1,721,266 research outputs found
A Quorum Sensing Active Matter in a Confined Geometry
No full textInspired by the problem of biofilm growth, we numerically investigate clustering in a two-dimensional suspension of active (Janus) particles of finite size confined in a circular cavity. Their dynamics is regulated by a non-reciprocal mechanism that causes them to switch from active to passive above a certain threshold of the perceived near-neighbor density (quorum sensing). A variety of cluster phases, i.e., glassy, solid (hexatic) and liquid, are observed, depending on the particle dynamics at the boundary, the quorum sensing range, and the level of noise
Enhanced buoyancy of active particles in convective flows
Full text linkWe numerically investigated the diffusion of a heavy active Brownian
particle in a linear periodic array of steady planar counter-rotating
convection rolls at high Peclet numbers. We show that, under certain
conditions, the particle rises to the surface even if it is denser than
the suspension fluid, and floats there for exceedingly long times. Such
an apparently counterintuitive phenomenon of ``enhanced buoyancy{''} is
a combined effect of gravity, advection, and shear torque
Consistent Hamiltonian models for space-momentum diffusion
No full textWe develop a unified Hamiltonian approach to the diffusion of a particle
coupled to a dissipative environment, an archetypal model widely invoked
to interpret condensed phase phenomena, such as polymerization and
cold-atom diffusion in optical lattices. By appropriate choices of the
coupling functions, we reformulate phenomenological diffusion models by
adding otherwise ignored space-momentum terms. We thus numerically
predict a variety of diffusion regimes, from diffusion saturation to
superballistic diffusion. With reference to ultracold atoms in optical
lattices, we also show that time correlated external noises prevent
superdiffusion from exceeding Richardson???s law. Some of these results
are unexpected and call for experimental validation
Hardness of T-carbon: Density functional theory calculations
No full textWe reconsider and interpret the mechanical properties of the recently proposed allotrope of carbon, T-carbon [Sheng et al., Phys. Rev. Lett. 106, 155703 (2011)], using density functional theory in combination with different empirical hardness models. In contrast with the early estimation based on Gao et al.'s model, which attributes to T-carbon a high Vickers hardness of 61 GPa comparable to that of superhard cubic boron nitride (c-BN), we find that T-carbon is not a superhard material, since its Vickers hardness does not exceed 10 GPa. Besides providing clear evidence for the absence of superhardness in T-carbon, we discuss the physical reasons behind the failure of Gao et al.'s and Simunek and Vackar's (SV) models in predicting the hardness of T-carbon, residing in their improper treatment of the highly anisotropic distribution of quasi-sp(3)-like C-C hybrids. A possible remedy for the Gao et al. and SV models based on the concept of the superatom is suggested, which indeed yields a Vickers hardness of about 8 GPa
Advection-enhanced diffusion in biased convection arrays
No full textWe numerically investigated the transport of a passive colloidal
particle in a one-dimensional periodic array of planar counter-rotating
convection rolls at high Peclet numbers. We show that advection-enhanced
diffusion is drastically suppressed by an external transverse bias but
strongly reinforced by a longitudinal drive of appropriate intensity.
Both effects are magnified by imposing free-slip flows at the array's
edges. The dependence of the diffusion constant on an external forcing
is interpreted as a measure of the fluid-mechanical robustness of the
flow boundary layer mechanism governing diffusion in convection rolls
Ratcheting by Stochastic Resetting With Fat-Tailed Time Distributions
Full text linkWe investigated both numerically and analytically the drift of a Brownian particle in a ratchet potential under stochastic resetting with fat-tailed distributions. As a study case we chose a Pareto time distribution with tail index beta. We observed that for 1 =2 < beta < 1 rectification occurs even if for beta < 1 the mean resetting time is infinite. However,for beta <= 1 = 2 rectification is completely suppressed. For low noise levels, the drift speed attains a maximum for beta immediately above 1, that is for finite but large mean resetting times. In correspondence with such an optimal drift the particle diffusion over the ratchet potential turns from normal to super diffusive ,a property also related to the fat tails of the resetting time distributio
Diffusion of active particles in convective flows
No full textWe numerically investigated the diffusion of an active Janus particle in
periodic arrays of planar counter-rotating convection rolls at high
Peclet numbers. We considered convection patterns with distinct
longitudinal and transverse advection properties and characterized the
dependence of the relevant diffusion constants on the particle's
dynamical parameters, namely, self-propulsion speed, correlation time
and chirality. Numerical results are interpreted analytically based on
qualitative arguments of classical transport theory. The purpose of the
present analysis is controlling active matter transport in microfluidic
devices
Non-Gaussian normal diffusion in low dimensional systems
Full text linkBrownian particles suspended in disordered crowded environments often
exhibit non-Gaussian normal diffusion (NGND), whereby their
displacements grow with mean square proportional to the observation time
and non-Gaussian statistics. Their distributions appear to decay almost
exponentially according to ``universal{''} laws largely insensitive to
the observation time. This effect is generically attributed to slow
environmental fluctuations, which perturb the local configuration of the
suspension medium. To investigate the microscopic mechanisms responsible
for the NGND phenomenon, we study Brownian diffusion in low dimensional
systems, like the free diffusion of ellipsoidal and active particles,
the diffusion of colloidal particles in fluctuating corrugated channels
and Brownian motion in arrays of planar convective rolls. NGND appears
to be a transient effect related to the time modulation of the
instantaneous particle's diffusivity, which can occur even under
equilibrium conditions. Consequently, we propose to generalize the
definition of NGND to include transient displacement distributions which
vary continuously with the observation time. To this purpose, we provide
a heuristic one-parameter function, which fits all time-dependent
transient displacement distributions corresponding to the same diffusion
constant. Moreover, we reveal the existence of low dimensional systems
where the NGND distributions are not leptokurtic (fat exponential
tails), as often reported in the literature, but platykurtic (thin
sub-Gaussian tails), i.e., with negative excess kurtosis. The actual
nature of the NGND transients is related to the specific microscopic
dynamics of the diffusing particle
Colloidal clustering and diffusion in a convection cell array
No full textWe numerically investigated the clustering of a uniform suspension of
finite-size disks in a linear array of two-dimensional convection cells.
We observed that, due to steric interactions, the disks tend to form
coherently rotating spatial structures at the center of each cell, as a
combined effect of advection and pair collisions. Micellar, ring-like
and hexatic patterns emerge in the deterministic regime, depending on
the suspension density, but dissolve in the presence of thermal
fluctuations. Moreover, pair collisions suffice to activate cell
crossings even by noiseless disks and, therefore, cause athermal
diffusion. The robustness of such collision induced effects is studied
against the opposing action of thermal noise, transverse biases, and
particle self-propulsion
Excess Diffusion of a Driven Colloidal Particle in a Convection Array
No full textWe numerically investigate the transport of a passive colloidal particle
in a periodic array of planar counter-rotating convection rolls, at high
Peclet numbers. It is shown that an external bias, oriented parallel to
the array, produces a huge excess diffusion peak, in cases where bias
and advection drag become comparable. This effect is not restricted to
one-dimensional convection geometries, and occurs independently of the
array's boundary conditions
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