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Abstract

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
Δρp 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.