Public defence in Engineering Physics, M.Sc. Hakimeh Koochi

Title of the doctoral thesis: Dynamics of low-density gels

Doctoral student: Hakimeh Koochi
Opponent: Associate Professor Outi Tammisola, KTH, Sweden
Custos: Prof. Mikko Alava, Aalto University School of Science, Department of Applied Physics

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

The research work of this thesis is targeted at a fundamental understanding of the non-linear flow behavior of low-density gels. Low-density gels are thixotropic, yield-stress, and viscoelastic fluids, and such peculiar rheological characteristics, along with a lightweight porous microstructure, render them interesting among researchers. As such, TEMPO-oxidized cellulose nanofibrilated gels (TEMPO-CNFs) are an environmentally friendly alternative material for a wide range of applications. At the laboratory level, rheological measurements and Stokes experiments of dropping solid objects into the fluid were used to characterize the flow behavior of aged samples of TEMPO-CNFs. In the interaction between the solid object and TEMPO-CNFs, the settling velocity exhibited irregular semi-oscillatory or logarithmic variation over time. Such behavior has been already reported for the free-falling motion of solid spheres in Laponite, a well-known thixotropic colloidal gel, and aqueous superabsorbent polymer suspensions, an elastic yield stress hydrogel. Then, considering a well-defined rheological model based on the structural dynamics of inelastic thixotropic fluids, computational rheology and Computational Fluid Dynamics (CFD) numerical simulations are employed to explain the experimental results. This study illustrates that the solid object needs to overcome the initial yield stress of the fluid to penetrate it, where the yield stress non-monotonically increases with the aging time. When the sphere finally finds its pass through the gel, it could enter either the same, a more structured, or a more broken layer of the fluid over time, depending on its initial degree of structuring and the gravity-induced stress. Therefore, the settling velocity of the sphere could be in a steady state, continuously decrease, or increase over time. In addition, depending on the speed of the sphere, it could capture the spatial heterogeneity of the microstructure, and accordingly, its settling velocity profile shows irregular fluctuation. These mechanisms are ruled by competition between kernels of orthokinetic and perikinetic buildup and shear-induced breakdown of the microstructure. Such competition also links different settling regimes to the yield stress, non-monotonic, and Newotonin plateau regions of flow curves. Moreover, the creep tests reveal a halting time in response to the imposed stress, which explains irregular settling profiles.

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Doctoral theses in the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52