COLLOIDAL TRANSPORT IN LIQUID CRYSTALS AND CONNECTIONS BETWEEN RHEOLOGY AND DYNAMICS OF SOFT DISORDERED SOLIDS

colloidal transport;liquid crystals;rheology;dynamics;disordered solids.;Physics

Chen, KuiLeheny, Robert L. ;

Johns Hopkins University

关键词: colloidal transport;    liquid crystals;    rheology;    dynamics;    disordered solids.;    Physics;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/59319/Kui%20Chen%27s%20thesis.docx?sequence=2&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

In this thesis, I report experimental studies on soft matter physics.Two fields were investigated. The first part of the thesis concerns colloidal transport within liquid crystals.When a colloidal particle translates through a liquid crystal under an external force, its mobility is not only affected by viscous forces, but also by the elastic properties of the liquid crystal. For example, the anisotropic viscous and elastic interaction in the liquid crystals can make the mobility highly dependent on the direction of motion. To explore such hydrodynamics, I have performed a series of experiments examining the transport behavior of colloidal particles suspended in liquid crystals with spatially periodic order.In Chapter 3, I describe experiments of colloids transport in cholesteric finger textures. When a cholesteric liquid crystal is confined within a homeotropic cell, the helical alignment is distorted into a finger texture, which includes a periodic array of disclinations. Interactions between colloids and the defects affect the particle translation. Under an external constant driving force, such interactions, along with the anisotropic viscosity of the cholesteric, lead to highly different particle mobility along the directions parallel and perpendicular to the cholesteric pitch. We characterized experimentally this mobility, which included stick-slip motion, a built a model that accounts for it quantitatively.In Chapter 4, I report investigations of colloidal transport within nematic liquid crystals within mircrofluidic arrays of obstacles. In contrast with the behavior in isotropic liquids, the interaction between the particles and obstacles mediated by the elasticity of the liquid crystal, along with the spatially varying anisotropic viscosity of the nematic, contributes to the mobility. A quantitative analysis distinguishes the viscous and elastic contributions to the mobility. In addition, the directional locking phenomenon was has been observed previously in isotropic liquids is shown to be altered by the liquid crystal forces, indicating a mechanism for particle separation technology. My research in the second field is focused on the nonlinear rheology and shear-induced dynamics of soft disordered solids.Nonlinear viscoelasticity is widely observed in these systems. In particular, when an applied stress exceeds the solid’s elastic limit, the solid yields, and irreversible changes to the material’s structure at the nanoscale to microscale occur. As described in Chapter 5, I have applied X-ray photon correlation spectroscopy (XPCS) to interrogate how shear-induced microscopic structural dynamics connects with the macroscopic deformation and flow properties in a set of soft disordered solids including concentrated nanocolloidal gels. This work was conducted under the supervision of Prof. Robert L. Leheny. The author also acknowledges Prof. Daniel H. Reich for his role as co-advisor.