Dr. Xiao Li, Assistant Professor in Materials Science and Engineering Department at University of North Texas will give a seminar titled "Liquid Crystals Self-Assembly and Topological Defects for Functional Material Design" to the interested faculty and students at Discovery Park.

Abstract

Chiral nematic or cholesteric phases in liquid crystals (LCs) exhibit asymmetrical packing of molecules, resulting in a finite twist angle between adjacent molecules, and long-range chiral ordering similar to helical superstructures in DNA. The increase in chirality leads to the formation of three-dimensional (3D) cubic symmetry known as blue phases (BPs)1,2, consisting of double twisted cylinders (DTCs). The chiral properties of LCs3,4 (from nano to micrometers) are widely used in display technologies, electro-optics, and sensors. However, exploiting the full potential of such beneficial chiral structures to adapt to diverse engineering conditions, e.g., temperature, stress, and chemical environments, needs to overcome the inherently weak mechanistic nature of LCs. In this seminar, I will present directed self-assembled hierarchical helix structure on chemical patterned surface. By increasing chirality and confining on an alternate patterned substrate of homeotropic and planar anchoring stripes, the cholesteric phase generates hierarchical helix structure, which is in pairs of left-handed and right-handed twisted band as a rope like structure. The morphology exhibits change in colors by temperature due to the slight change in helical pitch. It mimics natural Bouligand structures, with long-range order helical hierarchies to exhibit excellent mechanical properties including fracture resistance, crack orientation insensitivity, and damage resistance. To facilitate the soft materials to withstand substantial mechanical deformation, sequential infiltration synthesis5 based on atomic layer deposition tool is applied to liquid crystal formed structural template. The resulting hybrid or fully inorganic framework are expected to realize many potential applications beyond scope of traditional soft material. Furthermore, by combining liquid crystals and elastomeric polymers, liquid crystal elastomers (LCEs)6 are created to include both inherit the elastic, deformable nature of polymers and the directional, responsive behavior of liquid crystals. The incorporation of homopolymers into LCEs facilitated porous structure in LCEs, which exhibit reduced density, improved processability, and increased surface area for interaction with external stimuli, thereby enhancing their actuation performance. Based on different LC phases, including nematic, chiral nematic, and blue phase, various new structures across multi length scale are created for optical devices, sensors, and microactuator designs.

Bio

Xiao Li is an Assistant Professor in Materials Science and Engineering Department at University of North Texas. Prior to joining UNT in August 2019, She received her Ph.D. in Polymer Chemistry and Physics from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. She did postdoctoral research in Chemical & Biological Engineering at University of Wisconsin-Madison (2012-2013), and Pritzker School of Molecular Engineering at University of Chicago (2013-2015). She was also a research scientist at University of Chicago (2015-2019), and a visiting associate at Argonne National Laboratory (2015-2019). Her research is based on soft materials, including polymers, liquid crystals, elastomers and colloidal particle, and aims to develop a wide range of applications for sensors, optoelectronics, optical coatings, micro-actuators, etc. Her research focuses on understanding the fundamental thermodynamics and hydrodynamics of organic/inorganic species within multidimensional soft matter templates, to reveal the underlying structure-property relationships. Dr. Li is a recipient of the NSF CAREER award (2024), and received funding supports from Army Research Office and ACS Petroleum Reasech Fund.