The Settling Behavior of Viscoelastic Gel Polymer Particles in Weak Turbulence

Particle transport in natural waterways is very important to the overall health of the environment. This transport mechanism is responsible for the movement of sediment, particulate pollutants such as microplastics, and organic material such as microalgae, which play a major role in the global carbon cycle. Particle transport in nature is highly influenced by biology, where the presence of sticky and viscoelastic biofilms and extracellular polymer exudates influence the size, porosity, and shape of both biotic (e.g. microalgae) and abiotic (e.g. microplastic, sediment) particles. Thus, the ability to understand and predict the settling dynamics of these complex, biologically-influenced particles is essential for monitoring particulate material in aquatic environments. In this investigation we examined the effects of viscoelasticity, characterized by the Deborah number, on the settling dynamics of model biological gel polymer particles in turbulence. We performed experiments in a turbulence tank designed to generate homogeneous and isotropic turbulence at low dissipation rates common in natural aquatic environments (ε <= O(10-4 ) m2 /s3 ). We determined the particle size, morphology, and settling velocity through Lagrangian particle tracking. We found that highly-deformable particles have unique settling characteristics compared to traditionally-studied, non-deformable particles. We discuss the implications of these findings on particle transport in the natural environment.