What are "Blue Phase" liquid crystals?
Cholesteric blue phase liquid crystals (BPLCs) are chiral liquid crystals with unique cubic structures that have optical isotropy and very fast response times to electrical fields, making them interesting candidates for display and other electro-optical applications. Their structure is characterized by a local director field that forms double-twisted supramolecular helicoids (Figure 1).
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Since the double twisted arrangement cannot perfectly occupy a 3D space, line defects known as twist disclinations are formed, as shown in Figure 2. In other words, the blue phase spontaneously forms an ordered defect structure.
The three different structures of BP are characterized by decreasing temperature: these are denoted as BP III, BP II and BP I. BP I self-arranges into body-centered cubic (BCC) and BP II into simple cubic (SC). The “blue fog” (BP III) has an isotropic symmetry and reflects light in a broad manner, which can be difficult to detect due to its extremely narrow temperature range (~0.1 â—¦C). BP I and BP II appear as multi-color platelets under the polarized microscope due to optical Bragg reflection
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Inorganic Nanoparticles in Blue Phase Liquid Crystals
Nanoparticles or polymers are added to BPLCs to increase the BP temperature range by filling up the high energy defect lines. Inorganic nanoparticles can also lower the threshold voltage for display applications.
Using synchrotron small angle X-ray scattering (SAXS), we discovered that in the case of 4 nm gold nanoparticles, the blue phase spontaneously and reversibly templates the formation of giant cubic crystals of nanoparticles throughout the entire sample !! Furthermore the gold nanoparticles form different cubic lattices in BP I and BP II and reversibly switch between these lattices with temperature.
This unexpected result was contrary to theoretical predictions that nanoparticles are too small to be trapped at specific sites within the lattice of defect lines. They are indeed mobile but keep going in and out of the same robust trapping sites. As more NPs are added, the Bragg peaks intensities increase but remain sharp.
Current Blue Phase Liquid Crystal Projects
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How the NP assembly occurs remains a mystery. There are many papers on adding NPs to BPLCs but apart from our work, no one has characterized the NP spatial distribution.
Zhang, et al.. (2021). Polymer Functionalized Nanoparticles in Blue Phase LC: Effect of Particle Shape.
Nanomaterials, 12(1), 91.
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We will explore both the NP lattice formation and its effect on the BPLC properties by varying the nanoparticle size and shape as well as by applying electric fields to distort the BPLC lattice and its optical bandgap.