95 research outputs found
A continuum theory of phase separation kinetics for active Brownian particles
Active Brownian particles (ABPs), when subject to purely repulsive
interactions, are known to undergo activity-induced phase separation broadly
resembling an equilibrium (attraction-induced) gas-liquid coexistence. Here we
present an accurate continuum theory for the dynamics of phase-separating ABPs,
derived by direct coarse-graining, capturing leading-order density gradient
terms alongside an effective bulk free energy. Such gradient terms do not obey
detailed balance; yet we find coarsening dynamics closely resembling that of
equilibrium phase separation. Our continuum theory is numerically compared to
large-scale direct simulations of ABPs and accurately accounts for domain
growth kinetics, domain topologies and coexistence densities
Bulk simulation of polar liquids in spherical symmetry.
Molecular simulations of strongly coupled dipolar systems of varying size have been carried out, using particles confined inside a dielectric cavity and an image charge approach to treat the dielectric response from the surroundings. A simple method using penalty functions was employed to create an isotropic and homogeneous distribution of particles inside the cavity. The dielectric response of the molecular system was found to increase as the number of particles N was increased. Nevertheless, a significant surface effect remained even for the largest systems (N=10,000), manifesting itself through a decrease in the dielectric constant of the system as the confining surface was approached. The surface effect was significantly reduced by using a negative dielectric constant of the surrounding dielectric medium, although accomplishing a full dielectric solvation of the molecular system was not possible
Nonequilibrium dynamics of mixtures of active and passive colloidal particles
We develop a mesoscopic field theory for the collective nonequilibrium
dynamics of multicomponent mixtures of interacting active (i.e., motile) and
passive (i.e., nonmotile) colloidal particles with isometric shape in two
spatial dimensions. By a stability analysis of the field theory, we obtain
equations for the spinodal that describes the onset of a motility-induced
instability leading to cluster formation in such mixtures. The prediction for
the spinodal is found to be in good agreement with particle-resolved computer
simulations. Furthermore, we show that in active-passive mixtures the spinodal
instability can be of two different types. One type is associated with a
stationary bifurcation and occurs also in one-component active systems, whereas
the other type is associated with a Hopf bifurcation and can occur only in
active-passive mixtures. Remarkably, the Hopf bifurcation leads to moving
clusters. This explains recent results from simulations of active-passive
particle mixtures, where moving clusters and interfaces that are not seen in
the corresponding one-component systems have been observed.Comment: 17 pages, 3 figure
Structural Anisotropy in Polar Fluids Subjected to Periodic Boundary Conditions
A heuristic model based on dielectric continuum theory for the long-range solvation free energy of a dipolar system possessing periodic boundary conditions (PBCs) is presented. The predictions of the model are compared to simulation results for Stockmayer fluids simulated using three different cell geometries. The boundary effects induced by the PBCs are shown to lead to anisotropies in the apparent dielectric constant and the long-range solvation free energy of as much as 50%. However, the sum of all of the anisotropic energy contributions yields a value that is very close to the isotropic one derived from dielectric continuum theory, leading to a total system energy close to the dielectric value. It is finally shown that the leading-order contribution to the energetic and structural anisotropy is significantly smaller in the noncubic simulation cell geometries compared to when using a cubic simulation cell
Phase behaviour of active Brownian particles:the role of dimensionality
Recently, there has been much interest in activity-induced phase separations
in concentrated suspensions of "active Brownian particles" (ABPs),
self-propelled spherical particles whose direction of motion relaxes through
thermal rotational diffusion. To date, almost all these studies have been
restricted to 2 dimensions. In this work we study activity-induced phase
separation in 3D and compare the results with previous and new 2D simulations.
To this end, we performed state-of-the-art Brownian dynamics simulations of up
to 40 million ABPs -- such very large system sizes are unavoidable to evade
finite size effects in 3D. Our results confirm the picture established for 2D
systems in which an activity-induced phase separation occurs, with strong
analogies to equilibrium gas-liquid spinodal decomposition, in spite of the
purely non-equilibrium nature of the driving force behind the phase separation.
However, we also find important differences between the 2D and 3D cases.
Firstly, the shape and position of the phase boundaries is markedly different
for the two cases. Secondly, for the 3D coarsening kinetics we find that the
domain size grows in time according to the classical diffusive law,
in contrast to the nonstandard subdiffusive exponent observed in 2D
Nondielectric long-range solvation of polar liquids in cubic symmetry
Long-range solvation properties of strongly coupled dipolar systems simulated using the Ewald and reaction field methods are assessed by using electric fluctuation formulas for a dielectric medium. Some components of the fluctuating electric multipole moments are suppressed, whereas other components are favored as the boundary of the simulation box is approached. An analysis of electrostatic interactions in a periodic cubic system suggests that these structural effects are due to the periodicity embedded in the Ewald method. Furthermore, the results obtained using the reaction field method are very similar to those obtained using the Ewald method, an effect which we attribute to the use of toroidal boundary conditions in the former case. Thus, the long-range solvation properties of polar liquids simulated using either of the two methods are nondielectric in their character. (C) 2009 American Institute of Physics. [doi:10.1063/1.3250941
Dynamics-dependent density distribution in active suspensions
Self-propelled colloids constitute an important class of intrinsically
non-equilibrium matter. Typically, such a particle moves ballistically at short
times, but eventually changes its orientation, and displays random-walk
behavior in the long-time limit. Theory predicts that if the velocity of
non-interacting swimmers varies spatially in 1D, , then their density
satisfies , where is an arbitrary
reference point. Such a dependence of steady-state on the particle
dynamics, which was the qualitative basis of recent work demonstrating how to
`paint' with bacteria, is forbidden in thermal equilibrium. We verify this
prediction quantitatively by constructing bacteria that swim with an
intensity-dependent speed when illuminated. A spatial light pattern therefore
creates a speed profile, along which we find that, indeed, , provided that steady state is reached
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