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causes underlying an observed decline: changes can be traced back to reveal which taxonomic groups or ecosystems are losing populations of species the fastest, and whether the overall deterioration is due to many declining populations, a few localized extinctions, or a combination of the two.

The problem of data availability has been sidestepped rather than solved: Scholes and Biggs’ calculation is based on expert opinion about how various species fare under differ- ent land use in each ecosystem. Clearly, real data would be preferable. But this method might also help to encourage the collection of data, because sampling systems estab- lished against this framework would be both achievable and useful, and might therefore be more likely to be implemented.

In addition, because land-use change is incorporated into the index, the results sug- gest where best to direct efforts to mitigate loss of biodiversity. For example, Scholes and Biggs’ BIIs for different taxa (Fig. 1 on page 47) show the relative sensitivity of birds, mammals and amphibians to a change in land use from moderate to degraded — that

Sonoluminescence

Cavitation hots up

Detlef Lohse

is, use at a rate exceeding replenishment and causing widespread disturbance. Thus, this method has already moved beyond the stage of designing measures to suggesting actions to achieve the target. ■ Georgina M. Mace is at the Institute of Zoology, Zoological Society of London, Regent’s Park,

London NW1 4RY, UK.

e-mail: georgina.mace@ioz.ac.uk

1. The Royal Society Measuring Biodiversity for Conservation (The Royal Society, London, 2003).

2. Scholes, R. J. & Biggs, R. Nature 434, 45–49 (2005).

3. ConventiononBiologicalDiversitywww.biodiv.org/2010-target

(2002).

4. Balmford, A. et al. Science 307, 212–213 (2005).

5. Mace,G.M.etal.inEcosystemsandHumanWell-being:

A Framework for Assessment Vol. 1 (Millennium Ecosystem

Assessment Ser., Island Press, Washington DC, 2005).

6. Luck,W.G.,Daily,G.C.&Ehrlich,P.R.TrendsEcol.Evol.18,

331–336 (2003).

7. Balmford,A.,Green,R.E.&Jenkins,M.TrendsEcol.Evol.18,

326–330 (2003).

8. Balmford,A.,Crane,P.,Dobson,A.,Green,R.E.&Mace,G.M.

Phil. Trans. R. Soc. Lond. B (in the press).

9. Loh, J. & Wackermagel, M. Living Planet (WWF Int., Gland,

Switzerland, 2004).

10. Butchart, S. H. M. et al. PLoS Biol. 2, e383 (2004).

11. Brooks, T. & Kennedy, E. Nature 431, 1046–1047 (2004). 12. Convention on Biological Diversity www.biodiv.org/2010-

target/indicators.aspx (2004).

news and views

100 YEARS AGO

“Charge carried by the Rays from Radium.” I have recently attacked this problem again, using the methods and apparatus previously described, but, in addition, employing

a strong magnetic field to remove the slow-moving electrons present with

the particles. The apparatus was

placed between the pole-pieces of an electromagnet, so that the field was parallel to the plane of the plates. In such a case, most of the escaping electrons describe curved paths and return to the plate from which they set out. On application of the magnetic field, a very striking alteration was observed in the magnitude of the current. The positive and negative currents for a given voltage were greatly reduced. The upper plate, into which the particles were fired, rapidly gained a positive charge... I think these experiments undoubtedly show that the particles

do carry a positive charge, and that the previous failures to detect this charge

were due to the masking action of the large number of slow-moving electrons emitted from the plates... Since the film of radium bromide is so thin that all the particles escape from its surface, it is easy to deduce from the observed charge from a known weight of radium the total number of

particles expelled per second from one gram of radium bromide... a most important constant, for on it depends all calculations to determine the volume of the emanation, and of helium, the heat emission of radium, and also the probable life of radium and the other radio-elements. E. Rutherford From Nature 2 March 1905.

50 YEARS AGO

While recognizing the greatness of its opportunities and responsibilities in Europe, the [British] Council remarks: “It would be an exaggeration but not an untruth to say that a much closer understanding of the Englishman and his ways exists at Karachi than at Lyons, partly because Englishmen are a more familiar sight in one city than

in the other, and partly because an outward similarity of culture helps to mask a basic difference of mental approach.”... The Council exists as a body which helps to interpret overseas the permanent features of the British way of national life and to make available to the rest of the world the British contribution to knowledge, welfare or enjoyment.

From Nature 5 March 1955.

Gas inside collapsing bubbles can become very hot and, as a result, emit light. It turns out that temperatures of more than 15,000 kelvin can be reached — as hot as the surface of a bright star.

In 1917, Britain’s Royal Navy had problems with bubble cavitation. This is a process in which tiny bubbles grow in size and then

collapse as a result of pressure variations in the turbulent water around ships’ propellers. The process is so violent that it was causing considerable damage to the propellers1, so the navy asked the renowned physicist Lord Rayleigh to analyse the problem2. His research led to what is now called the Rayleigh equation, which describes the dynamics of the collapsing bubble walls1,2. However, the solution to the equation pro- duced a singularity. It implied that, during collapse, the gas inside the bubble is com- pressed so fast that it cannot equilibrate with the surrounding liquid, leading to energy focusing and an infinite temperature increase. In reality, of course, this cannot happen, so the question is: what limits the temperature increase, or, in other words, how hot does the bubble get? On page 52 of this issue3, Flannigan and Suslick report a study of light emission from single bubbles during cavitation, and provide a direct answer to this question.

The temperature reached by the collapsing bubble depends on how much of the focused energy is lost by sound emission at the collapse

and how much is consumed by internal processes such as vibrations, rotations, disso- ciation and eventually ionization. If there are many collapsing bubbles, they disturb each other, which leads to a less-spherical collapse and therefore less-efficient energy focusing. Nonetheless, temperatures can rise so high that the bubbles start to glow. This phenom- enon has already been investigated intensively by using sound waves to drive bubble produc- tion in liquids and then detecting the light emitted; the sound waves cause a temporarily reduced pressure in the liquid, which makes the bubbles grow and eventually collapse again (Fig. 1, overleaf ). So far, emission spectra with a detailed line structure have only been observed for many transient bubbles together (so-called multi-bubble sonoluminescence). Analysis of the emitted spectral lines4 indicates that the temperature reached inside these bubbles is around 5,000 kelvin.

In single-bubble sonoluminescence5,6, an isolated and stable bubble is studied; disturbances from other bubbles are absent. The light emission from such a bubble can be more than 107 photons per flash7. As the bubble is driven periodically with sound waves at frequencies of typically 20–40 kHz, the emitted light is visible to the naked eye.

NATURE | VOL 434 | 3 MARCH 2005 | www.nature.com/nature

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©2005 NaturePublishingGroup

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