199 research outputs found
Anisotropic particles near surfaces: Self-propulsion and friction
We theoretically study the phenomenon of self-propulsion through Casimir
forces in thermal non-equilibrium. Using fluctuational electrodynamics, we
derive a formula for the self-propulsion force for an arbitrary small object in
two scenarios, i) for the object being isolated, and ii) for the object being
close to a planar surface. In the latter case, the self-propulsion force (i.e.,
the force parallel to the surface) increases with decreasing distance, i.e., it
couples to the near-field. We numerically calculate the lateral force acting on
a hot spheroid near a surface and show that it can be as large as the
gravitational force, thus being potentially measurable in fly-by experiments.
We close by linking our results to well-known relations of linear response
theory in fluctuational electrodynamics: Looking at the friction of the
anisotropic object for constant velocity, we identify a correction term that is
additional to the typically used approach.Comment: 13 pages, 8 figures (v2: References updated
Heat radiation and transfer for point particles in arbitrary geometries
We study heat radiation and heat transfer for pointlike particles in a system
of other objects. Starting from exact many-body expressions found from
scattering theory and fluctuational electrodynamics, we find that transfer and
radiation for point particles are given in terms of the Green's function of the
system in the absence of the point particles. These general expressions contain
no approximation for the surrounding objects. As an application, we compute the
heat transfer between two point particles in the presence of a sphere of
arbitrary size and show that the transfer is enhanced by several orders of
magnitude through the presence of the sphere, depending on the materials.
Furthermore, we compute the heat emission of a point particle in front of a
planar mirror. Finally, we show that a particle placed inside a spherical
mirror cavity does not radiate energy.Comment: 14 pages, 9 figures (v2: Sec. IIIE was added; explanation of Eq. (29)
was added; sentence in Acknowledgments was added; Ref. [69] was added; minor
changes in text
Oscillating Modes of Driven Colloids in Overdamped Systems
Microscopic particles suspended in liquids are the prime example of an
overdamped system because viscous forces dominate over inertial effects. Apart
from their use as model systems, they receive considerable attention as
sensitive probes from which forces on molecular scales can be inferred. The
interpretation of such experiments rests on the assumption, that, even if the
particles are driven, the liquid remains in equilibrium, and all modes are
overdamped. Here, we experimentally demonstrate that this is no longer valid
when a particle is forced through a viscoelastic fluid. Even at small driving
velocities where Stokes law remains valid, we observe particle oscillations
with periods up to several tens of seconds. We attribute these to
non-equilibrium fluctuations of the fluid, which are excited by the particle's
motion. The observed oscillatory dynamics is in quantitative agreement with an
overdamped Langevin equation with negative friction-memory term and which is
equivalent to the motion of a stochastically driven underdamped oscillator.
This fundamentally new oscillatory mode will largely expand the variety of
model systems but has also considerable implications on how molecular forces
are determined by colloidal probe particles under natural viscoelastic
conditions.Comment: Accepted with Nat. Comm. (originally submitted version, complying
with Nature policies). 10 pages, 8 figure
Properties of a nonlinear bath: Experiments, theory, and a stochastic Prandtl-Tomlinson model
A colloidal particle is a prominent example of a stochastic system, and, if
suspended in a simple viscous liquid, very closely resembles the case of an
ideal random walker. A variety of new phenomena have been observed when such
colloid is suspended in a viscoelastic fluid instead, for example pronounced
nonlinear responses when the viscoelastic bath is driven out of equilibrium.
Here, using a micron-sized particle in a micellar solution, we investigate in
detail, how these nonlinear bath properties leave their fingerprints already in
equilibrium measurements, for the cases where the particle is unconfined or
trapped in a harmonic potential. We find that the coefficients in an effective
linear (generalized) Langevin equation show intriguing inter-dependencies,
which can be shown to arise only in nonlinear baths: For example, the friction
memory can depend on the external potential that acts only on the colloidal
particle (as recently noted in simulations of molecular tracers in water in
Phys. Rev. X 7, 041065 (2017)), it can depend on the mass of the colloid, or,
in an overdamped setting, on its bare diffusivity. These inter-dependencies,
caused by so-called fluctuation renormalizations, are seen in an exact small
time expansion of the friction memory based on microscopic starting points.
Using linear response theory, they can be interpreted in terms of
microrheological modes of force-controlled or velocity-controlled driving. The
mentioned nonlinear markers are observed in our experiments, which are
astonishingly well reproduced by a stochastic Prandtl-Tomlinson model mimicking
the nonlinear viscoelastic bath. The pronounced nonlinearities seen in our
experiments together with the good understanding in a simple theoretical model
make this system a promising candidate for exploration of colloidal motion in
nonlinear stochastic environments.Comment: 14 pages, 12 figures changes v2: minor changes regarding abstract,
and Fig. 1
Many-body heat radiation and heat transfer in the presence of a nonabsorbing background medium
Heat radiation and near-field radiative heat transfer can be strongly manipulated by adjusting geometrical shapes, optical properties, or the relative positions of the objects involved. Typically, these objects are considered as embedded in vacuum. By applying the methods of fluctuational electrodynamics, we derive general closed-form expressions for heat radiation and heat transfer in a system of N arbitrary objects embedded in a passive nonabsorbing background medium. Taking into account the principle of reciprocity, we explicitly prove the symmetry and positivity of transfer in any such system. Regarding applications, we find that the heat radiation of a sphere as well as the heat transfer between two parallel plates is strongly enhanced by the presence of a background medium. Regarding near- and far-field transfer through a gas like air, we show that a microscopic model (based on gas particles) and a macroscopic model (using a dielectric contrast) yield identical results. We also compare the radiative transfer through a medium like air and the energy transfer found from kinetic gas theory
Slow compressional wave in porous media: Finite difference simulations on micro-scale
We perform wave propagation simulations in porous media on microscale in which a slow compressional wave can be observed. Since the theory of dynamic poroelasticity was developed by Biot (1956), the existence of the type II or Biot's slow compressional wave (SCW) remains the most controversial of its predictions. However, this prediction was confirmed experimentally in ultrasonic experiments. The purpose of this paper is to observe the SCW by applying a recently developed viscoelastic displacement-stress rotated staggered finite-difference (FD) grid technique to solve the elastodynamic wave equation. To our knowledge this is the first time that the slow compressional wave is simulated on first principles
Hantavirus in African Wood Mouse, Guinea
Hantaviruses are rodentborne, emerging viruses that cause life-threatening human diseases in Eurasia and the Americas. We detected hantavirus genome sequences in an African wood mouse (Hylomyscus simus) captured in Sangassou, Guinea. Sequence and phylogenetic analyses of the genetic material demonstrate a novel hantavirus species, which we propose to name "Sangassou virus.
Attenuated epigenetic suppression of muscle stem cell necroptosis is required for efficient regeneration of dystrophic muscles
Somatic stem cells expand massively during tissue regeneration, which might require control of cell fitness, allowing elimination of non-competitive, potentially harmful cells. How or if such cells are removed to restore organ function is not fully understood. Here, we show that a substantial fraction of muscle stem cells (MuSCs) undergo necroptosis because of epigenetic rewiring during chronic skeletal muscle regeneration, which is required for efficient regeneration of dystrophic muscles. Inhibition of necroptosis strongly enhances suppression of MuSC expansion in a non-cell-autonomous manner. Prevention of necroptosis in MuSCs of healthy muscles is mediated by the chromatin remodeler CHD4, which directly represses the necroptotic effector Ripk3, while CHD4-dependent Ripk3 repression is dramatically attenuated in dystrophic muscles. Loss of Ripk3 repression by inactivation of Chd4 causes massive necroptosis of MuSCs, abolishing regeneration. Our study demonstrates how programmed cell death in MuSCs is tightly controlled to achieve optimal tissue regeneration
Clinical characterization of two severe cases of hemorrhagic fever with renal syndrome (HFRS) caused by hantaviruses Puumala and Dobrava-Belgrade genotype Sochi
Background: Hantavirus disease belongs to the emerging infections. The clinical picture and severity of infections differ between hantavirus species and may even vary between hantavirus genotypes. The mechanisms that lead to the broad variance of severity in infected patients are not completely understood. Host- and virus-specific factors are considered. Case presentation: We analyzed severe cases of hantavirus disease in two young women. The first case was caused by Puumala virus (PUUV) infection in Germany; the second case describes the infection with Dobrava-Belgrade virus (DOBV) in Russia. Symptoms, laboratory parameters and cytokine levels were analyzed and compared between the two patients. Serological and sequence analysis revealed that PUUV was the infecting agent for the German patient and the infection of the Russian patient was caused by Dobrava-Belgrade virus genotype Sochi (DOBV-Sochi). The symptoms in the initial phase of the diseases did not differ noticeably between both patients. However, deterioration of laboratory parameter values was prolonged and stronger in DOBV-Sochi than in PUUV infection. Circulating endothelial progenitor cells (cEPCs), known to be responsible for endothelial repair, were mobilized in both infections. Striking differences were observed in the temporal course and level of cytokine upregulation. Levels of angiopoietin-2 (Ang-2), vascular endothelial growth factor (VEGF), and stromal derived factor-1 (SDF-1α) were increased in both infections; but, sustained and more pronounced elevation was observed in DOBV-Sochi infection. Conclusions: Severe hantavirus disease caused by different hantavirus species did not differ in the general symptoms and clinical characteristics. However, we observed a prolonged clinical course and a late and enhanced mobilization of cytokines in DOBV-Sochi infection. The differences in cytokine deregulation may contribute to the observed variation in the clinical course
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