5 research outputs found
Migration reversal of soft particles in vertical flows
Non-neutrally buoyant soft particles in vertical microflows are investigated.
We find, soft particles lighter than the liquid migrate to off-center
streamlines in a downward Poiseuille flow (buoyancy-force antiparallel to
flow). In contrast, heavy soft particles migrate to the center of the downward
(and vanishing) Poiseuille flow. A reversal of the flow direction causes in
both cases a reversal of the migration direction, i. e. heavier (lighter)
particles migrate away from (to) the center of a parabolic flow profile.
Non-neutrally buoyant particles migrate also in a linear shear flow across the
parallel streamlines: heavy (light) particles migrate along (antiparallel to)
the local shear gradient. This surprising, flow-dependent migration is
characterized by simulations and analytical calculations for small particle
deformations, confirming our plausible explanation of the effect. This density
dependent migration reversal may be useful for separating particles.Comment: 8 pages, 7 figure
Deflection of phototactic microswimmers through obstacle arrays
We study the effect of inhomogeneous environments on the swimming direction
of the microalgae \textit{Chlamydomonas Reinhardtii} (CR) in the presence of a
light stimulus. Positive or negative phototaxis describe the ability of
microorganisms to bias their swimming towards or away from a light source. Here
we consider microswimmers with negative phototaxis in a microfluidic device
with a microfabricated square lattice of pillars as obstacles. We measured a
mean deflection of microswimmers that shows an interesting nonlinear dependence
on the direction of the guiding light beam with respect to the symmetry axes of
the pillar lattice. By simulating a model swimmer in a pillar lattice and
analyzing its scattering behavior, we identified the width of the reorientation
distribution of swimmers to be also crucial for the nonlinear behavior of the
swimmer deflection. On the basis of these results we suggest in addition an
analytical model for microswimmers, where the pillar lattice is replaced by an
anisotropic scattering medium, that depends only on a scattering rate and the
width of the reorientation distribution of swimmers. This flexible and handy
model fits the experimental results as well. The presented analysis of the
deflection of light guided swimmers through pillar lattice may be used for
separating swimmers having different reorientation distributions
Engineering passive swimmers by shaking liquids
The locomotion and design of microswimmers are topical issues of current
fundamental and applied research. In addition to numerous living and artificial
active microswimmers, a passive microswimmer was identified only recently: a
soft, Lambda-shaped, non-buoyant particle propagates in a shaken liquid of
zero-mean velocity [Jo et al. Phys. Rev. E 94, 063116 (2016)]. We show that
this novel passive locomotion mechanism works for realistic non-buoyant,
asymmetric Janus microcapsules as well. According to our analytical
approximation, this locomotion requires a symmetry breaking caused by different
Stokes drags of soft particles during the two half periods of the oscillatory
liquid motion. It is the intrinsic anisotropy of Janus capsules and
Lambda-shaped particles that break this symmetry for sinusoidal liquid motion.
Further, we show that this passive locomotion mechanism also works for the
wider class of symmetric soft particles, e.g., capsules, by breaking the
symmetry via an appropriate liquid shaking. The swimming direction can be
uniquely selected by a suitable choice of the liquid motion. Numerical studies,
including lattice Boltzmann simulations, also show that this locomotion can
outweigh gravity, i.e., non-buoyant particles may be either elevated in shaken
liquids or concentrated at the bottom of a container. This novel propulsion
mechanism is relevant to many applications, including the sorting of soft
particles like healthy and malignant (cancer) cells, which serves medical
purposes, or the use of non-buoyant soft particles as directed microswimmers .Comment: This is the version of the article before peer review or editing, as
submitted by an author to New Journal of Physics. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at
https://doi.org/10.1088/1367-2630/ab240