MEAM Ph.D. Thesis Defense: “Multi-Scale Probing of Colloidal Sedimentation Dynamics in Active Suspensions”

Microorganisms, although challenging to observe, are ubiquitous in both natural ecosystems and industries. They inhabit diverse environments: natural ones ranging from small river tributaries and lakes to oceans, as well as in industrial settings, like wastewater treatment plants and food manufacturing. In these diverse contexts, microorganisms coexist with settling particles, a process heavily influenced by gravity. Consequently, gravity significantly influences microorganisms’ behaviors, impacting aspects such as locomotion and nutrient uptake. The presence of microorganisms alongside colloidal particles, specifically, can influence industrial processes and transport properties. The comprehension of microorganisms’ physical and biological behaviors in aquatic environments, especially under external forces like gravity, remains elusive and challenging.

In this thesis, I explore the dynamics of spherical colloidal sedimentation in the presence of swimming Escherichia coli across various concentrations within the dilute regime. The sedimentation processes receive comprehensive characterization across length scales, through the examination of macro-scale settling speeds, meso-scale concentration profiles, and micro-scale particle diffusivities. First, I showcase how bacterial activity affects the concentration profiles of spherical particles—an alteration describable through an advection-diffusion equation with an added population dynamics term. Subsequently, I characterize the sedimentation speed of spherical particles across different bacterial concentrations, unveiling the emergence of two sedimentation fronts: particle- and bacteria-rich fronts. Even passive systems of poly-dispersed (by size) particles are known to show segregating sedimentation fronts; larger (and faster) settling particles will separate from smaller (and slower) ones, given enough time. In this context, heightened activity influences sedimentation speeds and the associated timescales tied to the appearance of the bacteria front. These timescales pertaining to the second front yield a phenomenological model that captures the sedimentation of passive particles within active fluids. Lastly, I explored the interactions between particles and bacteria in the presence of gravity, uncovering that bacterial-induced convective motions reduce the convective transport of colloidal particles. By increasing bacterial concentration, particle convection diminishes and nearly stabilizes. These observations reveal a correlation with measurements of macro-scale settling speeds. I demonstrate that the emergence of this phenomenon is associated with the development of complex bioconvection patterns. Overall, this dissertation illuminates the intricate interplay between microorganisms and particles in the presence of gravity, revealing nontrivial effects.