Lava Lamp Physics
Lava lamps are widely known and beloved gadgets decorating living rooms
and offices. Besides the commercial products, hundreds of recipes can be
found in the web on how to tinker a working piece at home. It might be
surprising, but the public literature contains practically nothing about the
physics of lava lamp convection. This work reports on experiments with
a laboratory specimen designed to permit quantitative measurements, since
the main ingredients (silicone oil and ordinary salt solution) are chemically
stable, non-toxic, and heat resistant. The dynamics is found to be quite
regular in the given parameter range. The characteristic behavior is a single
blob exchange, where a superheated ball of silicone oil rises from the bottom,
sticks to the top, cools down and sinks back to the bottom. One period is
shown in the picture, the time stamp format is min:sec. This two-fluid system
seems to be very simple, however a precise physical understanding
represents a real challenge: all the material parameters (density,
viscosity, heat transfer properties, and interfacial tension) have nontrivial
temperature dependence and apparently essential role in the dynamics.
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BE11433
Universal limit of below vacuum thermal conductance in multi-layer
photonic crystals
Vacuum is commonly thought to be the best thermal insulator. Can we
engineer the vacuum to achieve even lower thermal conductance?
Intuitively, since heat is entirely carried by photons in vacuum, the
vacuum thermal conductance can be suppressed by using photonic band gap
nanostructures. The simplest of such nanostructures is the multi-layer
photonic crystal, consisted of alternate layers of vacuum and dielectrics.
In general, the overall thermal-conducting behavior is determined by the
detail geometry of the crystal, and it would be interesting to find out
the extent of the best achievable thermal insulation, and the corresponding
structural design. In this paper, we derive the analytical expression for
the lower limit of normalized thermal conductance with respect to vacuum
for the multi-layer photonic crystal, where the best thermal insulation
occurs. This limit, however, is universal, since it is independent of the
relative thicknesses of the layers, and depends only on the choice of the
dielectric material. Strikingly for such crystals, while highly
thermally-insulating, are optically transparent for narrow bands of
light with spectrum outside the photonic band gaps. From theoretical
viewpoint, such geometric independence reveals the deep fact that the
distribution of the underlying photonic bands in frequency space is
ergodic.
Figure Caption:
Normalized thermal conductance of silicon-vacuum multi-layer photonic
crystal versus normalized temperature, for different relative layer
thicknesses of the silicon and vacuum layers. The lower limits of all
curves converge to the same value, where the best thermally-insulating
effect occurs. The inset is the geometry of the structure.
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LG12505
Does the Universe change as fast as it can?
Our work supports the idea that the very early universe shows
behavior in periods of rapid change that can be mimicked in systems
undergoing rapid change in the laboratory, in that each change as
fast as they can.
When we change a system it cannot respond in its entirety
immediately. There is a maximum speed (e.g. speed of light) at which
information about one part of the system can reach another
part. This is particularly true in the first millionth of a second
of the universe, when rapid cooling turns its primordial constituents
into the ingredients of everyday matter. If the universe changes as
fast as it can, it will only be uniform in domains over which
information about its state can be communicated in the time it takes
to change, which can lead to the formation of intergalactic 'defects'
at the boundaries. We have looked at simpler changes in the
laboratory, cooling conductors into showing superconducting
behavior. By counting the defects (magnetic flux lines) we have
shown that our laboratory system does, indeed, change as fast as it can.
