The deep ocean remains one of the most enigmatic and dynamic environments on our planet, hosting complex physical and biological processes that influence global ecosystems and climate. Among the many phenomena shaping this vast underwater world is the settling of particulate matter through stratified fluids — layers of water where density varies with depth. In groundbreaking new research, scientists from Brown University and the University of North Carolina at Chapel Hill have unveiled surprising mechanisms that govern the sinking of porous particles in such stratified oceanic conditions, overturning long-held assumptions and offering fresh insights that could compel a paradigm shift in marine science.
Stratified fluids, like the ocean, exhibit a density gradient: the water near the surface is less dense due to lower salinity and higher temperature, while deeper water is denser, influenced by increased salt content and lower temperatures. These gradients complicate the behavior of sinking particles, often organic aggregates commonly referred to as “marine snow.” These particulate aggregates descend from the ocean surface, transporting carbon and nutrients vital for marine life and global biogeochemical cycles. Until now, the physics describing their descent primarily focused on drag forces opposing sedimentation, assuming larger particles invariably sink faster due to gravity overcoming fluid resistance.
Challenging this notion, the interdisciplinary team revealed that the permeability and porosity of such particles, specifically their ability to absorb salt from surrounding water, play a crucial role in determining sinking velocity. This phenomenon arises because salt diffusion into the particle alters its effective density as it sinks through stratified layers. The research showed that the absorption rate of salt relative to particle volume fundamentally shapes how rapidly it settles, a revelation that upends the intuitive expectation that larger particles always descend faster.
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Robert Hunt, a postdoctoral researcher at Brown’s School of Engineering who spearheaded the investigation, explains, “Our findings demonstrate that in stratified fluids, particle size alone does not dictate sinking speed. Instead, smaller, highly porous particles can sink faster than bigger ones due to enhanced salt diffusion and absorption.” This counterintuitive behavior emphasizes the importance of appreciating microscopic physical and chemical interactions driving macroscopic oceanic processes.
The team’s theoretical framework builds upon hydrodynamics and mass transfer principles, connecting fluid drag, buoyancy, and diffusion-limited salt absorption. By treating porous particles as dynamic systems interacting chemically and physically with their environment, the model predicts settling rates that vary nonlinearly with size and shape. Particularly for spherical particles, diffusion-limited settling results in smaller spheres achieving higher terminal velocities than their larger counterparts, defying classic sedimentation theory valid for homogeneous fluids.
To empirically verify their model, researchers devised a controlled laboratory setup featuring a large tank capable of maintaining a precise linear stratification of fluid density. This was achieved by continuously feeding fresh and saltwater from separate reservoirs, meticulously pumping to maintain the gradient. Such an apparatus enabled the simulation of ocean-like conditions on a manageable scale, allowing meticulous observation of particle behavior within realistic stratification profiles.
Utilizing advanced 3D printing techniques, the team fabricated porous agar particles of varying geometries and dimensions. Agar, a gelatinous polysaccharide derived from red seaweed, served as an ideal analog due to its tunable porosity and salt-absorbing characteristics. High-speed cameras tracked the descent trajectories and velocities of these particles through the stratified tank, providing valuable data to compare against model predictions.
The experimental results corroborated the theory convincingly. Smaller spherical particles settled systematically faster than larger ones, while non-spherical shapes exhibited settling velocities predominantly influenced by their smallest dimension. Elongated or flattened particles, for example, demonstrated a propensity to sink more swiftly than volumetrically equivalent spheres. This phenomenon is attributable to the differential salt absorption rates and interaction between particle geometry and stratification-induced resistive forces.
These discoveries have profound implications beyond fundamental fluid dynamics, touching upon critical ecological and environmental challenges. Marine snow plays a pivotal role in sequestering atmospheric carbon dioxide by ferrying organic material from surface waters to the deep ocean. Accurately characterizing the sinking behavior of these particles therefore directly impacts our understanding of carbon flux and storage, key components in climate modeling and ocean health assessments.
Moreover, the findings offer valuable perspectives on the fate of anthropogenic particulates, such as microplastics, whose environmental distribution and longevity remain urgent global concerns. Understanding how porosity and stratified fluid interactions influence their sinking can inform mitigation strategies and pollution models, aiding efforts to preserve marine ecosystems.
Daniel Harris, an associate professor of engineering at Brown University overseeing the project, highlights the broader significance: “By distilling complex natural phenomena into tractable physical principles, we provide a predictive tool that can be readily integrated into larger scale ecological models. This fusion of experimentation, theory, and engineering has the potential to refine how we interpret particle dynamics in oceans and other stratified systems.”
This research also exemplifies effective collaboration across disciplines and institutions. Co-authors Roberto Camassa and Richard McLaughlin from UNC Chapel Hill contributed mathematical expertise fundamental to model development. The study’s funding from agencies including the National Science Foundation and the Office of Naval Research underscores the strategic importance of understanding stratified fluid dynamics in both scientific and applied contexts.
Despite replicating only simplified oceanic conditions, the lab-based approach adopted here serves as a powerful foundation for future investigations. The reductionist methodology enables researchers to isolate and elucidate key mechanisms without confounding variables present in the open sea. Such insights can then be tested and contextualized through field measurements, fostering an iterative dialogue between theory and observation crucial for advancing marine science.
Looking forward, the research team hopes to extend their work by engaging with oceanographers, climate scientists, and environmental policy makers to explore practical applications and refine models that connect microscale particle physics with macroscale oceanographic processes. As climate change and human activity continue to alter ocean stratification and chemistry, this work offers timely tools to anticipate and mitigate impacts on planetary health.
In summary, this study reveals how the interplay between particle porosity, salt diffusion, and fluid stratification governs surprisingly intricate sinking behaviors. Moving beyond classical sedimentation paradigms, it opens new avenues for understanding the fate of organic and inorganic particulates in oceans, with far-reaching consequences for the global carbon cycle, pollution management, and ecological forecasting. The elegant fusion of theory, innovative experimentation, and sophisticated modeling sets a new benchmark for investigating sedimentation within complex natural fluids.
Subject of Research: Particle sinking dynamics in stratified fluids and the influence of porous particle salt absorption on sedimentation rates.
Article Title: Diffusion-limited settling of highly porous particles in density-stratified fluids
News Publication Date: 20-Jun-2025
Web References:
https://www.pnas.org/doi/10.1073/pnas.2505085122
http://dx.doi.org/10.1073/pnas.2505085122
Image Credits: Harris Lab / Brown University
Keywords
Engineering, Environmental sciences, Marine engineering, Carbon cycle
Tags: biogeochemical cycles and marine lifeBrown University marine scienceclimate influence of ocean processesdensity gradients in oceanmarine aggregates and carbon transportmarine snow phenomenonparadigm shift in oceanographysettling of particulate mattersinking porous particles researchstratified ocean dynamicsunexpected physics in marine environmentsUniversity of North Carolina ocean studies
