Gravitational settling of particles through density interfaces

Srdic-Mitrovic, A.; Mohamed, N. A.; Fernando, H. J. S.

doi:10.1017/s0022112098003590

Cited by 83 publications

(116 citation statements)

References 37 publications

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“…Above and below the center of the pycnocline the model somewhat overpredicts sinking velocity; i.e., it overpredicts the rate of diffusive adaptation. This overprediction can be rationalized by considering that porous spheres entrain a wake of lighter fluid during their descent, as observed for solid spheres (11)(12)(13). The wake represents a layer of intermediate density that shields the interstitial fluid from direct diffusive exchange of the stratifying agent with the ambient fluid.…”

Section: Resultsmentioning

confidence: 98%

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Diffusion-limited retention of porous particles at density interfaces

Kindler

Khalili

Stocker

2010

Proc. Natl. Acad. Sci. U.S.A.

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Downward carbon flux in the ocean is largely governed by particle settling. Most marine particles settle at low Reynolds numbers and are highly porous, yet the fluid dynamics of this regime have remained unexplored. We present results of an experimental investigation of porous particles settling through a density interface at Reynolds numbers between 0.1 and 1. We tracked 100 to 500 μm hydrogel spheres with 95.5% porosity and negligible permeability. We found that a small negative initial excess density Δρ p relative to the lower (denser) fluid layer, a common scenario in the ocean, results in long retention times of particles at the interface. We hypothesized that the retention time was determined by the diffusive exchange of the stratifying agent between interstitial and ambient fluid, which increases excess density of particles that have stalled at the interface, enabling their settling to resume. This hypothesis was confirmed by observations, which revealed a quadratic dependence of retention time on particle size, consistent with diffusive exchange. These results demonstrate that porosity can control retention times and therefore accumulation of particles at density interfaces, a mechanism that could underpin the formation of particle layers frequently observed at pycnoclines in the ocean. We estimate retention times of 3 min to 3.3 d for the characteristic size range of marine particles. This enhancement in retention time can affect carbon transformation through increased microbial colonization and utilization of particles and release of dissolved organics. The observed size dependence of the retention time could further contribute to improve quantifications of vertical carbon flux.marine snow | stratification | biological pump T he settling of particulate organic matter in the ocean is the main vector of carbon export from surface waters toward the deep sea (1, 2). Besides its key role in the oceanic carbon cycle, this particle flux supplies resources to waters below the surface layer, impacting ocean productivity and trophic dynamics (3, 4). The sinking speed of marine particles and their residence time in the epipelagic control the rate at which particulate organic matter is exported from the upper ocean or consumed and remineralized by microbes.Marine snow particles are 0.5 to 20 mm amorphous aggregates of various origins, both organic and inorganic, bound together by exopolymeric material (5, 6). Their small excess density relative to ambient water [typically

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Section: Resultsmentioning

confidence: 98%

“…Although porosity ε often exceeds 95% (4), marine snow has been modeled primarily as solid particles, either in a homogeneous (10) or in a stratified (11)(12)(13) fluid. Porous particles have been studied exclusively in the context of homogeneous fluids (14).…”

mentioning

confidence: 99%

Diffusion-limited retention of porous particles at density interfaces

Kindler

Khalili

Stocker

2010

Proc. Natl. Acad. Sci. U.S.A.

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“…The effect of buoyancy is often quantified by means of the Froude number (23,25,26), Fr ¼ U∕ðNaÞ, where N ¼ ffiffiffiffiffiffiffiffiffiffiffi ffi γg∕ρ 0 p is the Brunt-Väisälä frequency, the natural frequency of oscillation of a vertically displaced particle in a stratified fluid, and γ ¼ −dρ∕dz is the background density gradient. However, Fr measures the relative importance of inertial and buoyancy forces, whereas in the inertialess world of microorganisms it is more appropriate to compare viscous and buoyancy forces.…”

Section: A Squirmer In a Stratified Fluidmentioning

confidence: 99%

Low-Reynolds-number swimming at pycnoclines

Doostmohammadi

Stocker²,

Ardekani³

2012

Proc. Natl. Acad. Sci. U.S.A.

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Microorganisms play pivotal functions in the trophic dynamics and biogeochemistry of aquatic ecosystems. Their concentrations and activities often peak at localized hotspots, an important example of which are pycnoclines, where water density increases sharply with depth due to gradients in temperature or salinity. At pycnoclines organisms are exposed to different environmental conditions compared to the bulk water column, including reduced turbulence, slow mass transfer, and high particle and predator concentrations. Here we show that, at an even more fundamental level, the density stratification itself can affect microbial ecology at pycnoclines, by quenching the flow signature, increasing the energetic expenditure, and stifling the nutrient uptake of motile organisms. We demonstrate this through numerical simulations of an archetypal low-Reynolds-number swimmer, the "squirmer." We identify the Richardson number-the ratio of buoyancy forces to viscous forces-as the fundamental parameter that quantifies the effects of stratification. These results demonstrate an unexpected effect of buoyancy on low-Reynolds-number swimming, potentially affecting a broad range of abundant organisms living at pycnoclines in oceans and lakes.stratified fluid | bio-locomotion V ertical variations in water density, or "pycnoclines," occur ubiquitously in aquatic and marine environments (1), due to gradients in temperature (thermoclines) or salinity (haloclines). Pycnoclines can trigger a wide range of environmental and oceanographic processes. In oceans and lakes, intense biological activity and accumulation of organisms and particles are associated with pycnoclines (2, 3). For example, formation of phytoplankton blooms is often correlated with stratification (3), and these blooms can enhance CO 2 sequestration (4) or disrupt water supply systems (5). Stratification can also affect organism migration: Some species of euphausiids do not cross thermoclines (6) and haloclines can act as a barrier to the vertical migration of dinoflagellates (7).Despite the widespread ecological implications of stratification, its hydrodynamic effects on organisms remain poorly understood. This is partly due to the notion that most organisms are too small to be affected by stratification, because the water density varies on a length scale, L ρ ¼ ρ 0 ∕γ ∼ OðkmÞ, much larger than the size of the organism, where ρ 0 is a reference density (e.g., 1;000 kg m −3 ) and γ is the vertical gradient in water density [typical values of γ range from Oð0.01Þ kg m −4 at ocean thermocline (8) to Oð1Þ kg m −4 in fjords and lakes (2, 3)].This notion is incorrect. It was recently found that the appropriate length scale to determine whether stratification affects motion is L ¼ ðμκ∕γgÞ 1∕4 , where μ is the dynamic viscosity, κ the diffusivity of the stratifying agent, and g the acceleration of gravity (9). [This length scale was earlier derived, in a different context, by List (10)]. Organisms larger than L are affected by stratification. For typical stratifications, L is in...

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“…Experiments on liquid and solid drops falling in a stably stratified fluid have been made for geophysical applications by Manga & Stone (1995) and Srdic-Mitrovic et al (1999) using two-layered and continuously stratified fluid, respectively. The basic phenomena are illustrated by the sequence of images in figure 1.…”

Section: Single-drop Experimentsmentioning

confidence: 99%

Experiments on metal–silicate plumes and core formation

Olson

Weeraratne²

2008

Phil. Trans. R. Soc. A.

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Short-lived isotope systematics, mantle siderophile abundances and the power requirements of the geodynamo favour an early and high-temperature core-formation process, in which metals concentrate and partially equilibrate with silicates in a deep magma ocean before descending to the core. We report results of laboratory experiments on liquid metal dynamics in a two-layer stratified viscous fluid, using sucrose solutions to represent the magma ocean and the crystalline, more primitive mantle and liquid gallium to represent the core-forming metals. Single gallium drop experiments and experiments on Rayleigh-Taylor instabilities with gallium layers and gallium mixtures produce metal diapirs that entrain the less viscous upper layer fluid and produce trailing plume conduits in the high-viscosity lower layer. Calculations indicate that viscous dissipation in metal-silicate plumes in the early Earth would result in a large initial core superheat. Our experiments suggest that metal-silicate mantle plumes facilitate high-pressure metal-silicate interaction and may later evolve into buoyant thermal plumes, connecting core formation to ancient hotspot activity on the Earth and possibly on other terrestrial planets.

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Gravitational settling of particles through density interfaces

Cited by 83 publications

References 37 publications

Diffusion-limited retention of porous particles at density interfaces

Diffusion-limited retention of porous particles at density interfaces

Low-Reynolds-number swimming at pycnoclines

Experiments on metal–silicate plumes and core formation

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