Color Space Transformation-Based Algorithm for Evaluation of Thermochromic Behavior of Cholesteric Liquid Crystals Using Polarized Light Microscopy

Abstract

Cholesteryl ester liquid crystals exhibit thermochromic properties related to the existence of a twisted nematic phase. When used in applications such as thermal mapping, a color change is often monitored by video cameras. Thus, quantitative methods to evaluate thermochromic behavior (e.g., blue-start, red-start, red-end, color play and bandwidth) from video analysis are desirable. However, obtaining quantitative color measurements from digital images remains a significant technical challenge, especially for highly reflective samples such as liquid crystals (for which ultraviolet−visible (UV−vis) reflectance spectroscopy is typically used). We developed a method to determine thermochromic properties from videos of liquid crystal cooling under polarized light microscopy. We relate observed color transitions to quantifiable changes in the cumulative color difference in the International Commission on Illumination (CIE) L*a*b* color space and validate this method with UV−vis reflectance spectroscopy. The measured thermochromic behavior and associated measurement uncertainties (coefficient of variations) were comparable to UV−vis reflectance measurements.

Thermochromic Fibers via Electrospinning

“…Polarized light microscopy was performed on the 20 wt.% PS, 20 wt.% LC-2 fibers and compared to 20 wt.% PS only fibers (Figure 7). Portions of the fiber containing LC-2 appear blue which are not apparent in the PS only fiber suggesting the successful incorporation of the liquid crystal as the mesophase transition temperature associated with the thermochromic behavior is reported to be around 27 • C [41]. However, no color change was observed upon heating the stage to 50 • C; the liquid crystal is expected to change color between 26-30 • C [41].…”

“…Portions of the fiber containing LC-2 appear blue which are not apparent in the PS only fiber suggesting the successful incorporation of the liquid crystal as the mesophase transition temperature associated with the thermochromic behavior is reported to be around 27 • C [41]. However, no color change was observed upon heating the stage to 50 • C; the liquid crystal is expected to change color between 26-30 • C [41]. We note, upon heating the stage from room temperature to~50 • C, there was evidence of a phase change in the fibers containing LC representative data from fibers containing LC-1 are shown in Figure S4 whereas no change was observed in the PS only fibers ( Figure S5).…”

Smart Supramolecular Self-Assembled Nanosystem: Stimulus-Responsive Hydrogen-Bonded Liquid Crystals

“…Cholesteric LCs (CLCs) possess outstanding reflection characteristics and are widely used in colorimetric sensors and display devices [ 115 , 116 , 117 , 118 ]. This characteristic comes from changes of helical pitch ( p ), namely, uniformly periodical layer deformations.…”

Thermochromic Microcapsules Containing Chiral Mesogens Enclosed by Hydrogel Shell for Colorimetric Temperature Reporters

“…Moreover, the cholesteric phase is less sensitive to the compositions, which renders the chiral mesogens easier for processing and useful for various optical applications. [ 41,42 ] However, there has been no systematic development on elaborate encapsulation technology for the chiral mesogens; only bulk encapsulation technique is available, which provides uncontrolled size and composition. [ 43–46 ]…”

“…Moreover, the cholesteric phase is less sensitive to the compositions, which renders the chiral mesogens easier for processing and useful for various optical applications. [41,42] However, there has been no systematic development on elaborate encapsulation technology for the chiral mesogens; only bulk encapsulation technique is available, which provides uncontrolled size and composition. [43][44][45][46] In this work, we present a microfluidic approach to produce photonic microcapsules composed of a cholesteryl ester core and an alginate hydrogel shell and the use of the microcapsules as colorimetric temperature reporters.…”