168 research outputs found
Non-additivity of van der Waals forces on liquid surfaces
We present an approach for modeling nanoscale wetting and dewetting of liquid
surfaces that exploits recently developed, sophisticated techniques for
computing van der Waals (vdW) or (more generally) Casimir forces in arbitrary
geometries. We solve the variational formulation of the Young--Laplace equation
to predict the equilibrium shapes of fluid--vacuum interfaces near solid
gratings and show that the non-additivity of vdW interactions can have a
significant impact on the shape and wetting properties of the liquid surface,
leading to very different surface profiles and wetting transitions compared to
predictions based on commonly employed additive approximations, such as Hamaker
or Derjaguin approximations.Comment: 5 pages (including abstract, acknowledgments, and references), 3
figure
Fluctuational Electrodynamics in Atomic and Macroscopic Systems: van der Waals Interactions and Radiative Heat Transfer
We present an approach to describing fluctuational electrodynamic (FED)
interactions, particularly van der Waals (vdW) interactions as well as
radiative heat transfer (RHT), between material bodies of vastly different
length scales, allowing for going between atomistic and continuum treatments of
the response of each of these bodies as desired. Any local continuum
description of electromagnetic (EM) response is compatible with our approach,
while atomistic descriptions in our approach are based on effective electronic
and nuclear oscillator degrees of freedom, encapsulating dissipation,
short-range electronic correlations, and collective nuclear vibrations
(phonons). While our previous works using this approach have focused on
presenting novel results, this work focuses on the derivations underlying these
methods. First, we show how the distinction between "atomic" and "macroscopic"
bodies is ultimately somewhat arbitrary, as formulas for vdW free energies and
RHT look very similar regardless of how the distinction is drawn. Next, we
demonstrate that the atomistic description of material response in our approach
yields EM interaction matrix elements which are expressed in terms of
analytical formulas for compact bodies or semianalytical formulas based on
Ewald summation for periodic media; we use this to compute vdW interaction free
energies as well as RHT powers among small biological molecules in the presence
of a metallic plate as well as between parallel graphene sheets in vacuum,
showing strong deviations from conventional macroscopic theories due to the
confluence of geometry, phonons, and EM retardation effects. Finally, we
propose formulas for efficient computation of FED interactions among material
bodies in which those that are treated atomistically as well as those treated
through continuum methods may have arbitrary shapes, extending previous
surface-integral techniques.Comment: 25 pages, 5 figures, 2 appendice
Impact of nuclear vibrations on van der Waals and Casimir interactions at zero and finite temperature
Van der Waals (vdW) and Casimir interactions depend crucially on material
properties and geometry, especially at molecular scales, and temperature can
produce noticeable relative shifts in interaction characteristics. Despite
this, common treatments of these interactions ignore electromagnetic
retardation, atomism, or contributions of collective mechanical vibrations
(phonons) to the infrared response, which can interplay with temperature in
nontrivial ways. We present a theoretical framework for computing
electromagnetic interactions among molecular structures, accounting for their
geometry, electronic delocalization, short-range interatomic correlations,
dissipation, and phonons at atomic scales, along with long-range
electromagnetic interactions among themselves or in the vicinity of continuous
macroscopic bodies. We find that in carbon allotropes, particularly fullerenes,
carbyne wires, and graphene sheets, phonons can couple strongly with long-range
electromagnetic fields, especially at mesoscopic scales (nanometers), to create
delocalized phonon polaritons that significantly modify the infrared molecular
response. These polaritons especially depend on the molecular dimensionality
and dissipation, and in turn affect the vdW interaction free energies of these
bodies above a macroscopic gold surface, producing nonmonotonic power laws and
nontrivial temperature variations at nanometer separations that are within the
reach of current Casimir force experiments.Comment: 11 pages, 4 figures (3 single-column, 1 double-column), 2 appendice
Mechanical relations between conductive and radiative heat transfer
We present a general nonequilibrium Green's function formalism for modeling
heat transfer in systems characterized by linear response that establishes the
formal algebraic relationships between phonon and radiative conduction, and
reveals how upper bounds for the former can also be applied to the latter. We
also propose an extension of this formalism to treat systems susceptible to the
interplay of conductive and radiative heat transfer, which becomes relevant in
atomic systems and at nanometric and smaller separations where theoretical
descriptions which treat each phenomenon separately may be insufficient. We
illustrate the need for such coupled descriptions by providing predictions for
a low-dimensional system of carbyne wires in which the total heat transfer can
differ from the sum of its radiative and conductive contributions. Our
framework has ramifications for understanding heat transfer between large
bodies that may approach direct contact with each other or that may be coupled
by atomic, molecular, or interfacial film junctions.Comment: 16 pages, 2 figures, 1 table, 2 appendice
-operator bounds on angle-integrated absorption and thermal radiation for arbitrary objects
We derive fundamental per-channel bounds on angle-integrated absorption and
thermal radiation for arbitrary bodies---for any given material susceptibility
and bounding region---that simultaneously encode both the per-volume limit on
polarization set by passivity and geometric constraints on radiative
efficiencies set by finite object sizes through the scattering
-operator. We then analyze these bounds in two practical settings,
comparing against prior limits as well as near optimal structures discovered
through topology optimization. Principally, we show that the bounds properly
capture the physically observed transition from the volume scaling of
absorptivity seen in deeply subwavelength objects (nanoparticle radius or thin
film thickness) to the area scaling of absorptivity seen in ray optics
(blackbody limits).Comment: 9 pages including appendices, 2 figures, 1 tabl
Fundamental limits to attractive and repulsive Casimir--Polder forces
We derive upper and lower bounds on the Casimir--Polder force between an
anisotropic dipolar body and a macroscopic body separated by vacuum via
algebraic properties of Maxwell's equations. These bounds require only a coarse
characterization of the system---the material composition of the macroscopic
object, the polarizability of the dipole, and any convenient partition between
the two objects---to encompass all structuring possibilities. We find that the
attractive Casimir--Polder force between a polarizable dipole and a uniform
planar semi-infinite bulk medium always comes within 10% of the lower bound,
implying that nanostructuring is of limited use for increasing attraction. In
contrast, the possibility of repulsion is observed even for isotropic dipoles,
and is routinely found to be several orders of magnitude larger than any known
design, including recently predicted geometries involving conductors with sharp
edges. Our results have ramifications for the design of surfaces to trap,
suspend, or adsorb ultracold gases.Comment: 6 pages, 3 figure
Fundamental limits to radiative heat transfer: theory
Near-field radiative heat transfer between bodies at the nanoscale can
surpass blackbody limits on thermal radiation by orders of magnitude due to
contributions from evanescent electromagnetic fields, which carry no energy to
the far-field. Thus far, principles guiding explorations of larger heat
transfer beyond planar structures have assumed utility in surface
nanostructuring, which can enhance the density of states, and further assumed
that such design paradigms can approach Landauer limits, in analogy to
conduction. We derive fundamental shape-independent limits to radiative heat
transfer, applicable in near- through far-field regimes, that incorporate
material and geometric constraints such as intrinsic dissipation and finite
object sizes, and show that these preclude reaching the Landauer limits in all
but a few restrictive scenarios. Additionally, we show that the interplay of
material response and electromagnetic scattering among proximate bodies means
that bodies which maximize radiative heat transfer actually maximize scattering
rather than absorption. Finally, we compare our new bounds to existing Landauer
limits, as well as limits involving bodies maximizing far-field absorption, and
show that these lead to overly optimistic predictions. Our results have
ramifications for the ultimate performance of thermophotovoltaics and nanoscale
cooling, as well as related incandescent and luminescent devices.Comment: 12 pages including appendices, 1 figure; SM and PSV contributed
equall
Phonon-polariton mediated thermal radiation and heat transfer among molecules and macroscopic bodies: nonlocal electromagnetic response at mesoscopic scales
Thermal radiative phenomena can be strongly influenced by the coupling of
phonons and long-range electromagnetic fields at infrared frequencies.
Typically employed macroscopic descriptions of thermal fluctuations tend to
ignore atomistic effects that become relevant at nanometric scales, whereas
purely microscopic treatments ignore long-range, geometry-dependent
electromagnetic effects. We describe a mesoscopic framework for modeling
thermal fluctuation phenomena among molecules in the vicinity of macroscopic
bodies, conjoining atomistic treatments of electronic and vibrational
fluctuations obtained from ab-initio density functional theory in the former
with continuum descriptions of electromagnetic scattering in the latter. The
interplay of these effects becomes particularly important at mesoscopic scales,
where phonon polaritons can be strongly influenced by the finite sizes, shapes,
and non-local/many-body response of the bodies to electromagnetic fluctuations.
We show that even in small but especially in elongated low-dimensional
molecular systems, such effects can modify thermal emission and heat transfer
by orders of magnitude and produce qualitatively different behavior compared to
predictions based on local, dipolar, or pairwise approximations valid only in
dilute media.Comment: 7 pages, 2 figures, includes supplement as appendi
Channel-based algebraic limits to conductive heat transfer
Recent experimental advances probing coherent phonon and electron transport
in nanoscale devices at contact have motivated theoretical channel-based
analyses of conduction based on the nonequilibrium Green's function formalism.
The transmission through each channel has been known to be bounded above by
unity, yet actual transmissions in typical systems often fall far below these
limits. Building upon recently derived radiative heat transfer limits and a
unified formalism characterizing heat transport for arbitrary bosonic systems
in the linear regime, we propose new bounds on conductive heat transfer. In
particular, we demonstrate that our limits are typically far tighter than the
Landauer limits per channel and are close to actual transmission eigenvalues by
examining a model of phonon conduction in a 1-dimensional chain. Our limits
have ramifications for designing molecular junctions to optimize conduction.Comment: 10 pages, 2 figures, 2 appendice
Ayurveda and Epilepsy
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