Viscous constraints on zooplankton approach and interaction

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Organisms that swim in water need to approach suspended particles for a variety of life processes, such as feeding, mating, fertilization, and finding favorable habitats. This research focuses on plankton, which despite their small size play an outsize role in ocean ecology due to their numerical abundance. For example, phytoplankton provide the base of the food web by converting sunlight into organic matter, which is transferred further up to the food chain, primarily by zooplankton which eat the phytoplankton. Hydrodynamics at the micro- to millimeter scales of zooplankton is dominated by viscosity, and severely constrains their ability to approach other particles. Attempts to approach smaller food particles are made difficult by viscous flows that tend to push the food particle away. However, zooplankton also approach similar size particles during mating, as well as larger particles during egg fertilization or when exploring new habitats. The goal of the research is to understand what general constraints viscous hydrodynamics place on the approach of plankton to suspended particles and how those constraints determine biological form and function. The results will increase our understanding of how plankton eat, reproduce, and spread to new habitats. The project will also enhance engineering education through graduate and undergraduate research training, and an outreach program will communicate our research results to local high school students. While the way that viscous hydrodynamics constrains propulsion and the form of swimming mechanisms of microorganisms is well appreciated, this research extends our understanding of the implications of viscous hydrodynamics to include constraints on approach which is as or more important than swimming for many life processes. To deduce how hydrodynamic constraints on approach determine biological form and behavior, the research uses numerical studies, flow visualization, and dynamically scaled experiments to investigate a range of swimmer geometries, propulsion methods, target particle sizes, and approach strategies to identify which enable close approach in the presence of viscous constraints. The results will be used to analyze observations of approach of various types of organisms reported in the literature to understand when viscous constraints determine the biological form of those organisms.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.