Collaborative Research: Scaling of ciliary flows at intermediate Reynolds number

Cilia are flexible hair-like appendages commonly used to create fluid motion in biological systems, facilitating swimming, feeding, reproduction, and other functional behaviors. Typical cilia are tens of microns long, but ctenophores (comb jellies) use cilia at much larger scales—around a millimeter in length. At small scales, ciliary flow is highly constrained by fluid viscosity. However, at larger scales, inertia becomes more important, leading to quantitative and qualitative differences in the velocities and forces produced by the cilia. These differences will be explored with a combination of laboratory experiments and computational simulations, using ctenophores as a model system for large-scale cilia. A better understanding of the fluid dynamics of cilia across scales will provide new tools to ask and answer questions related to biology, ecology, and the fundamental physics of how flexible structures create flow across scales. This knowledge may lead to new developments in engineering, including bioinspired devices, sensors, and robots. The project will also include the development of several educational components, including a new module on the viscous-inertial transition for high school physics students and outreach activities for young women interested in engineering. The overall goal of the project is to understand the physical principles that govern ciliary flows from low to intermediate Reynolds numbers. This study will explicitly examine the effects of substrate geometry and deformability on ciliary flows. A combined experimental-numerical approach will be used to investigate hydrodynamic interactions of multiple flexible propulsors at low-to-intermediate Reynolds numbers and develop useful scaling laws. The experimental approach will employ both planar and volumetric particle image velocimetry to visualize the flows generated by living ctenophores across a range of animal and propulsor sizes. The material properties of the ciliary substrate (mesoglea) will also be characterized during the investigation. These results will guide the development of a scalable computational fluid dynamics model, which will be used to investigate the larger parameter space of ciliary flow generation across scales. The project will focus on the effects of three key variables: (i) propulsor kinematics, including the degree of bending and spatiotemporal asymmetry; (ii) substrate geometry, from flat to curved; and (iii) substrate deformability, from rigid to highly deformable. This integrated approach will enable an in-depth investigation of how flexible structures generate flow across the viscous-inertial transition, and the development of broadly applicable scaling principles to guide future technology development. 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.