6 research outputs found
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
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
Using the fluctuation-dissipation theorem for nonconservative forces
An equilibrium system which is perturbed by an external potential relaxes to
a new equilibrium state, a process obeying the fluctuation-dissipation theorem.
In contrast, perturbing by nonconservative forces yields a nonequilibrium
steady state, and the fluctuation-dissipation theorem can in general not be
applied. Here we exploit a freedom inherent to linear response theory: Force
fields which perform work that does not couple statistically to the considered
observable can be added without changing the response. Using this freedom, we
demonstrate that the fluctuation-dissipation theorem can be applied for certain
nonconservative forces. We discuss the case of a nonconservative force field
linear in particle coordinates, where the mentioned freedom can be formulated
in terms of symmetries. In particular, for the case of shear, this yields a
response formula, which we find advantageous over the known Green-Kubo relation
in terms of statistical accuracy.Comment: 5 pages, 3 figures (v2: Acknowledgments have been extended. v3: minor
changes in the abstract, text, and Fig. 3; Ref. [22] has been added
Response of active Brownian particles to shear flow
We study the linear response of interacting active Brownian particles in an
external potential to simple shear flow. Using a path integral approach, we
derive the linear response of any state observable to initiating shear in terms
of correlation functions evaluated in the unperturbed system. For systems and
observables which are symmetric under exchange of the and coordinates,
the response formula can be drastically simplified to a form containing only
state variables in the corresponding correlation functions (compared to the
generic formula containing also time derivatives). In general, the shear
couples to the particles by translational as well as rotational advection, but
in the aforementioned case of symmetry only translational advection is
relevant in the linear regime. We apply the response formulas analytically in
solvable cases and numerically in a specific setup. In particular, we
investigate the effect of a shear flow on the morphology and the stress of
confined active particles in interaction, where we find that the activity as
well as additional alignment interactions generally increase the response.Comment: 13 pages, 4 figure