Experimental Investigation of Settling and Deformation of Extracellular Polymeric Substances-Analog Xanthan Gum Particles under Still-Water and Turbulent Conditions
Open Access DepositedThis thesis investigates the settling and deformation behavior of Xanthan Gum (XG) particles as mechanical analogs for Extracellular Polymeric Substances (EPS)-enriched marine aggregates under both still-water and turbulent conditions. The study was motivated by the need to understand how soft aggregate-like particles respond to quiescent flow and to weak, ocean-relevant turbulence, since marine snow transport in the ocean depends not only on settling velocity but also on particle deformation and flow-induced trajectory modification. Two XG concentrations, 1.25 g/L and 2.5 g/L, were selected as controlled viscoelastic analog conditions, and particles of 500 µL, 200 µL, and 20 µL were examined. Experiments were conducted in an octagonal turbulence tank equipped with synchronized orthogonal high-speed cameras, and three-dimensional particle motion was reconstructed using Direct Linear Transform (DLT)-based image analysis. The turbulent condition was based on a previously characterized low-dissipation flow field generated at 30 rpm, representative of small-scale oceanic turbulence, with dissipation rate on the order of ɛ∼10^(-5) m^2/〖,s〗^3. In still water, settling velocity decreased systematically with particle size for both concentrations, and the 2.5 g/L particles settled faster than the corresponding 1.25 g/L particles, indicating stronger cohesion and better shape retention at higher concentration. In turbulence, the measured net downward velocities were higher than in still water, but the trajectories became irregular, truncated, and strongly influenced by background flow and velocity fluctuations, especially for the smallest particles. Deformation analysis from both camera views showed that particle elongation, projected-axis reduction, and reorientation were strongly dependent on particle size, concentration, and flow condition. Overall, the results demonstrate that settling of soft marine-aggregate analogs is governed by the coupled effects of particle size, viscoelastic concentration, turbulence, and time-dependent deformation, and that still-water drag-based models are insufficient to fully describe turbulent settling of deformable particles.
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