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to the MSUERL HomePageNucleation is a term used to describe the formation of the first bubbles of vapor during a vaporization process. Our current research emphasis is on developing better predictive models for the onset of nucleation of gases dissolved in liquids, particularly when effects of confinement within small volumes or small flow cross-sectional areas elevate nucleation thresholds above their equilibrium levels. As an example of the effectiveness of confinement in elevating nucleation thresholds, we have been able to elevate the boiling point of water at atmospheric pressure to 240'C simply by purifying it (by short exposure to a very high pressure) then heating it within a 10-micron capillary tube. Clearly, equilibrium thermodynamic relationships are not useful in such metastable situations.
Models for Nucleation:
Existing models for nucleation and the conditions under which nucleation takes
place typically combine experimental calibration with some elements of
nucleation theory to yield semi-predictive models for the rate of
nucleation --- huge nucleation rates are taken to mean that bubbles will be
seen whereas small ones infer one has to wait forever to see the formation of a
single bubble. Moreover, such models usually require recalibration for each
different dissolved gas used and take little account of surface chemistry,
diffusion to/from the gas-liquid interface, and other factors that are known to
be important. They also include many implicit assumptions such as the ready
availability of sites for nuclation at surfaces and so cannot predicts effects
of confinement on the conditions under which nucleation takes place.
Consequently, it is very difficult to design equipment in which
gas-supersaturated liquids are delivered along capillary tubes without better
theoretical guidance.
Modeling Nucleation in Confined Regions:
As one outcome of some preliminary studies of effects of confinement on
nucleation, we have developed an improved theory of nucleation which does take
into account effects of confinement and, using two experimentally calibrated
coefficients, appears to predict quite well in narrow tubes of different sizes
both: i) nucleation of pure water under superheat; and ii) nucleation of oxygen
dissolved in water under pressure reduction (cavitation). Current efforts are
focused on generalizing these theories to reduce the level of calibration
required and exploring computational approaches to nucleation studies such as
density functional methods.
Applications of Delayed Nucleation:
The effect of confinement in raising the nucleation threshold can be exploited
in numerous applications, one of the most important of which is remediating
hypoxia in patients. By forcibly dissolving oxygen at high concentrations in
water or D5W at high pressures, a highly oxygenated liquid can be delivered,
bubble free, along a narrow capillary and mixed with the blood in either an
extracorporeal circuit or by direct infusion into a main artery. In preliminary
work, carried out with the research group of Dr. Richard Spears at Wayne State
University, we have demonstrated that, at the oxygenation levels we can
presently achieve, this technique is highly effective in providing beneficial
regional oxygenation to organs like the heart during reperfusion after
myocardial infarctions (heart attacks).
A Long-Term Goal:
The ultimate goal of this new paradigm in oxygenation --- by liquid-liquid
mixing, which is much more efficient than gas-liquid diffusion across membranes
(i.e. in the lungs) --- is to infuse oxygenated liquids at the levels required to
oxygenate the entire cardiac output, in which case it could be used for general
systemic treatments and obviate the need for catheterization laboratories
and fluoroscopy to position catheters for regional treatment of hypoxia.
Acknowledgements:
These research efforts are supported by the NIH, Therox Inc., and the Michigan
Life Sciences Corridor. The principal collaborators at MSU are Drs. Tom Shih
and Simon Garrett, at WSU, Drs. Richard Spears and Richard Crilly, and at
Therox, Dr. Paul Zaleski.
Some Selected Publications:
Brereton,
G. J., Spears, J. R., & Crilly, R. J.,
Nucleation in small capillary tubes.
Chemical Physics, 230, 253, 1998.
Spears, J. R., Crilly, R. J., Zalesky, P., Salley, S. &
Brereton, G. J.
Aqueous oxygen --- a highly O2-supersaturated infusate
for regional correction of hypoxemia and production of hyperoxemia.
Circulation, 96, 12, 4386, 1997.