192 research outputs found
Geometric control of bacterial surface accumulation
Controlling and suppressing bacterial accumulation at solid surfaces is
essential for preventing biofilm formation and biofouling. Whereas various
chemical surface treatments are known to reduce cell accumulation and
attachment, the role of complex surface geometries remains less well
understood. Here, we report experiments and simulations that explore the
effects of locally varying boundary curvature on the scattering and
accumulation dynamics of swimming Escherichia coli bacteria in
quasi-two-dimensional microfluidic channels. Our experimental and numerical
results show that a concave periodic boundary geometry can decrease the average
cell concentration at the boundary by more than 50% relative to a flat surface.Comment: 10 pages, 5 figure
Vesicle dynamics in elongation flow: Wrinkling instability and bud formation
We present experimental results on the relaxation dynamics of vesicles
subjected to a time-dependent elongation flow. We observed and characterized a
new instability, which results in the formation of higher order modes of the
vesicle shape (wrinkles), after a switch in the direction of the gradient of
the velocity. This surprising generation of membrane wrinkles can be explained
by the appearance of a negative surface tension during the vesicle deflation,
due to compression in a sign-switching transient. Moreover, the formation of
buds in the vesicle membrane has been observed in the vicinity of the dynamical
transition point.Comment: 4 pages, 4 figure
Ciliary contact interactions dominate surface scattering of swimming eukaryotes
Interactions between swimming cells and surfaces are essential to many
microbiological processes, from bacterial biofilm formation to human
fertilization. However, in spite of their fundamental importance, relatively
little is known about the physical mechanisms that govern the scattering of
flagellated or ciliated cells from solid surfaces. A more detailed
understanding of these interactions promises not only new biological insights
into structure and dynamics of flagella and cilia, but may also lead to new
microfluidic techniques for controlling cell motility and microbial locomotion,
with potential applications ranging from diagnostic tools to therapeutic
protein synthesis and photosynthetic biofuel production. Due to fundamental
differences in physiology and swimming strategies, it is an open question
whether microfluidic transport and rectification schemes that have recently
been demonstrated for pusher-type microswimmers such as bacteria and sperm
cells, can be transferred to puller-type algae and other motile eukaryotes, as
it is not known whether long-range hydrodynamic or short-range mechanical
forces dominate the surface interactions of these microorganisms. Here, using
high-speed microscopic imaging, we present direct experimental evidence that
the surface scattering of both mammalian sperm cells and unicellular green
algae is primarily governed by direct ciliary contact interactions. Building on
this insight, we predict and verify experimentally the existence of optimal
microfluidic ratchets that maximize rectification of initially uniform
Chlamydomonas reinhardtii suspensions. Since mechano-elastic properties of
cilia are conserved across eukaryotic species, we expect that our results apply
to a wide range of swimming microorganisms.Comment: Preprint as accepted for publication in PNAS, for published journal
version (open access) and Supporting Information see
http://dx.doi.org/10.1073/pnas.121054811
Dynamics of Vesicles in shear and rotational flows: Modal Dynamics and Phase Diagram
Despite the recent upsurge of theoretical reduced models for vesicle shape
dynamics, comparisons with experiments have not been accomplished. We review
the implications of some of the recently proposed models for vesicle dynamics,
especially the Tumbling-Trembling domain regions of the phase plane and show
that they all fail to capture the essential behavior of real vesicles for
excess areas, \Delta, greater than 0.4. We emphasize new observations of shape
harmonics and the role of thermal fluctuations.Comment: (removed forgotten leftover figure files
Fluid Velocity Fluctuations in a Suspension of Swimming Protists
In dilute suspensions of swimming microorganisms the local fluid velocity is
a random superposition of the flow fields set up by the individual organisms,
which in turn have multipole contributions decaying as inverse powers of
distance from the organism. Here we show that the conditions under which the
central limit theorem guarantees a Gaussian probability distribution function
of velocities are satisfied when the leading force singularity is a Stokeslet,
but are not when it is any higher multipole. These results are confirmed by
numerical studies and by experiments on suspensions of the alga Volvox carteri,
which show that deviations from Gaussianity arise from near-field effects.Comment: 4 pages, 3 figure
Particles held by springs in a linear shear flow exhibit oscillatory motion
The dynamics of small spheres, which are held by linear springs in a low
Reynolds number shear flow at neighboring locations is investigated. The flow
elongates the beads and the interplay of the shear gradient with the nonlinear
behavior of the hydrodynamic interaction among the spheres causes in a large
range of parameters a bifurcation to a surprising oscillatory bead motion. The
parameter ranges, wherein this bifurcation is either super- or subcritical, are
determined.Comment: 4 pages, 5 figure
Entrainment dominates the interaction of microalgae with micron-sized objects
The incessant activity of swimming microorganisms has a direct physical effect on surrounding microscopic objects, leading to enhanced diffusion far beyond the level of Brownian motion with possible influences on the spatial distribution of non-motile planktonic species and particulate drifters. Here we study in detail the effect of eukaryotic flagellates, represented by the green microalga Chlamydomonas reinhardtii, on microparticles. Macro- and micro-scopic experiments reveal that microorganism--colloid interactions are dominated by rare close encounters leading to large displacements through direct entrainment. Simulations and theoretical modelling show that the ensuing particle dynamics can be understood in terms of a simple jump-diffusion process, combining standard diffusion with Poisson-distributed jumps. This heterogeneous dynamics is likely to depend on generic features of the near-field of swimming microorganisms with front-mounted flagella
Bimodal rheotactic behavior reflects flagellar beat asymmetry in human sperm cells
Successful sperm navigation is essential for sexual reproduction, yet we still understand relatively little about how sperm cells are able to adapt their swimming motion in response to chemical and physical cues. This lack of knowledge is owed to the fact that it has been difficult to observe directly the full 3D dynamics of the whip-like flagellum that propels the cell through the fluid. To overcome this deficiency, we apply a new algorithm to reconstruct the 3D beat patterns of human sperm cells in experiments under varying flow conditions. Our analysis reveals that the swimming strokes of human sperm are considerably more complex than previously thought, and that sperm may use their heads as rudders to turn right or left.Swiss National Science Foundation (Grant 148743)Solomon Buchsbaum AT&T Research Fun
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