Chemical and Biomolecular Engineering - Theses

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End date: 2019 × Start date: 2000 × Type: PhD thesis × Subject: flow × Subject: nanoparticle × Subject: polymer ×

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    Nanoparticles and macromolecules in flow
    Xie, Donglin ( 2016)
    The optimised fabrication of nanomaterials requires understanding of their behaviors in flow. Here we report on investigations into the flow induced structured changes in colloidal suspensions and polymer solutions. Fabrication of nano-devices requires understanding of the fundamental aspects of flow induced changes in suspensions or polymers. Understanding the behaviors of the dynamics of particles in flow gives insights into fluid mechanics at these length scales. In this work, rheo-optics are combined with numerical simulations to study flow induced phenomena in the colloidal systems. The rheo-optical methods are used to measure the real time optical changes of the sheared nanomaterial suspensions. The simulations are developed to derive the optical changes from the microstructure or motion changes of the suspending nanomaterials in flow. The thesis reports on three main areas of study. The first two studies are on the flow induced alignment of prolate nanoparticles in the red form poly-4BCMU solutions and gold nanorods aqueous sucrose solutions. The absorption spectra have been measured over a range of shear rates using polarized and unpolarized incident light, and the reversible optical changes indicate that the nanoparticles do not undergo aggregation during measurement. The measured absorbance anisotropy is attributed to the flow induced particle alignment which reach the limitation at high Peclet numbers. The spectral changes are consistent with the Jeffery's orbits (cooperating with the Brownian rotation) for large nanoparticles. While, for the nanorods with the long axis <100nm, the spectral shifts are no longer consistent with the modified Jeffery's orbits, but with the rods flipping between extreme orientations of the Jeffery's orbits. This indicates that the effect of the Brownian motion and hydrodynamic forces on the nanoscale rods needs being reconsidered. The viscoelastic effect on the flow-induced aggregation is studied in the dilute colloidal polystyrene nanoparticles suspensions. The real-time aggregation processes have been recorded via measuring optical absorption/scattering in flow. The observed absorbance decreases over time are attributed to the flow-induced coagulation. The aggregation processes still follow the Smoluchowski coagulation equation in a revised version. Suspensions in a series of media are studied to evaluate the effect of the media rheological properties on the particle aggregation. The data shows that elasticity reduces the aggregation while the solution viscosity increases the aggregation rate. In conclusion, the flow induced alignment and aggregation in the nanomaterials suspensions were studied using the rheo-optical method. The classical hydrodynamic theory describes the rotation of larger prolate nanoparticles in flow, but is no longer efficient for the rod like particles with the long axis 100nm. The solution viscosity accelerates the aggregation of nanoparticles while the elasticity has the opposite effect.