387 research outputs found
Self-propelled rods exhibit a novel phase-separated state characterized by the presence of active stresses and the ejection of polar clusters
We study collections of self-propelled rods (SPR) moving in two dimensions
for packing fractions less than or equal to 0.3. We find that in the
thermodynamical limit the SPR undergo a phase transition between a disordered
gas and a novel phase-separated system state. Interestingly, (global)
orientational order patterns -- contrary to what has been suggested -- vanish
in this limit. In the found novel state, the SPR self-organize into a highly
dynamical, high-density, compact region - which we call aggregate - which is
surrounded by a disordered gas. Active stresses build inside aggregates as
result of the combined effect of local orientational order and active forces.
This leads to the most distinctive feature of these aggregates: constant
ejection of polar clusters of SPR. This novel phase-separated state represents
a novel state of matter characterized by large fluctuations in volume and
shape, related to mass ejection, and exhibits positional as well as
orientational local order. SPR systems display new physics unseen in other
active matter systems due to the coupling between density, active stresses, and
orientational order (such coupling cannot be reduced simply to a coupling
between speed and density).Comment: to appear in PR
Recycling probability and dynamical properties of germinal center reactions
We introduce a new model for the dynamics of centroblasts and centrocytes in
a germinal center. The model reduces the germinal center reaction to the
elements considered as essential and embeds proliferation of centroblasts,
point mutations of the corresponding antibody types represented in a shape
space, differentiation to centrocytes, selection with respect to initial
antigens, differentiation of positively selected centrocytes to plasma or
memory cells and recycling of centrocytes to centroblasts. We use exclusively
parameters with a direct biological interpretation such that, once determined
by experimental data, the model gains predictive power. Based on the experiment
of Han et al. (1995b) we predict that a high rate of recycling of centrocytes
to centroblasts is necessary for the germinal center reaction to work reliably.
Furthermore, we find a delayed start of the production of plasma and memory
cells with respect to the start of point mutations, which turns out to be
necessary for the optimization process during the germinal center reaction. The
dependence of the germinal center reaction on the recycling probability is
analyzed.Comment: 30 pages, 8 figure
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
A mean-field theory for self-propelled particles interacting by velocity alignment mechanisms
A mean-field approach (MFA) is proposed for the analysis of orientational
order in a two-dimensional system of stochastic self-propelled particles
interacting by local velocity alignment mechanism. The treatment is applied to
the cases of ferromagnetic (F) and liquid-crystal (LC) alignment. In both
cases, MFA yields a second order phase transition for a critical noise strength
and a scaling exponent of 1/2 for the respective order parameters. We find that
the critical noise amplitude at which orientational order emerges in
the LC case is smaller than in the F-alignment case, i.e.
. A comparison with simulations of individual-based
models with F- resp. LC-alignment shows that the predictions about the critical
behavior and the qualitative relation between the respective critical noise
amplitudes are correct
- …