Topology Driven Flows in Chromonic Liquid Crystals

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical and computational research, and education on defects in liquid crystals. Liquid crystals are made of rod like molecules that display complex spatial arrangements, though intermediate between perfectly ordered crystals and completely disordered liquids. Their combined ability to flow like a fluid, while transmitting directional forces and possessing direction dependent properties like a crystal, make them suitable for a wide range of applications. However, because of their weak crystallinity, defects are abundant in laboratory samples. Although this property has long been considered deleterious for applications, recent research has uncovered a host of new and unexpected properties of these materials that follow precisely from the existence of defects, and from their manipulation. Examples include guided transport of other molecular species or biological agents, the development of mechanical switches activated by light or electric fields, and even analogies involving biological response. This project is aimed to improve existing theoretical models that describe defected liquid crystals, and any accompanying transport. New models detailing the structure and motion of defects will be developed, including their interaction with transport of mass or fluid flow. The aim is to develop a predictive framework that describes the response of the liquid crystal to operating conditions or external influences. The specific focus of the research is on lyotropic chromonic liquid crystals, substances in which the liquid crystalline behavior is induced by using an appropriate dilution solvent to change the concentration of the rod like molecules. They have been known for a long time as dyes (including food dyes), and are analogs of lung surfactant agents. They are also bio-compatible liquid crystals, and are currently being developed for live cell steering or sorting. This class of liquid crystals features unusually large defect features that make them amenable to optical analysis, and comparison to theory. The research will be used to further develop and enhance an interdisciplinary course on computation in the physical sciences. Students participating in the project will benefit from substantial training and collaboration opportunities at the Minnesota Supercomputing Institute. TECHNICAL SUMMARY This award supports theoretical and computational research, and education focused on the systematic study of topological defects, equilibrium morphologies, and dynamical evolution of nematic-isotropic interfaces in lyotropic chromonic liquid crystals. Configurations considered include a pure nematic phase, or the biphasic region of coexistence between nematic and isotropic phases. This study is motivated by ongoing experiments that show that the characteristic microscopic scale of chromonics is on the order of microns, much larger than the 10 nm size in conventional thermotropic liquid crystals. Such large scales allow modern optical imaging techniques to resolve defect cores and two-phase interfaces, and hence to extract the parameters of mesoscopic free energy models down to an unprecedented spatial scale. This information opens the door to critical and quantitative tests of current nonlinear, gradient theories of nonequilibrium for nematics. In addition, the unusual morphology of nematic domains (tactoids), and their nonequilibrium evolution, are expected to lead to novel behavior due to the complexity of interactions at this scale, including strong anisotropy, nonlocal elastic interactions due to topological defects, and mesocale biaxiality. Defect driven flows and the dynamics of biological matter suspended in the chromonic are among planned investigations. These are under active scrutiny because of potential applications in flow control in microfluidics and optobiological devices, in the biomedical field for cell sorting and bio sensing. The research will be used to further develop and enhance an interdisciplinary course on computation in the physical sciences. Students participating in the project will benefit from substantial training and collaboration opportunities at the Minnesota Supercomputing Institute. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.