660 research outputs found
Emergence of macroscopic directed motion in populations of motile colloids
From the formation of animal flocks to the emergence of coordinate motion in
bacterial swarms, at all scales populations of motile organisms display
coherent collective motion. This consistent behavior strongly contrasts with
the difference in communication abilities between the individuals. Guided by
this universal feature, physicists have proposed that solely alignment rules at
the individual level could account for the emergence of unidirectional motion
at the group level. This hypothesis has been supported by agent-based
simulations. However, more complex collective behaviors have been
systematically found in experiments including the formation of vortices,
fluctuating swarms, clustering and swirling. All these model systems
predominantly rely on actual collisions to display collective motion. As a
result, the potential local alignment rules are entangled with more complex,
often unknown, interactions. The large-scale behavior of the populations
therefore depends on these uncontrolled microscopic couplings. Here, we
demonstrate a new phase of active matter. We reveal that dilute populations of
millions of colloidal rollers self-organize to achieve coherent motion along a
unique direction, with very few density and velocity fluctuations. Identifying
the microscopic interactions between the rollers allows a theoretical
description of this polar-liquid state. Comparison of the theory with
experiment suggests that hydrodynamic interactions promote the emergence of
collective motion either in the form of a single macroscopic flock at low
densities, or in that of a homogenous polar phase at higher densities.
Furthermore, hydrodynamics protects the polar-liquid state from the giant
density fluctuations. Our experiments demonstrate that genuine physical
interactions at the individual level are sufficient to set homogeneous active
populations into stable directed motion
Symmetry as a sufficient condition for a finite flex
We show that if the joints of a bar and joint framework are
positioned as `generically' as possible subject to given symmetry constraints
and possesses a `fully-symmetric' infinitesimal flex (i.e., the
velocity vectors of the infinitesimal flex remain unaltered under all symmetry
operations of ), then also possesses a finite flex which
preserves the symmetry of throughout the path. This and other related
results are obtained by symmetrizing techniques described by L. Asimov and B.
Roth in their paper `The Rigidity Of Graphs' from 1978 and by using the fact
that the rigidity matrix of a symmetric framework can be transformed into a
block-diagonalized form by means of group representation theory. The finite
flexes that can be detected with these symmetry-based methods can in general
not be found with the analogous non-symmetric methods.Comment: 26 pages, 10 figure
Collective beating of artificial microcilia
We combine technical, experimental and theoretical efforts to investigate the
collective dynamics of artificial microcilia in a viscous fluid. We take
advantage of soft-lithography and colloidal self-assembly to devise microcapets
made of hundreds of slender magnetic rods. This novel experimental setup is
used to investigate the dynamics of extended cilia arrays driven by a
precessing magnetic field. Whereas the dynamics of an isolated cilium is a
rigid body rotation, collective beating results in a symmetry breaking of the
precession patterns. The trajectories of the cilia are anisotropic and
experience a significant structural evolution as the actuation frequency
increases. We present a minimal model to account for our experimental findings
and demonstrate how the global geometry of the array imposes the shape of the
trajectories via long range hydrodynamic interactions.Comment: 5 pages, 3 figure
Emergent vortices in populations of colloidal rollers
Coherent vortical motion has been reported in a wide variety of populations
including living organisms (bacteria, fishes, human crowds) and synthetic
active matter (shaken grains, mixtures of biopolymers), yet a unified
description of the formation and structure of this pattern remains lacking.
Here we report the self-organization of motile colloids into a macroscopic
steadily rotating vortex. Combining physical experiments and numerical
simulations, we elucidate this collective behavior. We demonstrate that the
emergent-vortex structure lives on the verge of a phase separation, and single
out the very constituents responsible for this state of polar active matter.
Building on this observation, we establish a continuum theory and lay out a
strong foundation for the description of vortical collective motion in a broad
class of motile populations constrained by geometrical boundaries
A necessary flexibility condition of a nondegenerate suspension in Lobachevsky 3-space
We show that some combination of the lengths of all edges of the equator of a
flexible suspension in Lobachevsky 3-space is equal to zero (each length is
taken either positive or negative in this combination).Comment: 20 pages, 13 figure
Kinetics and cellular site of glycolipid loading control
CD1d-restricted natural killer T cells (NKT cells) possess a wide range of effector and regulatory
activities that are related to their ability to secrete both T helper 1 (Th1) cell- and Th2 cell-type
cytokines. We analyzed presentation of NKT cell activating α galactosylceramide (αGalCer) analogs
that give predominantly Th2 cell-type cytokine responses to determine how ligand structure controls
the outcome of NKT cell activation. Using a monoclonal antibody specific for αGalCer-CD1d
complexes to visualize and quantitate glycolipid presentation, we found that Th2 cell-type cytokinebiasing
ligands were characterized by rapid and direct loading of cell-surface CD1d proteins.
Complexes formed by association of these Th2 cell-type cytokine-biasing αGalCer analogs with
CD1d showed a distinctive exclusion from ganglioside-enriched, detergent-resistant plasma
membrane microdomains of antigen-presenting cells. These findings help to explain how subtle
alterations in glycolipid ligand structure can control the balance of proinflammatory and antiinflammatory
activities of NKT cells
Synthesis and biological activity of α-glucosyl C24:0 and C20:2 ceramides
a-Glucosyl ceramides 4 and 5 have been synthesised and evaluated for their ability to stimulate the activation
and expansion of human iNKT cells. The key challenge in the synthesis of both target molecules was the stereoselective synthesis of the a-glycosidic linkage. Of the methods examined, glycosylation using per-TMS-protected glucosyl iodide 16 was completely a-selective and provided gram quantities of amine 11, from which a-glucosyl ceramides 4 and 5 were obtained by N-acylation. a-GlcCer 4, containing a C24 saturated acyl chain, stimulated a marked proliferation and expansion of human circulating iNKT cells in short-term cultures. a-GlcCer 5, which contains a C20 11,14-cis-diene acyl chain (C20:2),induced extremely similar levels of iNKT cell activation and expansion
- …