65 research outputs found
Active and driven hydrodynamic crystals
Motivated by the experimental ability to produce monodisperse particles in
microfluidic devices, we study theoretically the hydrodynamic stability of
driven and active crystals. We first recall the theoretical tools allowing to
quantify the dynamics of elongated particles in a confined fluid. In this
regime hydrodynamic interactions between particles arise from a superposition
of potential dipolar singularities. We exploit this feature to derive the
equations of motion for the particle positions and orientations. After showing
that all five planar Bravais lattices are stationary solutions of the equations
of motion, we consider separately the case where the particles are passively
driven by an external force, and the situation where they are self-propelling.
We first demonstrate that phonon modes propagate in driven crystals, which are
always marginally stable. The spatial structure of the eigenmodes depend solely
on the symmetries of the lattices, and on the orientation of the driving force.
For active crystals, the stability of the particle positions and orientations
depends not only on the symmetry of the crystals but also on the perturbation
wavelengths and on the crystal density. Unlike unconfined fluids, the stability
of active crystals is independent of the nature of the propulsion mechanism at
the single particle level. The square and rectangular lattices are found to be
linearly unstable at short wavelengths provided the volume fraction of the
crystals is high enough. Differently, hexagonal, oblique, and face-centered
crystals are always unstable. Our work provides a theoretical basis for future
experimental work on flowing microfluidic crystals.Comment: 10 pages, 10 figure
Biochar application differentially affects soil micro-, meso-macro-fauna and plant productivity within a nature restoration grassland
Biochar is proposed as an option to sequester carbon (C) in soils and promote other soil-based ecosystem services. However, its impact on soil biota from micro to macroscale remains poorly understood. We investigated biochar effects on the soil biota across the soil food web, on plant community composition and on biomass production. We conducted a field experiment in a nature restoration grassland testing four treatments: two biochar types (herbaceous feedstock pyrolyzed at 400 °C or 600 °C – hereafter B400 and B600), and a positive (i.e. unpyrolysed biochar feedstock, hereafter Hay) and negative (no addition) control. Responses of plants and soil biota were evaluated one and three years after establishing the treatments.
Soil pH and K concentrations increased significantly in the B600 treatment. Mite abundances were significantly higher in B400 whereas nematode abundances were highest in Hay (1st year) and lowest in B400 (3rd year). Other soil fauna groups (enchytraeids and earthworms) varied more between years than between treatments. Legume cover increased significantly in the biochar treatments but this effect was transient. Legumes, grasses and primary productivity also showed a statistically significant Treatment x Year interaction due to transitory effects that were no longer present by the 3rd year.
Our results suggest that biochar produced from meadow cuttings and applied at the 10 t/ha rate cause transitory impacts on soil biota abundance and plant communities over the 3-year timeframe used for this experiment. Therefore, this type of biochar could potentially be used for soil carbon sequestration, with minimal impacts on soil biota abundance or diversity, within the groups studied here, or plant biodiversity and productivity. Further research is required to investigate the longer-term impacts of this potential soil C storage sink
Restructuring of colloidal aggregates in shear flow: Coupling interparticle contact models with Stokesian dynamics
A method to couple interparticle contact models with Stokesian dynamics (SD)
is introduced to simulate colloidal aggregates under flow conditions. The
contact model mimics both the elastic and plastic behavior of the cohesive
connections between particles within clusters. Owing to this, clusters can
maintain their structures under low stress while restructuring or even breakage
may occur under sufficiently high stress conditions. SD is an efficient method
to deal with the long-ranged and many-body nature of hydrodynamic interactions
for low Reynolds number flows. By using such a coupled model, the restructuring
of colloidal aggregates under stepwise increasing shear flows was studied.
Irreversible compaction occurs due to the increase of hydrodynamic stress on
clusters. Results show that the greater part of the fractal clusters are
compacted to rod-shaped packed structures, while the others show isotropic
compaction.Comment: A simulation movie be found at
http://www-levich.engr.ccny.cuny.edu/~seto/sites/colloidal_aggregates_shearflow.htm
The long-time dynamics of two hydrodynamically-coupled swimming cells
Swimming micro-organisms such as bacteria or spermatozoa are typically found
in dense suspensions, and exhibit collective modes of locomotion qualitatively
different from that displayed by isolated cells. In the dilute limit where
fluid-mediated interactions can be treated rigorously, the long-time
hydrodynamics of a collection of cells result from interactions with many other
cells, and as such typically eludes an analytical approach. Here we consider
the only case where such problem can be treated rigorously analytically, namely
when the cells have spatially confined trajectories, such as the spermatozoa of
some marine invertebrates. We consider two spherical cells swimming, when
isolated, with arbitrary circular trajectories, and derive the long-time
kinematics of their relative locomotion. We show that in the dilute limit where
the cells are much further away than their size, and the size of their circular
motion, a separation of time scale occurs between a fast (intrinsic) swimming
time, and a slow time where hydrodynamic interactions lead to change in the
relative position and orientation of the swimmers. We perform a multiple-scale
analysis and derive the effective dynamical system - of dimension two -
describing the long-time behavior of the pair of cells. We show that the system
displays one type of equilibrium, and two types of rotational equilibrium, all
of which are found to be unstable. A detailed mathematical analysis of the
dynamical systems further allows us to show that only two cell-cell behaviors
are possible in the limit of , either the cells are attracted to
each other (possibly monotonically), or they are repelled (possibly
monotonically as well), which we confirm with numerical computations
Periodic and Quasiperiodic Motion of an Elongated Microswimmer in Poiseuille Flow
We study the dynamics of a prolate spheroidal microswimmer in Poiseuille flow
for different flow geometries. When moving between two parallel plates or in a
cylindrical microchannel, the swimmer performs either periodic swinging or
periodic tumbling motion. Although the trajectories of spherical and elongated
swimmers are qualitatively similar, the swinging and tumbling frequency
strongly depends on the aspect ratio of the swimmer. In channels with reduced
symmetry the swimmers perform quasiperiodic motion which we demonstrate
explicitely for swimming in a channel with elliptical cross section
Asymptotic Behavior for a Nematic Liquid Crystal Model with Different Kinematic Transport Properties
We study the asymptotic behavior of global solutions to hydrodynamical
systems modeling the nematic liquid crystal flows under kinematic transports
for molecules of different shapes. The coupling system consists of
Navier-Stokes equations and kinematic transport equations for the molecular
orientations. We prove the convergence of global strong solutions to single
steady states as time tends to infinity as well as estimates on the convergence
rate both in 2D for arbitrary regular initial data and in 3D for certain
particular cases
Flow of foam through a convergent channel
International audienceWe study experimentally the flow of a foam confined as a bubble monolayer between two plates through a convergent channel. We quantify the velocity, the distribution and orientation of plastic events, and the elastic stress, using image analysis. We use two different soap solutions: a sodium dodecyl sulfate (SDS) solution, with a negligible wall friction between the bubbles and the confining plates, and a mixture containing a fatty acid, giving a large wall friction. We show that for SDS solutions, the velocity profile obeys a self-similar form which results from the superposition of plastic events, and the elastic deformation is uniform. For the other solution, the velocity field differs and the elastic deformation increases towards the exit of the channel. We discuss and quantify the role of wall friction on the velocity profile, the elastic deformation, and the rate of plastic events
Rotational propulsion enabled by inertia
The fluid mechanics of small-scale locomotion has recently attracted considerable attention, due to its importance in cell motility and the design of artificial micro-swimmers for biomedical applications. Most studies on the topic consider the ideal limit of zero Reynolds number. In this paper, we investigate a simple propulsion mechanism --an up-down asymmetric dumbbell rotating about its axis of symmetry-- unable to propel in the absence of inertia in a Newtonian fluid. Inertial forces lead to continuous propulsion for all finite values of the Reynolds number. We study computationally its propulsive characteristics as well as analytically in the small-Reynolds-number limit. We also derive the optimal dumbbell geometry. The direction of propulsion enabled by inertia is opposite to that induced by viscoelasticity
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