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Science Seminar

Directed Self-Assembly of Soft Materials - Liquid Crystals by Chemically Patterned Surfaces
Xiao Li
Material Science and Engineering Department, University of North Texas
Liquid crystals (LCs) are a state matter intermediate between the solid and liquid phases. The unique properties of LCs, such as inherent ordering as liquid phase, optical anisotropy, the ability to response to an external field, make them be widely used for electronic displays, laser devices, photonics, biosensors, metastructures, etc. Fundamental understanding of the morphology and through-film optical properties of LC system, as well as precisely controlling the orientation of LC molecules towards contacting surfaces, are therefore play the central roles in device design and performance. In this seminar, I will present a generalizable platform based on anchoring contrast from chemically patterned surfaces to directed self-assemble (DSA) LC system. I will start with an investigation on the morphology of LCs under confined system to have an in-depth interpretation of three LC morphologies and sharp transitions as a function of strip width. On the basis of such fundamental understanding, LC topological defect structures by multiple director orientations, are created through rational design of chemical patterns on 2D surfaces, thereby providing new opportunities for the assembly of colloidal particles through defect-particle interactions over large areas. Furthermore, chirality is induced into the LC system to expect more complex liquid crystalline morphologies and behaviors. More specifically, I will focus on blue phase liquid crystals (BPLCs), which exhibit ordered cubic arrangements of topological defects. The highly ordered morphology of BPs gives rise to unusual physical properties, such as Bragg reflection of visible light and fast optical response. However, polycrystalline structures limit their performance in applications. Chemically patterned surfaces are presented for the first time to obtain stable, lattice selective, macroscopic single-crystal BP materials. By studying the chemical pattern assisted heterogeneous crystal nucleation and growth process of BPs, the transformation between BPs is found to be martensitic in nature as the result of the collective behavior of the double-twist-cylinders.