80 research outputs found
Repulsive Casimir forces between solid materials with high refractive index intervening liquids
In order to explore repulsive Casimir/van der Waals forces between solid
materials with liquid as the intervening medium, we analyze dielectric data for
a wide range of materials as for example PTFE, polystyrene, silica and more
than twenty liquids. Although significant variation in the dielectric data from
different sources exist, we provide a scheme based on measured static
dielectric constants, refractive indices, and applying Kramers Kronig (KK)
consistency to dielectric data to create accurate dielectric functions at
imaginary frequencies. The latter is necessary for more accurate force
calculations via the Lifshitz theory allowing reliable predictions of repulsive
Casimir forces.Comment: 20 pages, 7 figures, 1 tabl
Tuning near field radiative heat flux through surface excitations with a metal insulator transition
The control of heat flow is a formidable challenge due to lack of good
thermal insulators. Promising new oppor-tunities for heat flow control were
recently theoretically discovered for radiative heat flow in near field, where
large heat flow contrasts may be achieved by tuning electronic excitations on
surfaces. Here we show experi-mentally that the phase transition of VO2 entails
a change of surface polariton states that significantly affects radiative heat
transfer in near field. In all cases the Derjaguin approximation correctly
predicted radiative heat transfer in near field, but it underestimated the
farfield limit. Our results indicate that heat flow contrasts can be realized
in near field that can be larger than those obtained in farfield.Comment: 3 figure
Experimental observation of nanoscale radiative heat flow due to surface plasmons in graphene and doped silicon
Owing to its two dimensional electronic structure, graphene exhibits many
unique properties. One of them is a wave vector and temperature dependent
plasmon in the infrared range. Theory predicts that due to these plasmons,
graphene can be used as a universal material to enhance nanoscale radiative
heat exchange for any dielectric substrate. Here we report on radiative heat
transfer experiments between SiC and a SiO2 sphere which have non matching
phonon polariton frequencies, and thus only weakly exchange heat in near field.
We observed that the heat flux contribution of graphene epitaxially grown on
SiC dominates at short distances. The influence of plasmons on radiative heat
transfer is further supported with measurements for doped silicon. These
results highlight graphenes strong potential in photonic nearfield and energy
conversion devices.Comment: 4 pages, 3 figure
Weak dispersive forces between glass-gold macroscopic surfaces in alcohols
In this work we concentrate on an experimental validation of the Lifshitz
theory for van der Waals and Casimir forces in gold-alcohol-glass systems. From
this theory weak dispersive forces are predicted when the dielectric properties
of the intervening medium become comparable to one of the interacting surfaces.
Using inverse colloid probe atomic force microscopy dispersive forces were
measured occasionally and under controlled conditions by addition of salt to
screen the electrostatic double layer force if present. The dispersive force
was found to be attractive, and an order of magnitude weaker than that in air.
Although the theoretical description of the forces becomes less precise for
these systems even with full knowledge of the dielectric properties, we find
still our results in reasonable agreement with Lifshitz theory.Comment: 19 pages, 7 figure
Influence of random roughness on the adhesion between metal surfaces due to capillary condensation
The capillary force was measured by atomic force microscopy between a gold
coated sphere mounted on a cantilever and gold surfaces with different
roughness. For smooth surfaces the capillary adhesive force surpasses in
magnitude any dispersion, e.g. van der Waals/Casimir, and/or electrostatic
forces. A substantial decrease in the capillary force was observed by
increasing the roughness ampltitude a few nanometers between 1-10 nm. From
these measurements two limits can be defined: a smooth limit where a closely
macroscopic size contact surface interacts through the capillary force, and the
rough limit where only a few asperities give a capillary contribution.Comment: 8 pages, 3 figures, To appear in Appl. Phys. Lett. (2007
Roughness corrections to the Casimir force: The importance of local surface slope
This paper concentrates on a study where finite conductivity corrections are
included in the theoretical description of the effects of roughness on the
Casimir force. The roughness data were taken from gold films evaporated onto
Silicon and polysterene spheres. We conclude that for a detailed comparison
with experimental data, i.e. at the level of at least 5 % at short separations
below 200 nm, the lateral dimensions of roughness for real films should be
included in the theoretical considerations. Moreover, if the RMS roughness is
considerable, high local surface slopes are shown to have a significant effect
on the Casimir force.Comment: 13 pages. To be publishe
Transition from Casimir to van der Waals force between macroscopic bodies
The transition of van der Waals to Casimir forces between macroscopic gold
surfaces is investigated by Atomic Force Microscopy in the plane-sphere
geometry. It was found that the transition appears to take place at separations
about 10 % the plasma wavelength for evaporated gold surfaces, which compares
to theoretical predictions by incorporation of experimental optical data and
roughness corrections. Moreover, the force data allow estimation of the Hamaker
constant AH in the van der Waals regime, which is in good agreement with the
Lifshitz theory predictions (even if roughness corrections are taken into
account) and former surface force apparatus measurements.Comment: 12 pages, 3 figure
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