title: Shear-induced orientational dynamics and spatial heterogeneity in suspensions of motile phytoplankton.
abstract: Fluid flow, ubiquitous in natural and man-made environments, has the potential to profoundly impact the transport of microorganisms, including phytoplankton in aquatic habitats and bioreactors. Yet, the effect of ambient flow on the swimming behaviour of phytoplankton has remained poorly understood, largely owing to the difficulty of observing cell-flow interactions at the microscale. Here, we present microfluidic experiments where we tracked individual cells for four species of motile phytoplankton exposed to a spatially non-uniform fluid shear rate, characteristic of many flows in natural and artificial environments. We observed that medium-to-high mean shear rates (1-25 s(-1)) produce heterogeneous cell concentrations in the form of regions of accumulation and regions of depletion. The location of these regions relative to the flow depends on the cells' propulsion mechanism, body shape and flagellar arrangement, as captured by an effective aspect ratio. Species having a large effective aspect ratio accumulated in the high-shear regions, owing to shear-induced alignment of the swimming orientation with the fluid streamlines. Species having an effective aspect ratio close to unity exhibited little preferential accumulation at low-to-moderate flow rates, but strongly accumulated in the low-shear regions under high flow conditions, potentially owing to an active, behavioural response of cells to shear. These observations demonstrate that ambient fluid flow can strongly affect the motility and spatial distribution of phytoplankton and highlight the rich dynamics emerging from the interaction between motility, morphology and flow.
authors.affiliation: Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA michael.barry@alum.mit.edu.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Barry
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
authors.forename: Roberto
authors.initials: R
authors.lastname: Rusconi
authors.affiliation: Department of Mechanical Engineering, Tufts University, 200 College Avenue, Medford, MA 02155, USA.
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Guasto
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Department of Civil, Environmental and Geomatic Engineering, ETH Zurich, Stefano Franscini Platz 5, 8093 Zurich, Switzerland romanstocker@ethz.ch.
authors.forename: Roman
authors.initials: R
authors.lastname: Stocker
mesh_terms: D017753:Ecosystem; D008954:Models, Biological; D010839:Phytoplankton / Q000502:physiology*
keywords: Jeffery orbits; elongation; hydrodynamics; motility; ocean patchiness; phytoplankton suspensions