Computer Simulation Study On Hierarchical Self-assembly Of Soft Disklike Particles Under Shear Flow

Hierarchical self-assembly offers a promising method to organize nanoscaleobjects into functional nanostructures. A revolution in novel anisotropicnanoparticles and colloidal building blocks has brought new excitement to thisfield. Soft anisotropic particles, such as soft patchy particles, soft disklikemicelles, and soft disklike molecules, may be also used as important buildingblocks for further hierarchical self-assembly due to their anisotropic interactionsand shapes. A lot of novel hierarchical nanostructures have been obtained by theself-assembly of these soft disklike particles in experiments and simulations,such as nematic columnar phase, hexagonal column structure, one-dimensionalthreads, hexagonal bundle structure, and so on.Actually, in experiments and during processing, the nonequilibriumconditions are commonly encountered. Shear-induced clustering, orderedpacking, orientational alignment, and structural changes in soft matter systemsare of considerable interest both from fundamental and application viewpoint. Amajority of studies have involved anisotropic particles because shear caninfluence both the texture and the orientation of the anisotropic species.Anisotropic particles are also of major practical relevance since the orientationof the particles in a fluid will substantially affect the physical and opticalproperties of the final products. Thus, it is very important to investigate theself-assembly of soft disklike particles under shear flow in order to design newhierarchical nanostructures.The thesis is dedicated to develop a mesoscale nonequilibrium simulationmodel that can reflect the interaction nature between soft disklike particles andtheir shear-alignment behavior and thus allows one to study hierarchicalself-assembly of soft disklike particles under shear flow. With this nonequilibrium model, we investigate the effect of steady shear on theself-assembled structures and the orientations of soft disklike particles in dilutesolutions.(1) A mesoscale nonequilibrium simulation model which is suitable to study thehierarchical self-assembly of soft disklike particles under shear flow. Byemploying the anisotropic potential which can reflect the interaction naturebetween soft disklike particles and the Lees-Edwards periodic boundaryconditions, we develop a mesoscale nonequilibrium simulation model that canreflect the interaction nature between soft disklike particles and theirshear-alignment behavior in a simple way, and allows one to study hierarchicalself-assembly of soft disklike particles under steady shear.(2) Effect of steady shear on the self-assembled thread structures. We keep softdisklike particles in athermal solvent condition, and examine the influence of theshear rate on the self-assembled thread structures. The weak attraction allows thedisklike particles to self-assemble into short threads, and then the shear flowtends to align the short threads along the flow direction, further leads tointerthread connection, and finally results in a relatively longer threads with theparticle direction vector parallel to the flow direction.(3) Effect of steady shear on the self-assembled bundle structures. We mainlyfocus on the influences of shear rate and solvent condition on the self-assembledbundle structures. By properly selecting the shear rates and the solventconditions, we can obtain the bundle-like structures along the flow direction.However, it is not easy to obtain the relatively well-defined bundle structuresalong the flow direction since shear flow suppresses the phase separation of thethreads into bundles. In other words, shear flow actually favors the formation oflong threads along the flow direction.