24 research outputs found
Viscotaxis: microswimmer navigation in viscosity gradients
The survival of many microorganisms, like \textit{Leptospira} or
\textit{Spiroplasma} bacteria, can depend on their ability to navigate towards
regions of favorable viscosity. While this ability, called viscotaxis, has been
observed in several bacterial experiments, the underlying mechanism remains
unclear. Here, we provide a framework to study viscotaxis of self-propelled
swimmers in slowly varying viscosity fields and show that suitable body shapes
create viscotaxis based on a systematic asymmetry of viscous forces acting on a
microswimmer. Our results shed new light on viscotaxis in \textit{Spiroplasma}
and \textit{Leptospira} and suggest that dynamic body shape changes exhibited
by both types of microorganisms may have an unrecognized functionality: to
prevent them from drifting to low viscosity regions where they swim poorly. The
present theory classifies microswimmers regarding their ability to show
viscotaxis and can be used to design synthetic viscotactic swimmers, e.g.\ for
delivering drugs to a target region distinguished by viscosity
Phototaxis of synthetic microswimmers in optical landscapes
Many microorganisms, with phytoplankton and zooplankton as prominent
examples, display phototactic behaviour, that is, the ability to perform
directed motion within a light gradient. Here we experimentally demonstrate
that sensing of light gradients can also be achieved in a system of synthetic
photo-activated microparticles being exposed to an inhomogeneous laser field.
We observe a strong orientational response of the particles because of
diffusiophoretic torques, which in combination with an intensity-dependent
particle motility eventually leads to phototaxis. Since the aligning torques
saturate at high gradients, a strongly rectified particle motion is found even
in periodic asymmetric intensity landscapes. Our results are in excellent
agreement with numerical simulations of a minimal model and should similarly
apply to other particle propulsion mechanisms. Because light fields can be
easily adjusted in space and time, this also allows to extend our approach to
dynamical environments.Comment: 10 pages, 7 figure
Colloidal Brazil nut effect in microswimmer mixtures induced by motility contrast
We numerically and experimentally study the segregation dynamics in a binary
mixture of microswimmers which move on a two-dimensional substrate in a static
periodic triangular-like light intensity field. The motility of the active
particles is proportional to the imposed light intensity and they possess a
motility contrast, i.e., the prefactor depends on the species. In addition, the
active particles also experience a torque aligning their motion towards the
direction of the negative intensity gradient. We find a segregation of active
particles near the intensity minima where typically one species is localized
close to the minimum and the other one is centered around in an outer shell.
For a very strong aligning torque, there is an exact mapping onto an
equilibrium system in an effective external potential that is minimal at the
intensity minima. This external potential is similar to (height-dependent)
gravity, such that one can define effective `heaviness' of the self-propelled
particles. In analogy to shaken granular matter in gravity, we define a
`colloidal Brazil nut effect' if the heavier particles are floating on top of
the lighter ones. Using extensive Brownian dynamics simulations, we identify
system parameters for the active colloidal Brazil nut effect to occur and
explain it based on a generalized Archimedes' principle within the effective
equilibrium model: heavy particles are levitated in a dense fluid of lighter
particles if their effective mass density is lower than that of the surrounding
fluid. We also perform real-space experiments on light-activated self-propelled
colloidal mixtures which confirm the theoretical predictions.Comment: 10 pages, 5 figures, JCP Special Topic on Chemical Physics of Active
Matte
Brownian motion of a circle swimmer in a harmonic trap
We study the dynamics of a Brownian circle swimmer with a time-dependent
self-propulsion velocity in an external temporally varying harmonic potential.
For several situations, the noise-free swimming paths, the noise-averaged mean
trajectories, and the mean square displacements are calculated analytically or
by computer simulation. Based on our results, we discuss optimal swimming
strategies in order to explore a maximum spatial range around the trap center.
In particular, we find a resonance situation for the maximum escape distance as
a function of the various frequencies in the system. Moreover, the influence of
the Brownian noise is analyzed by comparing noise-free trajectories at zero
temperature with the corresponding noise-averaged trajectories at finite
temperature. The latter reveal various complex self-similar spiral or
rosette-like patterns. Our predictions can be tested in experiments on
artificial and biological microswimmers under dynamical external confinement.Comment: 16 pages, 11 figure
Non-Gaussian behaviour of a self-propelled particle on a substrate
The overdamped Brownian motion of a self-propelled particle which is driven
by a projected internal force is studied by solving the Langevin equation
analytically. The "active" particle under study is restricted to move along a
linear channel. The direction of its internal force is orientationally
diffusing on a unit circle in a plane perpendicular to the substrate. An
additional time-dependent torque is acting on the internal force orientation.
The model is relevant for active particles like catalytically driven Janus
particles and bacteria moving on a substrate. Analytical results for the first
four time-dependent displacement moments are presented and analysed for several
special situations. For vanishing torque, there is a significant dynamical
non-Gaussian behaviour at finite times t as signalled by a non-vanishing
normalized kurtosis in the particle displacement which approaches zero for long
time with a 1/t long-time tail.Comment: 20 pages, 10 figure
Gravitaxis of asymmetric self-propelled colloidal particles
Many motile microorganisms adjust their swimming motion relative to the
gravitational field and thus counteract sedimentation to the ground. This
gravitactic behavior is often the result of an inhomogeneous mass distribution
which aligns the microorganism similar to a buoy. However, it has been
suggested that gravitaxis can also result from a geometric fore-rear asymmetry,
typical for many self-propelling organisms. Despite several attempts, no
conclusive evidence for such an asymmetry-induced gravitactic motion exists.
Here, we study the motion of asymmetric self-propelled colloidal particles
which have a homogeneous mass density and a well-defined shape. In experiments
and by theoretical modeling we demonstrate that a shape anisotropy alone is
sufficient to induce gravitactic motion with either preferential upward or
downward swimming. In addition, also trochoid-like trajectories transversal to
the direction of gravity are observed.Comment: 9 pages, 5 figures, 1 tabl