1,608 research outputs found
Physics of Microswimmers - Single Particle Motion and Collective Behavior
Locomotion and transport of microorganisms in fluids is an essential aspect
of life. Search for food, orientation toward light, spreading of off-spring,
and the formation of colonies are only possible due to locomotion. Swimming at
the microscale occurs at low Reynolds numbers, where fluid friction and
viscosity dominates over inertia. Here, evolution achieved propulsion
mechanisms, which overcome and even exploit drag. Prominent propulsion
mechanisms are rotating helical flagella, exploited by many bacteria, and
snake-like or whip-like motion of eukaryotic flagella, utilized by sperm and
algae. For artificial microswimmers, alternative concepts to convert chemical
energy or heat into directed motion can be employed, which are potentially more
efficient. The dynamics of microswimmers comprises many facets, which are all
required to achieve locomotion. In this article, we review the physics of
locomotion of biological and synthetic microswimmers, and the collective
behavior of their assemblies. Starting from individual microswimmers, we
describe the various propulsion mechanism of biological and synthetic systems
and address the hydrodynamic aspects of swimming. This comprises
synchronization and the concerted beating of flagella and cilia. In addition,
the swimming behavior next to surfaces is examined. Finally, collective and
cooperate phenomena of various types of isotropic and anisotropic swimmers with
and without hydrodynamic interactions are discussed.Comment: 54 pages, 59 figures, review article, Reports of Progress in Physics
(to appear
Meso-scale turbulence in living fluids
Turbulence is ubiquitous, from oceanic currents to small-scale biological and
quantum systems. Self-sustained turbulent motion in microbial suspensions
presents an intriguing example of collective dynamical behavior amongst the
simplest forms of life, and is important for fluid mixing and molecular
transport on the microscale. The mathematical characterization of turbulence
phenomena in active non-equilibrium fluids proves even more difficult than for
conventional liquids or gases. It is not known which features of turbulent
phases in living matter are universal or system-specific, or which
generalizations of the Navier-Stokes equations are able to describe them
adequately. Here, we combine experiments, particle simulations, and continuum
theory to identify the statistical properties of self-sustained meso-scale
turbulence in active systems. To study how dimensionality and boundary
conditions affect collective bacterial dynamics, we measured energy spectra and
structure functions in dense Bacillus subtilis suspensions in quasi-2D and 3D
geometries. Our experimental results for the bacterial flow statistics agree
well with predictions from a minimal model for self-propelled rods, suggesting
that at high concentrations the collective motion of the bacteria is dominated
by short-range interactions. To provide a basis for future theoretical studies,
we propose a minimal continuum model for incompressible bacterial flow. A
detailed numerical analysis of the 2D case shows that this theory can reproduce
many of the experimentally observed features of self-sustained active
turbulence.Comment: accepted PNAS version, 6 pages, click doi for Supplementary
Informatio
Collective motion and nonequilibrium cluster formation in colonies of gliding bacteria
We characterize cell motion in experiments and show that the transition to
collective motion in colonies of gliding bacterial cells confined to a
monolayer appears through the organization of cells into larger moving
clusters. Collective motion by non-equilibrium cluster formation is detected
for a critical cell packing fraction around 17%. This transition is
characterized by a scale-free power-law cluster size distribution, with an
exponent , and the appearance of giant number fluctuations. Our
findings are in quantitative agreement with simulations of self-propelled rods.
This suggests that the interplay of self-propulsion of bacteria and the
rod-shape of bacteria is sufficient to induce collective motion
Motility-induced phase separation and coarsening in active matter
Active systems, or active matter, are self-driven systems which live, or
function, far from equilibrium - a paradigmatic example which we focus on here
is provided by a suspension of self-motile particles. Active systems are far
from equilibrium because their microscopic constituents constantly consume
energy from the environment in order to do work, for instance to propel
themselves. The nonequilibrium nature of active matter leads to a variety of
non-trivial intriguing phenomena. An important one which has recently been the
subject of intense interest among biological and soft matter physicists is that
of the so-called "motility-induced phase separation", whereby self-propelled
particles accumulate into clusters in the absence of any explicit attractive
interactions between them. Here we review the physics of motility-induced phase
separation, and discuss this phenomenon within the framework of the classic
physics of phase separation and coarsening. We also discuss theories for
bacterial colonies where coarsening may be arrested. Most of this work will
focus on the case of run-and-tumble and active Brownian particles in the
absence of solvent-mediated hydrodynamic interactions - we will briefly discuss
at the end their role, which is not currently fully understood in this context.Comment: Contribution to the special issue "Coarsening dynamics", Comptes
Rendus de Physique, see
https://sites.google.com/site/ppoliti/crp-special-issu
Tuning the motility and directionality of self-propelled colloids
Microorganisms are able to overcome the thermal randomness of their
surroundings by harvesting energy to navigate in viscous fluid environments. In
a similar manner, synthetic colloidal microswimmers are capable of mimicking
complex biolocomotion by means of simple self-propulsion mechanisms. Although
experimentally the speed of active particles can be controlled by e.g.
self-generated chemical and thermal gradients, an in-situ change of swimming
direction remains a challenge. In this work, we study self-propulsion of
half-coated spherical colloids in critical binary mixtures and show that the
coupling of local body forces, induced by laser illumination, and the wetting
properties of the colloid, can be used to finely tune both the colloid's
swimming speed and its directionality. We experimentally and numerically
demonstrate that the direction of motion can be reversibly switched by means of
the size and shape of the droplet(s) nucleated around the colloid, depending on
the particle radius and the fluid's ambient temperature. Moreover, the
aforementioned features enable the possibility to realize both negative and
positive phototaxis in light intensity gradients. Our results can be extended
to other types of half-coated microswimmers, provided that both of their
hemispheres are selectively made active but with distinct physical properties.Comment: 12 pages, 5 figures. Scientific Reports (Received: 04 August 2017,
accepted: 04 October 2017, published online: 02 November 2017
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