76 research outputs found
Soft deformable self-propelled particles
In this work we investigate the collective behavior of self-propelled
particles that deform due to local pairwise interactions. We demonstrate that
this deformation alone can induce alignment of the velocity vectors. The onset
of collective motion is analyzed. Applying a Gaussian-core repulsion between
the particles, we find a transition to disordered non-collective motion under
compression. We here explain that this reflects the reentrant fluid behavior of
the general Gaussian-core model now applied to a self-propelled system.
Truncating the Gaussian potential can lead to cluster crystallization or more
disordered cluster states. For intermediate values of the Gaussian-core
potential we for the first time observe laning for deformable self-propelled
particles. Finally, without the core potential, but including orientational
noise, we connect our description to the Vicsek approach for self-propelled
particles with nematic alignment interactions.Comment: 6 pages, 7 figure
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Interfacial mechanisms in active emulsions
Active emulsions, i.e., emulsions whose droplets perform self-propelled motion, are of tremendous interest for mimicking collective phenomena in biological populations such as phytoplankton and bacterial colonies, but also for experimentally studying rheology, pattern formation, and phase transitions in systems far from thermal equilibrium. For fuelling such systems, molecular processes involving the surfactants which stabilize the emulsions are a straightforward concept. We outline and compare two different types of reactions, one which chemically modifies the surfactant molecules, the other which transfers them into a different colloidal state. While in the first case symmetry breaking follows a standard linear instability, the second case turns out to be more complex. Depending on the dissolution pathway, there is either an intrinsically nonlinear instability, or no symmetry breaking at all (and hence no locomotion)
Focusing by blocking: repeatedly generating central density peaks in self-propelled particle systems by exploiting diffusive processes
Over the past few years the displacement statistics of self-propelled
particles has been intensely studied, revealing their long-time diffusive
behavior. Here, we demonstrate that a concerted combination of boundary
conditions and switching on and off the self-propelling drive can generate and
afterwards arbitrarily often restore a non-stationary centered peak in their
spatial distribution. This corresponds to a partial reversibility of their
statistical behavior, in opposition to the above-mentioned long-time diffusive
nature. Interestingly, it is a diffusive process that mediates and makes
possible this procedure. It should be straightforward to verify our predictions
in a real experimental system.Comment: 6 pages, 6 figure
Whirligig beetles as corralled active Brownian particles
We study the collective dynamics of groups of whirligig beetles Dineutus discolor (Coleoptera: Gyrinidae) swimming freely on the surface of water. We extract individual trajectories for each beetle, including positions and orientations, and use this to discover (i) a density-dependent speed scaling like v ∼ ρ−ν with ν ≈ 0.4 over two orders of magnitude in density (ii) an inertial delay for velocity alignment of approximately 13 ms and (iii) coexisting high and low-density phases, consistent with motility-induced phase separation (MIPS). We modify a standard active Brownian particle (ABP) model to a corralled ABP (CABP) model that functions in open space by incorporating a density-dependent reorientation of the beetles, towards the cluster. We use our new model to test our hypothesis that an motility-induced phase separation (MIPS) (or a MIPS like effect) can explain the co-occurrence of high- and low-density phases we see in our data. The fitted model then successfully recovers a MIPS-like condensed phase for N = 200 and the absence of such a phase for smaller group sizes N = 50, 100
Active and driven hydrodynamic crystals
Motivated by the experimental ability to produce monodisperse particles in
microfluidic devices, we study theoretically the hydrodynamic stability of
driven and active crystals. We first recall the theoretical tools allowing to
quantify the dynamics of elongated particles in a confined fluid. In this
regime hydrodynamic interactions between particles arise from a superposition
of potential dipolar singularities. We exploit this feature to derive the
equations of motion for the particle positions and orientations. After showing
that all five planar Bravais lattices are stationary solutions of the equations
of motion, we consider separately the case where the particles are passively
driven by an external force, and the situation where they are self-propelling.
We first demonstrate that phonon modes propagate in driven crystals, which are
always marginally stable. The spatial structure of the eigenmodes depend solely
on the symmetries of the lattices, and on the orientation of the driving force.
For active crystals, the stability of the particle positions and orientations
depends not only on the symmetry of the crystals but also on the perturbation
wavelengths and on the crystal density. Unlike unconfined fluids, the stability
of active crystals is independent of the nature of the propulsion mechanism at
the single particle level. The square and rectangular lattices are found to be
linearly unstable at short wavelengths provided the volume fraction of the
crystals is high enough. Differently, hexagonal, oblique, and face-centered
crystals are always unstable. Our work provides a theoretical basis for future
experimental work on flowing microfluidic crystals.Comment: 10 pages, 10 figure
Periodic and Quasiperiodic Motion of an Elongated Microswimmer in Poiseuille Flow
We study the dynamics of a prolate spheroidal microswimmer in Poiseuille flow
for different flow geometries. When moving between two parallel plates or in a
cylindrical microchannel, the swimmer performs either periodic swinging or
periodic tumbling motion. Although the trajectories of spherical and elongated
swimmers are qualitatively similar, the swinging and tumbling frequency
strongly depends on the aspect ratio of the swimmer. In channels with reduced
symmetry the swimmers perform quasiperiodic motion which we demonstrate
explicitely for swimming in a channel with elliptical cross section
Osmosis with active solutes
Despite much current interest in active matter, little is known about osmosis
in active systems. Using molecular dynamics simulations, we investigate how
active solutes perturb osmotic steady states. We find that solute activity
increases the osmotic pressure, and can also expel solvent from the solution -
i.e. cause reverse osmosis. The latter effect cannot be described by an
effective temperature, but can be reproduced by mapping the active solution
onto a passive one with the same degree of local structuring as the passive
solvent component. Our results provide a basic framework for understanding
active osmosis, and suggest that activity-induced structuring of the passive
component may play a key role in the physics of active-passive mixtures.Comment: 6 page
Scalar <i>φ</i><sup>4</sup> field theory for active-particle phase separation
Recent theories predict phase separation among orientationally disordered
active particles whose propulsion speed decreases rapidly enough with density.
Coarse-grained models of this process show time-reversal symmetry (detailed
balance) to be restored for uniform states, but broken by gradient terms; hence
detailed-balance violation is strongly coupled to interfacial phenomena. To
explore the subtle generic physics resulting from such coupling we here
introduce `Active Model B'. This is a scalar field theory (or
phase-field model) that minimally violates detailed balance via a leading-order
square-gradient term. We find that this additional term has modest effects on
coarsening dynamics, but alters the static phase diagram by creating a jump in
(thermodynamic) pressure across flat interfaces. Both results are surprising,
since interfacial phenomena are always strongly implicated in coarsening
dynamics but are, in detailed-balance systems, irrelevant for phase equilibria.Comment: 15 pages, 7 figure
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