109 research outputs found
Living Liquid Crystals
Collective motion of self-propelled organisms or synthetic particles often
termed active fluid has attracted enormous attention in broad scientific
community because of it fundamentally non-equilibrium nature. Energy input and
interactions among the moving units and the medium lead to complex dynamics.
Here we introduce a new class of active matter, living liquid crystals (LLCs)
that combine living swimming bacteria with a lyotropic liquid crystal. The
physical properties of LLCs can be controlled by the amount of oxygen available
to bacteria, by concentration of ingredients, or by temperature. Our studies
reveal a wealth of new intriguing dynamic phenomena, caused by the coupling
between the activity-triggered flow and long-range orientational order of the
medium. Among these are (a) non-linear trajectories of bacterial motion guided
by non-uniform director, (b) local melting of the liquid crystal caused by the
bacteria-produced shear flows, (c) activity-triggered transition from a
non-flowing uniform state into a flowing one-dimensional periodic pattern and
its evolution into a turbulent array of topological defects, (d)
birefringence-enabled visualization of microflow generated by the
nanometers-thick bacterial flagella. Unlike their isotropic counterpart, the
LLCs show collective dynamic effects at very low volume fraction of bacteria,
on the order of 0.2%. Our work suggests an unorthodox design concept to control
and manipulate the dynamic behavior of soft active matter and opens the door
for potential biosensing and biomedical applications.Comment: 32 pages, 8 figures, Supporting Information include
Model of coarsening and vortex formation in vibrated granular rods
Neicu and Kudrolli observed experimentally spontaneous formation of the
long-range orientational order and large-scale vortices in a system of vibrated
macroscopic rods. We propose a phenomenological theory of this phenomenon,
based on a coupled system of equations for local rods density and tilt. The
density evolution is described by modified Cahn-Hilliard equation, while the
tilt is described by the Ginzburg-Landau type equation. Our analysis shows
that, in accordance to the Cahn-Hilliard dynamics, the islands of the ordered
phase appear spontaneously and grow due to coarsening. The generic vortex
solutions of the Ginzburg-Landau equation for the tilt correspond to the
vortical motion of the rods around the cores which are located near the centers
of the islands.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Suppression of bacterial rheotaxis in wavy channels
Controlling the swimming behavior of bacteria is crucial, for example, to
prevent contamination of ducts and catheters. We show the bacteria modeled by
deformable microswimmers can accumulate in flows through straight microchannels
either in their center or on previously unknown attractors near the channel
walls. We predict a novel resonance effect for semiflexible microswimmers in
flows through wavy microchannels. As a result, microswimmers can be deflected
in a controlled manner so that they swim in modulated channels distributed over
the channel cross-section rather than localized near the wall or the channel
center.
Thus, depending on the flow amplitude, both upstream orientation of swimmers
and their accumulation at the boundaries which can lead to surface rheotaxis
are suppressed. Our results suggest new strategies for controlling the behavior
of live and synthetic swimmers in microchannels
A particle-field representation unifies paradigms in active matter
Active matter research focuses on the emergent behavior among interacting
self-propelled particles. Unification of seemingly disconnected paradigms --
active phase-separation of repulsive discs and collective motion of
self-propelled rods -- is a major challenge in contemporary active matter.
Inspired by the quanto-mechanical wave-particle duality, we develop an approach
based on the representation of active particles by smoothed continuum fields.
On the basis of the collision kinetics, we demonstrate analytically and
numerically how nonequilibrium stresses acting among self-driven, anisotropic
objects hinder the formation of phase-separated states as observed for
self-propelled discs and facilitate the emergence of orientational order.
Besides particle shape, the rigidity of self-propelled objects controlling the
symmetry of emergent ordered states is as a crucial parameter: impenetrable,
anisotropic rods are found to form polar, moving clusters, whereas large-scale
nematic structures emerge for soft rods, notably separated by a bistable
coexistence regime. These results indicate that the symmetry of the ordered
state is not dictated by the symmetry of the interaction potential but is
rather a dynamical, emergent property of active systems. This unifying
theoretical framework can represent a variety of active systems: living cell
tissues, bacterial colonies, cytoskeletal extracts as well as shaken granular
media.Comment: 14 pages, 8 figure
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