1,248 research outputs found
Kinetic theory for strong uniform shear flow of granular media at high density
We discuss the uniform shear flow of a fluidized granular bed composed of
monodisperse Hertzian spheres. Considering high densities around the glass
transition density of inelastic Hertzian spheres, we report kinetic theory
expressions for the Newtonian viscosity as well as the Bagnold coefficient. We
discuss the dependence of the transport coefficients on density and coefficient
of restitution.Comment: Powders & Grains 201
The Glass Transition in Driven Granular Fluids: A Mode-Coupling Approach
We consider the stationary state of a fluid comprised of inelastic hard
spheres or disks under the influence of a random, momentum-conserving external
force. Starting from the microscopic description of the dynamics, we derive a
nonlinear equation of motion for the coherent scattering function in two and
three space dimensions. A glass transition is observed for all coefficients of
restitution, epsilon, at a critical packing fraction, phi_c(epsilon), below
random close packing. The divergence of timescales at the glass-transition
implies a dependence on compression rate upon further increase of the density -
similar to the cooling rate dependence of a thermal glass. The critical
dynamics for coherent motion as well as tagged particle dynamics is analyzed
and shown to be non-universal with exponents depending on space dimension and
degree of dissipation.Comment: 16 pages, 9 figure
Comment on "Explicit Analytical Solution for Random Close Packing in and "
I comment on Zaccone, Phys. Rev. Lett. {\bf 128}, 028002 (2022) highlighting
a flaw in the derivation that led to a spurious divergent factor. This renders
the derivation of the random close packing density invalid.Comment: Slightly cleaned up the presentation in response to the published
Erratu
Effective dynamics of microorganisms that interact with their own trail
Like ants, some microorganisms are known to leave trails on surfaces to
communicate. We explore how trail-mediated self-interaction could affect the
behavior of individual microorganisms when diffusive spreading of the trail is
negligible on the timescale of the microorganism using a simple
phenomenological model for an actively moving particle and a finite-width
trail. The effective dynamics of each microorganism takes on the form of a
stochastic integral equation with the trail interaction appearing in the form
of short-term memory. For moderate coupling strength below an emergent critical
value, the dynamics exhibits effective diffusion in both orientation and
position after a phase of superdiffusive reorientation. We report experimental
verification of a seemingly counterintuitive perpendicular alignment mechanism
that emerges from the model.Comment: new figure with experimental results; expanded appendi
Partitioning of energy in highly polydisperse granular gases
A highly polydisperse granular gas is modeled by a continuous distribution of
particle sizes, a, giving rise to a corresponding continuous temperature
profile, T(a), which we compute approximately, generalizing previous results
for binary or multicomponent mixtures. If the system is driven, it evolves
towards a stationary temperature profile, which is discussed for several
driving mechanisms in dependence on the variance of the size distribution. For
a uniform distribution of sizes, the stationary temperature profile is
nonuniform with either hot small particles (constant force driving) or hot
large particles (constant velocity or constant energy driving). Polydispersity
always gives rise to non-Gaussian velocity distributions. Depending on the
driving mechanism the tails can be either overpopulated or underpopulated as
compared to the molecular gas. The deviations are mainly due to small
particles. In the case of free cooling the decay rate depends continuously on
particle size, while all partial temperatures decay according to Haff's law.
The analytical results are supported by event driven simulations for a large,
but discrete number of species.Comment: 10 pages; 5 figure
The manifold rheology of fluidized granular media
Fluidized granular media have a rich rheology: measuring shear stress
as a function of shear rate , they exhibit Newtonian
behavior for low densities and shear rates, develop a
yield stress for intermediate shear rates and densities approaching the
granular glass transition, and finally, cross over to shear-thickening Bagnold
scaling, . This wealth of flow-behaviors makes
fluidized beds a fascinating material, but also one that is challenging to
encompass into a global theory, despite its relevance for optimizing industrial
processes and predicting natural hazards. We provide careful measurements
spanning eight orders of magnitude in shear rate, and show that all these
rheological regimes can be described qualitatively and quantitatively using the
granular integration through transient formalism, a theory for glassy dynamics
under shear adapted to granular fluids
Drosophila bloom helicase maintains genome integrity by inhibiting recombination between divergent DNA sequences
DNA double strand breaks (DSB) can be repaired either via a sequence independent joining of DNA ends or via homologous recombination. We established a detection system in D. melanogaster to investigate the impact of sequence constraints on the usage of the homology based DSB repair via single strand annealing (SSA), which leads to recombination between direct repeats with concomitant loss of one repeat copy. First of all, we find the SSA frequency to be inversely proportional to the spacer length between the repeats, for spacers up to 2.4 kb in length. We further show that SSA between divergent repeats (homeologous SSA) is suppressed in cell cultures and in vivo in a sensitive manner, recognizing sequence divergences smaller than 0.5%. Finally, we demonstrate that the suppression of homeologous SSA depends on the Bloom helicase (Blm), encoded by the Drosophila gene mus309. Suppression of homeologous recombination is a novel function of Blm in ensuring genomic integrity, not described to date in mammalian systems. Unexpectedly, distinct from its function in S. cerevisiae, the mismatch repair (MMR) factor Msh2 encoded by spel1 does not suppress homeologous SSA in Drosophil
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