2025-06-20 ブラウン大学
“Marine snow” is common in the deep ocean and plays a key role in nutrient cycling. New research reveals details about how these flakes sink. Credit: NOAA.
＜関連情報＞
密度成層流体中の高多孔質粒子の拡散制限沈降 Diffusion-limited settling of highly porous particles in density-stratified fluids
Robert Hunt, Roberto Camassa, Richard M. McLaughlin, and Daniel M. Harris
Proceedings of the National Academy of Sciences Published:June 20, 2025
DOI:https://doi.org/10.1073/pnas.2505085122
Significance
Vertical transport of solid materials in marine environments plays an important role in the carbon cycle and in the ocean ecosystem. Here, we study the diffusion-limited settling of highly porous particles in a density-stratified fluid and introduce an equation for the settling velocity as a function of the solid density, size, and solute diffusivity, as well as the density gradient of the background fluid. In this regime, smaller particles settle faster, in contrast with the standard approximation used in marine science. Particle size has important implications for bioavailability and microbial respiration, for instance, and thus the differential settling due to size is relevant to such natural processes.
Abstract
The vertical transport of solid material in a stratified medium is fundamental to a number of environmental applications, with implications for the carbon cycle and nutrient transport in marine ecosystems. In this work, we study the diffusion-limited settling of highly porous particles in a density-stratified fluid through a combination of experiment, analysis, and numerical simulation. By delineating and appealing to the diffusion-limited regime wherein buoyancy effects due to mass adaptation dominate hydrodynamic drag, we derive a simple expression for the steady settling velocity of a sphere as a function of the density, size, and diffusivity of the solid, as well as the density gradient of the background fluid. In this regime, smaller particles settle faster, in contrast with most conventional hydrodynamic drag mechanisms. Furthermore, we outline a general mathematical framework for computing the steady settling speed of a body of arbitrary shape in this regime and compute exact results for the case of general ellipsoids. Using hydrogels as a highly porous model system, we validate the predictions with laboratory experiments in linear stratification for a wide range of parameters. Last, we show how the predictions can be applied to arbitrary slowly varying background density profiles and demonstrate how a measured particle position over time can be used to reconstruct the background density profile.
