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Results for “Thermal Diffusion”

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At least 433 records · Page 24Linked to original sources

PubMed2005

Dendritic flux avalanches and nonlocal electrodynamics in thin superconducting films.

We report a mechanism of nonisothermal dendritic flux penetration in superconducting films. Our numerical and analytical analysis of coupled nonlinear Maxwell and thermal diffusion equations shows that dendritic flux pattern formation results from spontaneous branching of propagating flux filaments due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heating. The branching is triggered by a thermomagnetic edge instability, which causes stratification of the critical state. The resulting distribution of thermomagnetic microavalanches is not universal, because it depends on a spatial distribution of defects. Our results are in good agreement with experiments on Nb films.

Journal Article↗

PubMed2005

Universal dynamic exponent at the liquid-gas transition from molecular dynamics.

The liquid-gas system is expected to exhibit distinct dynamic behavior in the fluid's critical region (model H). We present molecular dynamics simulations of a Lennard-Jones fluid model starting from specially designed, near-equilibrium, initial conditions. By following the fluid's relaxation towards equilibrium, we calculate the requisite transport coefficients in the critical region. The results yield the scaling behavior of the thermal diffusion coefficient D(T) approximately xi(-1.023+/-0.018) (xi is the correlation length) and a nonconventional divergent heat conductivity, all of which are in accord with mode-coupling and renormalization group predictions, as well as some experimental data.

Journal Article↗

PubMed2006

Cross-stream migration of flexible molecules in a nanochannel.

Molecular dynamics simulations are used to examine the cross-stream chain migration phenomenon in dilute polymer solutions that are flowing in nanochannels. In particular, both uniform planar shear (Couette) and pressure driven (Poiseuille) flows of a dilute polymer solution are studied using a bead-spring representation of polymer chains and a coarse grained model for the solvent. Our results show that three mechanisms govern the migration of deformable molecules in a nanochannel: (1) chain-wall hydrodynamic interactions, (2) thermal diffusion, and (3) gradient in chain mobility. These results are discussed in the context of recent experimental, numerical and theoretical work.

Bacteriophage lambda↗

PubMed1999

Quantitative zone-axis convergent-beam electron diffraction (CBED) studies of metals. I. Structure-factor measurements.

The zone-axis CBED pattern-matching technique ZAPMATCH [Bird & Saunders (1992). Ultramicroscopy, 45, 241-251] has been applied to low-order structure-factor measurements in nickel and copper. Considerable disagreement exists between previously published results obtained with a variety of solid-state theories and experimental techniques. The nickel ZAPMATCH results confirm previous electron-diffraction critical-voltage measurements and are in excellent agreement with FLAPW (full-potential linearized augmented plane-wave) theory calculations. This is further proof of the accuracy achievable with ZAPMATCH analysis. For copper, however, while the results support the findings of previous experimental measurements, they are consistently higher than those given by a range of solid-state theories, perhaps demonstrating some limitation in the existing theory. Two extensions to the ZAPMATCH technique are also considered. First, rules are developed to determine the number of structure factors that can be refined accurately from a given CBED pattern. Second, the imaginary potential generally introduced to account for the effects of thermal diffuse scattering (TDS) is also refined. It is shown that, while the widely used Einstein model is a useful approximation, the refined values are consistently higher than the model predicts. In addition, the importance of a second-order (real) TDS correction arising from the Einstein model is investigated. Although its effects are limited in this instance, it may prove to be more significant at lower beam energies or for materials of higher atomic number.

Journal Article↗

PubMed1995

On the differences between X-ray and neutron thermal vibration parameters.

For crystal structures analyzed by both X-ray and neutron diffraction, the anisotropic mean-square displacement parameters of the non-H atoms are sometimes found to differ significantly. The differences can usually be adjusted by either: (1) an isotropic factor q, defined by UijX = qUijN, to correct for a temperature difference between the two experiments; (2) anisotropic factors qij, defined by UijX = qijUijN, to correct for a temperature difference and different anisotropic diffraction effects of absorption, extinction, thermal diffuse scattering, multiple reflection, or systematic measuring errors in the two experiments; (3) anisotropic diffraction correction terms delta Uij, defined by UijX = UijN + delta Uij; (4) the sum of an isotropic temperature correction and anisotropic diffraction corrections, defined by UijX = qUijN + delta Uij. Correction parameters q, qij and delta Uij are easily calculated by linear least-squares fit, and the corrections from (3) or (4) seem to be the most reliable. Corrections calculated from X-ray and neutron Uij's of the non-H atoms of a crystal can be useful for adjusting the neutron Uij's of the H atoms for adoption, along with the neutron coordinates of the H atoms, as fixed parameters in an X-ray analysis of the electron density distribution.

Crystallography, X-Ray↗

PubMed1996

A technique for identifying the total space or temperature dependent thermal parameters (TITP) of biological materials in vivo.

A new algorithm for simultaneously identifying the space or temperature dependent conductivity K, blood perfusion rate Wb, metabolic rate qm, and thermal diffusibility alpha in Pennes' equation is postulated. The obtained over determined equations are solved by way of Householder's transformation. The validity of the method is tested through an example.

Algorithms↗

PubMed1994

Differential vascular response to laser photothermolysis.

Individual blood vessels in the chick choriallantoic membrane were selectively coagulated through photothermolysis, using pulsed laser irradiation at 585 nm. Pulse durations were chosen to be 0.45 ms and 10 ms, which correspond to the thermal relaxation times in blood vessels of 30 microns and 150 microns diameter, respectively. The short pulses, at a light fluence F = 3 Jcm-2, caused permanent occlusion of vessels of 40 microns diameter or less, whereas larger caliber vessels (60-120 microns) required F = 4-5 Jcm-2. The long-duration pulses, at F = 7 Jcm-2, caused coagulation of the larger diameter vessels; the small-caliber vessels and capillaries showed resistance to photothermolysis and required multiple exposures to achieve coagulation. The fluence versus diameter (F versus d) relationship for coagulation was calculated for the two pulse durations. The energy deposited in a cylindrical absorber of diameter d by an optical field, incident perpendicular to the vessel, was expressed analytically and compared with the energy required to coagulate a blood vessel of the same lumen dimeter. When thermal diffusion is incorporated into the model, our findings can be accounted for quantitatively. This information will be of use for improving the laser treatment of port wine stains and other vasculopathies. A surprising observation was that arterioles were damaged at lower incident energy densities than venules having the same lumen diameter, despite the fact that absorbance in oxygen-rich and oxygen-poor blood is the same at 585 nm.

Allantois↗

PubMed1977

Stratum corneum studies with photoacoustic spectroscopy.

Conventional spectroscopic studies on opaque membranes are difficult to pursue because of excessive light scattering and complications arising from the specimen's surface. Recently, a new spectroscopic technique, photoacoustic spectroscopy, unlike conventional optical spectroscopy, has been demonstrated to be an informative technique amenable to spectroscopic studies of solids and membrane-like samples. We have applied photoacoustic spectroscopy to the study of hydration and maturation of newborn rat stratum corneum, and have obtained clean spectra in the 220 to 450 nm region indicative of a change in thermal diffusivity with increased hydration, and biochemical changes associated with the initial maturation period.

Animals↗

PubMed1998

Complications of cutaneous laser surgery. A review.

Continuous wave (CW) lasers, the first medical lasers to be used, continue to be effective, but are extremely operator dependent and can potentially result in significant risks, including scarring. In 1983, the theory of selective photothermolysis was introduced, which enabled physician-scientists to design lasers that were highly selective and safer to operate. These newer lasers are capable of affecting a specific target tissue without a high risk of scarring and pigmentary changes. They accomplish this task by producing a wavelength and pulse duration that is best absorbed by a specific target. However, not all modern lasers employ this theory and, therefore, may operate in either a CW, quasi-CW, pulsed, or Q-switched mode. CW lasers are least selective and tend to produce unwanted tissue damage and scarring through heat dissipation. Quasi-CW lasers attempt to limit unwanted thermal damage by producing a series of brief laser pulses or by the chopping of a CW beam; however, they still have a relatively high risk of causing nonspecific tissue damage and thermal injury. The pulsed and Q-switched systems adhere most closely to the principles of selective photothermolysis and result in the most selective destruction with the lowest risk of scarring and unwanted thermal diffusion. Of course, any laser system can potentially result in scarring and tissue damage; therefore, adequate operator education and skill are essential when utilizing medical lasers.

Dermatologic Surgical Procedures↗

PubMedDate not supplied

Absorbed dose water calorimeter.

Advantage was taken of the low thermal diffusivity of water and the imperviousness of polyethylene film to water to construct a calorimeter for directly measuring absorbed dose in that medium. An ultrasmall bead thermistor was sandwiched between two thin films stretched on polystyrene rings and immersed in an unregulated water bath. Ten cobalt-60 irradiation runs were made with a precision of 0.5% mean error of the mean at a dose rate of 66 mGy/s. Further development is directed toward a standard instrument that can be used in a medical therapy beam.

Calorimetry↗

PubMed2000

Acoustic streaming in annular thermoacoustic prime-movers

The theory of acoustic streaming in an annular thermoacoustic prime-mover is developed. It is predicted that above the threshold for traveling wave excitation the device considered (which does not contain any moving parts or externally imposed pressure gradients) produces circulation of fluid. The heat flux carried by this directional mass flow inside the thermoacoustic stack exceeds (or is comparable with) the heat flux associated with the acoustically induced increase of thermal diffusivity of the gas. The effects investigated are important for optimization of the performance of thermoacoustic devices.

Journal Article↗

PubMed2002

Saturation of thermoacoustic mixture separation.

The theory for thermoacoustic mixture separation is extended to include the effects of a nonzero concentration gradient. New data are presented, which are in excellent agreement with this theory. The maximum concentration gradient which may be achieved in a binary mixture of gases through this separation process is intrinsically limited by the fractional pressure amplitude, by the tidal displacement, and by the size of the thermal diffusion ratio. Ordinary diffusion further detracts from the attainable final concentration gradient and can become the dominant remixing process as the cross section of the duct is increased. Rayleigh streaming also works against thermoacoustic separation, and an estimate of the molar flux from streaming is given.

Journal Article↗

PubMed2004

Development of an optical trap for microparticle clouds in dilute gases.

Long-duration experiments with clouds of microparticles are planned for the ICAPS facility on board the International Space Station ISS. The scientific objectives of such experiments are widespread and are ranging from the simulation of aerosol behaviour in Earth's atmosphere to the formation of planets in the early solar system. It is, however, even under microgravity conditions, impossible to sustain a cloud of free-floating, microscopic particles for an extended period of time, due to thermal diffusion and due to unavoidable external accelerations. Therefore, a trap for dust clouds is required which prevents the diffusion of the particles, which provides a source of relative velocities between the dust grains and which can also concentrate the dust to higher number densities that are otherwise not achievable. We are planning to use the photophoretic effect for such a particle trap. First short-duration microgravity experiments on the photophoretic motion of microscopic particles show that such an optical particle-cloud trap is feasible. First tests of a two-dimensional trap were performed in the Bremen drop tower.

Journal Article↗

PubMedDate not supplied

Cortical ischemia: effect upon direct cortical response.

Correlation of cortical blood flow as measured by a thermal diffusion flow probe (CBFp) with the direct cortical response (DCR) was studied in 48 lightly anesthetized cats with global ischemia. Thresholds for attenuation and loss of DCR were 21.3 +/- 4.7 and 8.7 and 3.4 ml/100 g/min respectively. In abrupt ischemia, CBFp of 0-3 ml/100 g/min produced absence of DCR in 6 min or less; however, at CBFp of 5-10 ml/100 g/min, the time to obliteration of DCR varied from 5 to 180 min. DCR was unlikely to recover after 13 min of 0-2 ml/100 g/min and after 35 min of 4-5 ml/100 g/min. At higher flows, DCR could recover over 60 min or more of ischemia. With gradual production of ischemia, flows less than 20 ml/100 g/min for over 60 min had a detrimental effect upon recovery of DCR if DCR was lost for 7.5 min or more. Some evidence that implied adaptability of the cortex to ischemia was found.

Animals↗

PubMedDate not supplied

Cortical blood flow during extracranial-intracranial bypass surgery.

Cortical blood flow was evaluated at the time of surgery with a thermal diffusion flow probe in 25 extracranial-intracranial bypass operations. The procedures were performed 14 times for carotid occlusion, 6 times for inaccessible carotid stenosis, on 3 occasions for middle cerebral occlusion, once for basilar occlusion and once for middle cerebral stenosis. Fifty-seven different cortical regions were evaluated before and after completion of the anastomosis. During temporary occlusion of the cortical vessel, no significant changes in cortical blood flow were identified. The mean flow pre-bypass was 58 ml/100 g/min and postbypass was 66. This was a significant difference. When initial ischemic areas were evaluated, flows increased from 36 ml/100 g/min to 52 ml/100 g/min. These postbypass flows were statistically different than the flow values in the nonischemic areas of 75 ml/100 g/min. Blood pressure and arterial carbon dioxide tension did not vary significantly. Extracranial-intracranial bypass increases cortical flow in ischemic areas although this does not bring the flow into the normal range.

Cerebral Revascularization↗

PubMedDate not supplied

Effects of nimodipine on acute focal cerebral ischemia.

Nimodipine is a calcium slow channel blocker with several pharmacologic properties suggesting the potential to favorably modify outcome in focal cerebral ischemia. Thirty adult cats underwent unilateral middle cerebral artery (MCA) occlusion for 4 hours. Seventeen cats were treated with an ipsilateral intracarotid infusion of nimodipine (1 microgram kg-1 min -1) beginning 15 minutes before MCA occlusion and continuing throughout the occlusion period. Eight nimodipine treated cats maintaining MAP greater than 90 mmHg were assigned to a Higher Pressure Nimodipine (HPN) group. The remaining nine treated cats with MAP less than 90 mmHg were assigned to the Lower Pressure Nimodipine (LPN) group. Thirteen cats were untreated, receiving an isovolumetric amount of vehicle through the ipsilateral carotid artery. Local cerebral blood flow (ICBF) was continuously monitored using thermal diffusion probes. The brains, assessed for colloidal carbon perfusion, fluorescein and Evans blue staining, electroencephalographic activity (EEG), and histological changes, revealed no significant differences by any of these methods between the HPN and control animals with the exceptions of: HPN treated cats exhibited a preservation of EEG activity at 15 minutes post-occlusion compared to the untreated cats, and Post-ischemic surface colloidal carbon perfusion was better preserved in the treated cats than in the untreated cats. Mild hypotension, as demonstrated by the LPN group, negated these two positive effects. Prior to MCA occlusion, ICBF was bilaterally significantly increased after nimodipine infusion in the HPN group as compared to vehicle infusion. Intra-arterially infused nimodipine did not reduce infarct size.

Animals↗

PubMed2003

The acute cerebrovascular effects of intracarotid adenosine in nonhuman primates.

UNLABELLED: In this study we sought to determine the acute cerebrovascular effects of intracarotid adenosine by using real-time cerebral blood flow (CBF) measurements in nonhuman primates. The internal carotid arteries of healthy anesthetized baboons were transfemorally cannulated. Changes in CBF were continuously measured at baseline and with 6 increasing doses of adenosine (0.002 to 1.5 mg/min) by use of an intraparenchymal thermal diffusion (TD) probe. Each infusion lasted 5 min. At baseline and at the largest dose of adenosine, CBF was also determined by the intraarterial (133)Xe technique. TD measurements revealed a dose-dependent increase in CBF from 32 +/- 6 mL x l00 g(-1) x min(-1) at baseline to 90 +/- 38 mL x l00 g(-1) x min(-1) with the largest dose of adenosine (n = 5; P < 0.0001). A similar magnitude of increase in CBF was also observed with (133)Xe CBF measurements. No significant increases in intracranial pressure or adverse systemic hemodynamic side effects were observed during adenosine infusion. The increase in CBF after adenosine lasted only for the duration of drug infusion. In conclusion, the transient cerebrovascular effects of intracarotid adenosine make it suitable for a trial of intraarterial vasodilator therapy and for controlled manipulation of cerebrovascular resistance. IMPLICATIONS: Using a real-time cerebral blood flow (CBF) measurement technique, we evaluated the acute cerebrovascular effects of intracarotid adenosine in anesthetized baboons. The increase in CBF lasted only for the duration of the adenosine infusion. Adenosine might be a suitable drug for trial as an intraarterial vasodilator for the treatment of cerebral vasospasm.

Adenosine↗

PubMedDate not supplied

Regional cortical blood flow at craniotomy.

Regional cortical blood flow was monitored at craniotomy in 32 patients with a thermal diffusion flow probe in an attempt to assess the effects of surgical conditions on flow. Changes in flow due to vascular manipulation, retraction, hypotension, and hyperventilation were readily apparent. Eleven patients with supratentorial tumors had intermittent measurements of cerebral blood flow; the expected differences in blood flow with halothane anesthesia and hyperventilation were observed. Fourteen patients with aneurysms were monitored continuously and demonstrated a reduction in flow during hypotension; in two of these cases, the reduction was to disastrously low levels. Flow reduction occurred secondary to branch occlusion by the aneurysm clip in a patient with three middle cerebral artery aneurysms and was corrected by changing the clip position. Direct measurement of regional flow may be of value in estimating the safety of temporary clips and the degree of hypotension that can be tolerated.

Brain Diseases↗