Bridging advection and diffusion in the encounter dynamics of sedimenting marine snow

Abstract: Sinking marine snow particles, composed primarily of organic matter, control the global export of photosynthetically fixed carbon from the ocean surface to depth. The fate of sedimenting marine snow particles is in part regulated by their encounters with suspended, micron-sized objects, which leads to mass accretion by the particles and potentially alters their buoyancy, and with bacteria that can colonize the particles and degrade them. Their collision rates are typically calculated using two types of models focusing either on direct (ballistic) interception with a finite interaction range, or advective-diffusive capture with a zero interaction range. Since the range of applicability of the two models is unknown, and many relevant marine encounter scenarios span across both regimes, quantifying such encounters remains challenging, because the two models yield asymptotically different predictions at high P\'{e}clet numbers. Here, we reconcile the two limiting approaches by quantifying the encounters in the general case using a combination of theoretical analysis and numerical simulations. Solving the advection-diffusion equation in Stokes flow around a sphere to model mass transfer to a large sinking particle by small yet finite-sized objects, we determine a new formula for the Sherwood number as a function of two dimensionless parameters: the P\'{e}clet number and the ratio of small to large particle sizes. We find that diffusion can play a significant role in generating encounters even at high Pe. At Pe as high as $10^6$, the direct interception model underestimates the encounter rate by up to two orders of magnitude. This overlooked contribution of diffusion to encounters suggests that important processes affecting the fate of marine snow, such as colonization by bacteria and plankton or accretion of neutrally buoyant gels, may proceed at a rate much faster than previously thought.