Visualizing Invisible Phase Transitions in Blue Phase Liquid Crystals Using Early Warning Indicators

Abstract

In materials science, critical transitions are manifest as phase transitions, [15,16] in which the ordering of the constituent atoms or molecules changes. The study of EWSs with regard to phase transitions is, however, less common than in other fields, possibly because phase transitions are usually induced by changing control parameters such as temperature or external fields, and the changes in physical properties are relatively easy to detect using suitable approaches. [17][18][19] However, certain kinds of phase transitions produce only a subtle change in physical properties, making their detection, let alone spatial mapping, a challenging task. Phase transitions of optically isotropic liquid crystals (LCs) are one example; a blue phase (BP) LC is a phase in which the constituent molecules self-assemble into a 3D structure with body-centered cubic (bcc) or simple cubic (sc) symmetry, known as BP-I or BP-II, respectively. [20][21][22][23] Upon cooling, they show a BP-II to BP-I phase transition in addition to an isotropic to BP-II transition. The self-organized periodic structure gives rise to Bragg reflections at wavelengths related to the crystallographic Miller planes, but the cubic symmetry renders the crystals otherwise pseudo-isotropic. While their complex 3D structure has attracted interest as tunable photonic crystals, their most attractive feature is their electro-optical response, which is approximately one order of magnitude faster than that for standard nematic LCs used in displays. [24] However, a challenge arises since, for electro-optical applications of BPLCs, the Bragg peaks must be in the ultraviolet region, making all phases optically isotropic and hence transparent to visible light. Recent studies have shown that transmission electron microscopy and small-angle X-ray diffraction can be utilized to study the microscopic domain structure of different BPs; [25] however, such methods are destructive and can only provide limited information on the temporal evolution of phase transitions. Developing a non-destructive technique to observe optically isotropic samples with temporal and spatial resolution is therefore of fundamental and practical significance.In this work, we demonstrate non-destructive spatial mapping of phase transitions in featureless BPLCs using statistical indicators obtained from time-resolved optical microscopy. Schematic diagram of this work is represented in Figure 1. We obtain time-series images at intervals of 20 ms by timing synchronization of camera trigger and temperature control, [26] predominantly containing light scattering signals from the Changes in the statistical properties of data as a system approaches a critical transition is studied intensively as early warning signals, but their application to materials science, where phase transitions-a type of critical transitionare of fundamental importance, are limited. Here, a critical transition analysis is applied to time-series data from a microscopic 3D ordered soft materialblue phase liquid crystals (BPLC)-and demons...

Polymer networks which locally rotate to accommodate stresses, torques, and deformation

“…This suggests it may be possible to validate, via careful experimentation, which macro-to-micro kinematic assumptions are more or less physically accurate for a given polymer network; in other words, the inherent physical phenomena may be isolated from the fitting of model parameters. While in some cases it may be difficult to directly discern or differentiate stable phases of a multistable polymer network, it may be possible to leverage new statistical techniques inspired by work on early warning signals to aid in the measurement of phases [66].…”

Spontaneous formation of liquid-crystalline nuclei in blue-phase liquid crystals based on different chirality

“…Although BPIII is without lattice symmetry, the phase transitions of BPLCs involve Iso-BPI, Iso-BPII, and BPII-BPI, which were reported several decades ago. [18][19][20][21] The properties of BPs in these phase transitions have been reported from many aspects, including lattice symmetry, 22 phase diagrams, [23][24][25] Bragg reflectivity, 26 and lattice orientations. 27,28 When the phase transition changes from Iso-BPI to Iso-BPII as chirality increases, BPI and BPII emerge simultaneously between these two processes with a small chiral range.…”

Self-assembled liquid crystal architectures for soft matter photonics

“… 96 , with permission from Wiley-VCH), cholesteric phase LCs 296 , 297 (Reproduced from refs. 296 , 297 , with permissions from Wiley-VCH), blue phase LCs 144 , 298 , 299 (Reproduced from refs. 298 , 299 , with permissions from Wiley-VCH.…”

“… 296 , 297 , with permissions from Wiley-VCH), blue phase LCs 144 , 298 , 299 (Reproduced from refs. 298 , 299 , with permissions from Wiley-VCH. Reproduced from ref.…”