Mesoscale martensitic transformation in single crystals of topological defects

Abstract

Liquid-crystal blue phases (BPs) are highly ordered at two levels. Molecules exhibit orientational order at nanometer length scales, while chirality leads to ordered arrays of double-twisted cylinders over micrometer scales. Past studies of polycrystalline BPs were challenged by the existence of grain boundaries between randomly oriented crystalline nanodomains. Here, the nucleation of BPs is controlled with precision by relying on chemically nanopatterned surfaces, leading to macroscopic single-crystal BP specimens where the dynamics of mesocrystal formation can be directly observed. Theory and experiments show that transitions between two BPs having a different network structure proceed through local reorganization of the crystalline array, without diffusion of the doubletwisted cylinders. In solid crystals, martensitic transformations between crystal structures involve the concerted motion of a few atoms, without diffusion. The transformation between BPs, where crystal features arise in the submicron regime, is found to be martensitic in nature when one considers the collective behavior of the double-twist cylinders. Single-crystal BPs are shown to offer fertile grounds for the study of directed crystal nucleation and the controlled growth of soft matter.T here is considerable interest in understanding at a fundamental level the processes through which crystals are nucleated, and how they grow and transform between different lattice symmetries. Among crystal-crystal transformations, martensitic transitions are of particular interest, as they involve a collective and diffusionless atomic rearrangement that occurs at velocities comparable to the speed of sound in the material (1). Here, we use liquid crystals (LCs) that exhibit long-range order and crystalline symmetries with lattice constants in the submicron regime, the socalled blue phases (BPs), to study a liquid analog of a crystalcrystal transformation. BPs are thermodynamically stable liquid crystalline states that occur in a narrow range of temperatures between the cholesteric (Chol) and isotropic (I) phases of chiral LCs. In BPs, the molecules are locally oriented and organized in such a way as to form structures known as double-twist cylinders. These cylinders, which arise from a balance of long-range elastic distortions and short-range enthalpic contributions to the free energy, adopt a cubic lattice structure, which is interdispersed by an ordered network of topological defects (disclination lines). Depending on the thermodynamic conditions, the double-twist cylinders adopt a body-centered cubic structure (BCC), which produces the so-called blue phase I (or BPI), or they adopt a simple cubic structure (SC), which corresponds to the so-called blue phase II (or BPII). In both cases, the lattice parameters are on the order of a few hundred nanometers (2-4). BPs exhibit selective light reflections; they have fast electro-optical switching characteristics, with submillisecond response times, and their viscosity is relatively large (3,(5)(6)(7)(8). This c...

Sculpted grain boundaries in soft crystals

“…In addition, by successfully fabricating BP single crystals, we have been able to analyze soft crystal nucleation and growth on chemically patterned substrates at substantial larger length scales than of atomic crystals. This allowed the observation that the BPII-BPI phase transformation assisted by a chemically patterned surface is martensitic in nature, is therefore directly analogous to classic diffusionless phase transformations found in solid crystals (13), and enriches the understanding of soft crystal crystallization and growth. To further explore BP liquid crystals as mesoscale and soft matter analogs of atomic crystals in the context of grain boundaries, in this work, a "soft heteroepitaxy" strategy, based on the accurate design and implementation of lithographically patterned chemical surfaces, is introduced to manipulate the orientation of BPs.…”

“…The DTC array is interspersed by a well-defined network of liquid crystalline line defects, the lattice parameters of which are on the submicron scale for both BPI and BPII. Recent work has shown that, in the absence of grain boundaries, the crystal-crystal transition from BPI to the BPII can be extremely fast and can be understood as the liquid analog of a classic martensitic transformation (13). Because of the length scale of the lattice parameters, BPs exhibit Bragg reflections for visible light.…”

“…For the chiral liquid crystal considered in our experiments, the BPs can be obtained either upon heating or cooling with the following phase transition temperatures: T Chol-BPI = 39.9° ± 0.05°C, T BPI-BPII = 40.7° ± 0.05°C, and T BPII-Iso = 42.9° ± 0.05°C, where Chol denotes the cholesteric phase and Iso denotes the isotropic phase (13,19). For BPII, the unit cell size is a BPII = 150 nm, and light is reflected by the (100), (110), and (111) planes at the peak wavelengths  (100) = 450 nm (visible),  (110) = 320 nm (ultraviolet), and  (111) = 260 nm (ultraviolet), respectively.…”

Polymer‐Stabilized Monodomain Blue Phase Diffraction Grating

“…However, when a sample cell is filled with the BPLC with a well‐treated surface, the orientation of all the lattices is in a particular direction and single color appears (Figure 1b). [ 30,36 ]…”

Surface-patterning generated half-skyrmion lattices in cholesteric blue phase thin films

“…On a patterned surface, interactions between the surface and the liquid crystal have different forms depending on the regions, thus forming a specific pattern. Aided by patterned confining surfaces, the BPI-BPII transformation was found to be martensitic-like, taking place in a diffusionless manner [33]. It is reasonable to expect that, specifically designed patterned confining surfaces can help discover and stabilise various complex BP structures that could be relevant to optical and photonic applications.…”