A pinboard by
Xiao Li

I am a research scientist from Institute for Molecular Engineering, University of Chicago.


Directed Self-Assembly of Blue Phases Single Crystal By Chemically Patterned Surfaces

Chiral nematic liquid crystals are known to form blue phases –liquid states of matter that exhibit cubic crystalline structures of topological defects. These phases reflect light selectively, and exhibit fast response times to external cues, making them of interest for emerging electro-optical technologies. Blue-phase specimens, however, are generally polycrystalline and consist of randomly oriented domains. The result is a texture that limits their performance for potential applications. In this work, a strategy is presented for preparation of stable, macroscopic single-crystal blue phase materials. This strategy relies on nano-patterned substrates whose precise characteristics are designed on the basis of field-theoretic calculations. Different template designs are conceived and engineered to exert optimal control over different planes of the blue-phase lattice orientation with respect to the underlying substrate. Experiments are used to demonstrate that it is indeed possible to create stable monocrystalline blue-phase domains having a desired orientation over arbitrarily large, macroscopic areas. These results provide an avenue that might permit full exploitation of the structural and electro-optical properties of blue-phases, which, until now, has been hindered by polydomain structures and an abundance of grain boundaries.


Directed self-assembly of nematic liquid crystals on chemically patterned surfaces: morphological states and transitions

Abstract: The morphology and through-film optical properties of nematic liquid crystals (LCs) confined between two surfaces may be engineered to create switches that respond to external electric fields, thereby enabling applications in optoelectronics that require fast responses and low power. Interfacial properties between the confining surfaces and the LC play a central role in device design and performance. Here we investigate the morphology of LCs confined in hybrid cells with a top surface that exhibits uniform homeotropic anchoring and a bottom surface that is chemically patterned with sub-micron and micron- wide planar anchoring stripes in a background of homeotropic anchoring. In a departure from past work, we first investigate isolated stripes, as opposed to dense periodic arrays of stripes, thereby allowing for an in-depth interpretation of the effects of patterning on LC morphology. We observe three LC morphologies and sharp transitions between them as a function of stripe width in the submicron and micron regimes. Numerical simulations and theory help explain the roles of anchoring energy, elastic deformation, entropy, pattern geometry, and coherence length of the LC in the experimentally observed behavior. The knowledge and models developed from an analysis of results generated on isolated features are then used to design dense patterned substrates for high-contrast and efficient orientational switching of LCs in response to applied fields.

Pub.: 05 Sep '16, Pinned: 29 Jun '17