Soft Matter and Complex Systems Seminar

The settling of individual particles at low to moderate Reynolds numbers is a key mechanism for transporting particulate matter in oceans, seas, and lakes, which are often stratified by salinity- and temperature-driven density gradients. In such environments, particles exhibit complex behaviors, including transient dynamics and orientation instabilities. Density interfaces can also act as accumulation zones for biopolymers secreted by algae and bacteria, locally introducing non-Newtonian properties to the water column that further influence particle motion through viscoelastic and shear-dependent effects. These processes, though elusive in nature, are critical to understanding nutrient and carbon fluxes, microbial ecology, and pollutant transport.

To explore stratification effects, this presentation will demonstrate experimental results on disk settling through a two-layer water column with a non-linear density transition. The goal was to assess how stratification and disk shape influence settling velocity, reorientation behavior, and hydrodynamic interactions with wake. Flow visualization revealed five settling phases, marked by sequential reorientations and local velocity minima. Unique wake structures, including a bell-shaped one, were observed. Further experiments introduced model biopolymers to examine the combined impact of density and rheological gradients on the sinking of particles with various shapes. Results revealed that rheological gradients can play a dominant role in governing particle behavior. Overall, the findings raise further questions regarding the interplay of particle shape, stratification properties, and complex fluid rheology in sedimentation processes.