Reduction of solar infrared heating by using highly transparent thin films based on organic chiral nematic liquid crystal polymer

On the isomeric purity of endcap molecules in cholesteric liquid crystal oligomers for near-infrared thermochromic coatings

Structurally coloured responsive materials provide an interesting avenue for the development of autonomous temperature regulating window films. One interesting class of such thermochromic materials is cholesteric liquid crystals. However, cholesteric liquid crystals have rarely been applied in coatings for smart window applications. In this work, we report the synthesis of endcapped cholesteric liquid crystal oligomers and its application as near-infrared thermochromic coatings for windows. Two isomerically pure monoacrylate endcapping molecules and its isomeric mixture are synthesised. The molecules are used to synthesise a variety of endcapped cholesteric liquid crystal oligomers to study the effect of the isomeric purity on the thermochromic properties of the coatings. It is found that while the oligomers are almost identical in composition and phase behaviour, only one isomer produces a clear transparent coating, highlighting the significance of minute isomeric differences. Remarkably, the thermochromic behaviour of the coatings for all oligomers is the same. The best performing oligomer is able to reversibly blueshift by 250 nanometres when heated from room temperature to 100°C, opening the way of cholesteric liquid crystals for use in temperature regulating window films.

Impact of Endcap Molecules on Temperature-Responsive Cholesteric Liquid Crystal Oligomers in Structural Color Stability and Hypsochromic Shift

Cholesteric liquid crystal oligomers are an interesting class of temperature responsive structurally colored materials. However, the role of endcap molecules in these oligomers is rather unexplored. In this work, we demonstrate the role of endcap molecules on structural color stability and hypsochromic shift in temperature-responsive cholesteric liquid crystal oligomers. First, new liquid crystal monoacrylate endcap molecules are synthesized, which are then used to synthesize various cholesteric liquid crystal oligomers. In addition, cholesteric oligomers using commercial monoacrylate endcap molecules are also prepared. It is found that the molecular weight and the polydispersity of the oligomers can be tuned by the endcapping molecules. The oligomers are used to produce reflective, structurally colored coatings. It was found that the coatings using the commercial monoacrylate lose their color and crystallize over time, most likely due to the presence of crystalline dimers. The coatings containing the newly synthesized monoacrylate endcap molecules did not exhibit this crystallization, resulting in structurally colored coatings that remained stable over time. These latter coatings possessed temperature responsive hypochromic behavior, which makes them interesting for advanced optical applications.

Design and fabrication of a simple and cost-effective optical flow meter using liquid crystals and textile grid

The measurement of airflow velocity is crucial in various fields, and several sensing approaches have been developed for detecting airflow, including optical fiber-based flowmeters. However, these sensors often require complex fabrication processes and precise optical alignment. In this paper, a simpler and more cost-effective approach has been used to measure air flow rate by utilizing the birefringence property of liquid crystals (LCs). LCs possess distinct optical characteristics, and their reorientation due to airflow can be detected by observing the intensity of the output light between crossed polarizers. The novelty of this study is the utilization of a textile grid to hold the LC layer, which simplifies the fabrication process. This LC-based gas flowmeter offers a simple, low-cost setup and provides rapid performance. This research presents what we believe to be a new approach to calculate airflow by exploiting the optical properties of LCs, which is a new frontier in gas flow measurement. The proposed airflow meter is capable of detecting airflow rates ranging from 0 l/min to 7.5 l/min with an accuracy of 0.5 l/min. It exhibits a stable response time in 75 seconds, and the sensor maintains acceptable stability over time.

Optimizing liquid crystal cell thickness in electro-optical Fresnel lenses through theoretical calculations and experimental validation

Tunable liquid crystal (LC) lenses have gained significant attention in recent decades due to their lightweight, low cost, and versatility in applications such as augmented reality, ophthalmic devices, and astronomy. Although various structures have been proposed to improve the performance of LC lenses, the thickness of the LC cell is a critical design parameter that is often reported without sufficient justification. While increasing the cell thickness can lead to a shorter focal length, it also results in higher material response times and light scattering. To address this issue, the Fresnel structure has been introduced as a solution to achieve a higher focal length dynamic range without increasing the cell thickness. In this study, we numerically investigate, for the first time (to our knowledge) the relationship between the number of phase resets and the minimum required cell thickness to achieve a Fresnel phase profile. Our findings reveal that the diffraction efficiency (DE) of a Fresnel lens also depends on the cell thickness. Specifically, to achieve a fast response Fresnel-structured-based LC lens with high optical transmission and over 90% DE using E7 as the LC material, the cell thickness should fall within the range of 13 to 23 µm.

Fabrication of a spatially tunable band reject filter with a very wide tunability range based on a chiral nematic liquid crystal polymer

A thin, waterproof, and stable spatially tunable band reject filter is fabricated based on a chiral nematic liquid crystal polymer. The fabrication method for this filter is new, to the best of our knowledge, and straightforward. The photonic bandgap (PBG) of the proposed filter can be tuned from 350 nm to 760 nm by a mechanical movement of 6.5 mm. The filter reflects almost 50% of unpolarized incident light in the PBG and remains practically transparent for other wavelengths. The filter remains stable for four years and has acceptable resistance to polar protic solvents and thermal stability up to 90°C. The filter can be detached from the glass substrates, to be used as a thin 8-µm free-standing film or to be attached to a flexible substrate. This spatial tunable band reject filter may be used in displays, optical devices, and optical communication.