651 research outputs found
Non-Equilibrium relation between mobility and diffusivity of interacting Brownian particles under shear
We investigate the relation between mobility and diffusivity for Brownian
particles under steady shear near the glass transition, using mode coupling
approximations. For the two directions perpendicular to the shear direction,
the particle motion is diffusive at long times and the mobility reaches a
finite constant. Nevertheless, the Einstein relation holds only for the
short-time in-cage motion and is violated for long times. In order to get the
relation between diffusivity and mobility, we perform the limit of small
wavevector for the relations derived previously [Phys. Rev. Lett. 102 (2009),
135701], without further approximation. We find good agreement to simulation
results. Furthermore, we split the extra term in the mobility in an exact way
into three terms. Two of them are expressed in terms of mean squared
displacements. The third is given in terms of the (less handy) force-force
correlation function.Comment: 14 pages, 4 figures, accepted for Prog. Theor. Phys. Suppl., issue
for the workshop "Frontiers in Nonequilibrium Physics", Kyoto, 200
Heat radiation and transfer in confinement
Near-field heat radiation and transfer are rich in various exciting effects,
in particular, regarding the amplification due to the geometrical configuration
of the system. In this paper, we study heat exchange in situations where the
objects are confined by additional objects so that the dimensionality of heat
flow is reduced. In particular, we compute the heat transfer for spherical
point particles placed between two parallel plates. The presence of the plates
can enhance or reduce the transfer compared to the free case and provides a
slower power-law decay for large distance. We also compute the heat radiation
of a sphere placed inside a spherical cavity, finding that it can be larger or
smaller compared to the radiation of a free sphere. This radiation shows strong
resonances as a function of the cavity's size. For example, the cooling rate of
a nanosphere placed in a cavity varies by a factor of between cavity
radii and .Comment: 8 pages, 7 figures (v2: discussion about different heat flow
contributions was added, temperatures were added to the figures describing
configurations; Fig. 3 was added; discussions related to validity of the
point particle approximation were updated; meters were replaced with
micrometers in figures and text; minor changes in text
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
Driven colloidal suspensions in confinement and density functional theory: Microstructure and wall-slip
We theoretically investigate general properties of driven (sheared) colloidal
suspensions in confinement, based on methods of classical density functional
theory. In the absence of an exact closed (Smoluchowski-) equation for the
one-particle density under shear, we formulate a set of general conditions for
approximations, and show that a simple closure fulfills them. The exact
microscopic stress tensor is identified. Exemplifying the situation near a wall
(oriented parallel to the direction of shear), we note that the microscopic
shear stress is not necessarily homogeneous. Formulating a second equation
additional to the Smoluchowski equation, we achieve a homogeneous shear stress,
and thereby compute the local flow velocity near the wall. This finally leads
to a slip length of the complex fluid at the wall.Comment: 11 pages, 8 figure
Extrapolation to nonequilibrium from coarse grained response theory
Nonlinear response theory, in contrast to linear cases, involves (dynamical)
details, and this makes application to many body systems challenging. From the
microscopic starting point we obtain an exact response theory for a small
number of coarse grained degrees of freedom. With it, an extrapolation scheme
uses near-equilibrium measurements to predict far from equilibrium properties
(here, second order responses). Because it does not involve system details,
this approach can be applied to many body systems. It is illustrated in a four
state model and in the near critical Ising model.Comment: Accepted for publication in Phys. Rev. Let
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
Overshoots in stress strain curves: Colloid experiments and schematic mode coupling theory
The stress versus strain curves in dense colloidal dispersions under start-up
shear flow are investigated combining experiments on model core-shell
microgels, computer simulations of hard disk mixtures, and mode coupling
theory. In dense fluid and glassy states, the transient stresses exhibit first
a linear increase with the accumulated strain, then a maximum ('stress
overshoot') for strain values around 5%, before finally approaching the
stationary value, which makes up the flow curve. These phenomena arise in
well-equilibrated systems and for homogeneous flows, indicating that they are
generic phenomena of the shear-driven transient structural relaxation.
Microscopic mode coupling theory (generalized to flowing states by integration
through the transients) derives them from the transient stress correlations,
which first exhibit a plateau (corresponding to the solid-like elastic shear
modulus) at intermediate times, and then negative stress correlations during
the final decay. We introduce and validate a schematic model within mode
coupling theory which captures all of these phenomena and handily can be used
to jointly analyse linear and large-amplitude moduli, flow curves, and
stress-strain curves. This is done by introducing a new strain- and
time-dependent vertex into the relation between the the generalized shear
modulus and the transient density correlator.Comment: 21 pages, 13 figure
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