteric phases are usually formed by molecules with several chiral centres, but here the molecule has only one chiral centre," explains Dr. Smekhova. It is a simple molecular chain with one lactate unit.

Self-assembly of responsive photonic biobased materials in liquid marbles

A surface displays structural colors when light is reflected by tiny, regular structural elements in a transparent material. Researchers have now developed a method to make structural colors from cellulose-based polymers by using coated droplets that exist in a surrounding fluid—so-called liquid marbles. The system readily responds to environmental changes, which makes it interesting for applications in bio-based sensors and soft photonic elements, according to the study published in the journal Angewandte Chemie.

Structural colors are a way to colorize a material without using a dye. Instead, the transparent material generates color through the regular arrangement of its molecules or other elements, as seen, for instance, in the ripples in the scales of colorful fish and butterflies, or in nanocrystals arranged at certain distances, as in the color-changing skin of chameleons.

Manos Anyfantakis and colleagues at the University of Luxembourg have identified a means to control the pitch, the distance of a full helical turn in a polymer, as a structural element on which reflection might occur and structural colors appear. Scientists can prepare liquid crystalline phases of biopolymers with pitches generating structural colors—called cholesteric phases—but these preparations depend on many parameters and need a long time to reach equilibrium.

Self-assembly of responsive photonic biobased materials in liquid marbles

A surface displays structural colors when light is reflected by tiny, regular structural elements in a transparent material. Researchers have now developed a method to make structural colors from cellulose-based polymers by using coated droplets that exist in a surrounding fluid -- so-called liquid marbles. The system readily responds to environmental changes, which makes it interesting for applications in bio-based sensors and soft photonic elements, according to the study published in the journal Angewandte Chemie.

Structural colors are a way to colorize a material without using a dye. Instead, the transparent material generates color through the regular arrangement of its molecules or other elements, as seen, for instance, in the ripples in the scales of colorful fish and butterflies, or in nanocrystals arranged at certain distances, as in the color-changing skin of chameleons.

Manos Anyfantakis and colleagues at the University of Luxembourg have identified a means to control the pitch, the distance of a full helical turn in a polymer, as a structural element on which reflection might occur and structural colors appear. Scientists can prepare liquid crystalline phases of biopolymers with pitches generating structural colors -- called cholesteric phases -- but these preparations depend on many parameters and need a long time to reach equilibrium.

Now, Anyfantakis and colleagues have discovered a faster and better controllable method, using liquid marbles as a platform for the controlled self-assembly of biopolymer-based structural colors. Liquid marbles are millimeter-sized droplets of liquid crystalline solutions, which are coated with nanoparticles. The coating protects the liquid from mixing with the outside fluid, but still allows for some interaction, dependent on the nature of both liquids.