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At least 181 records · Page 10Linked to original sources
PubMed2005

Reverse nonequilibrium molecular-dynamics calculation of the Soret coefficient in liquid benzene/cyclohexane mixtures.

Reverse nonequilibrium molecular dynamics is the method applied here for the investigation of thermal diffusion in realistic molecular fluids. The Soret coefficients of benzene/cyclohexane mixtures are calculated using an all-atom model. The autocorrelation functions indicate that the mole fraction gradient converges much slower than the temperature gradient. Compared to experimental data, the results show the same tendency of the Soret coefficient variation versus the mole fraction. Although a systematic error exists for the magnitude of the Soret coefficient, a meanwhile systematic error for both the mutual diffusion and thermal diffusion coefficients provides some explanation of it; and the calculation with different force field parameters indicates a possibility to annihilate the systematic error. The influences of algorithm variables such as cutoff lengths and perturbation intensities are tested. Furthermore the temperature dependence of the Soret effect is observed, yielding the same trend as previous studies.

Journal Article↗
PubMed2001

Coupling of thermal and mass diffusion in regular binary thermal lattice gases.

We have constructed a regular binary thermal lattice gas model in which the thermal diffusion and mass diffusion are coupled and form two nonpropagating diffusive modes. The power spectrum is shown to be similar in structure as for the one in real fluids, in which the central peak becomes a combination of coupled entropy and concentration contributions. Our theoretical findings for the power spectra are confirmed by computer simulations performed on this model.

Journal Article↗
PubMed2003

Anisotropic thermally activated diffusion in percolation systems.

We present a study of static and frequency-dependent diffusion with anisotropic thermally activated transition rates in a two-dimensional bond percolation system. The approach accounts for temperature effects on diffusion coefficients in disordered anisotropic systems. Static diffusion shows an Arrhenius behavior for low temperatures with an activation energy given by the highest energy barrier of the system. From the frequency-dependent diffusion coefficients, we calculate a characteristic frequency omega(c) approximately 1/t(c), related to the time t(c) needed to overcome a characteristic barrier. We find that omega(c) follows an Arrhenius behavior with different activation energies in each direction.

Anisotropy↗
PubMed1999

Convection and thermodiffusion of colloidal gold tracers by laser light scattering.

In a dynamic light scattering experiment, we have investigated the time intensity correlation function and the profile of the transmitted laser beam for organic dispersions of light absorbing colloidal particles containing tiny gold clusters. The correlation functions have been found to show a superposition of an exponential decay, corresponding to Brownian motion of the tracers, and well-defined oscillations. These oscillations are caused by convection due to local heating of the sample by the incident laser beam, which has been confirmed independently via measurements of the local temperature within the sample. It will be shown how the particle convection velocity, which is the order of 1 mm/s, can be obtained quantitatively from the oscillating correlation functions. The profile of the transmitted beam allowed us to determine the Soret coefficient, which is a measure for the thermal diffusion of the particles. This article shows how tracer diffusion, convection, and thermal diffusion can be determined simultaneously by one single experiment, laser light scattering of light absorbing colloidal particles in dispersion.

Journal Article↗
PubMed1985

A transient heating technique for the measurement of thermal properties of perfused biological tissue.

Knowledge of tissue thermal transport properties is imperative for any therapeutic medical tool which employs the localized application of heat to perfused biological tissue. In this study, several techniques are proposed to measure local tissue thermal diffusion by heating with a focused ultrasound field. Transient as well as near steady-state heat inputs are discussed and examined for their suitability as a measurement technique for either tissue thermal diffusivity or perfusion rate. It is shown that steady-state methods are better suited for the measurement of perfusion; however the uncertainty in the perfusion measurement is directly related to knowledge of the tissue's intrinsic thermal diffusivity. Results are presented for a transient thermal pulse technique for the measurement of the thermal diffusivity of perfused and nonperfused tissues, in vitro and in vivo. Measurements conducted in plexiglas, animal muscle, kidney and brain concur with tabulated values and show a scatter from 5-15 percent from the mean; measurements made in perfused muscle and brain compare well with the nonperfused values. An estimate of the error introduced by the effect of perfusion shows that except for highly perfused kidney tissue the effect of perfusion is less than the experimental scatter. This validation of the tissue heat transfer model will allow its eventual extension to the simultaneous measurement of local tissue thermal diffusivity and perfusion.

Animals↗
PubMed1995

Involvement of endothelin in peripheral circulatory change induced by hand-arm vibration.

Physiologic mechanisms involving local vasoregulating factor, endothelium-derived relaxing factor (EDRF) and endothelium-derived constricting factors (EDCF) have been postulated to play a role in VWF. Recent evidence that endothelin is a potent vasoconstrictor peptide indicates that it may play a role in vasoregulation during vibration exposure through the local actions of EDRF or EDCF. Therefore, we examined the effects of grasping (50 N) and hand-arm vibration with an unweighted acceleration of 50 m/s2 rms at a frequency of 120 Hz in the direction of X-axis on digital blood flow (DBF) and on the level of plasma endothelin in 7 healthy male office workers. DBF was measured by an apparatus based on the thermal diffusion method where the thermal diffusion flow probe was incorporated in the Peltier's stack. In the grasping test, DBF decreased by about 40% from resting level within 1 minute after the grasping was begun, and was maintained at almost the same level during grasping, but increased immediately after cessation of grasping. In the vibration exposure test, initial decrease in DBF due to grasping was the same as that observed in the grasping test, but DBF gradually increased when exposure to vibration was performed simultaneously. No significant change in plasma endothelin level was induced by grasping. Endothelin was significantly lower after exposure to vibration than at rest and after grasping. The grasping-induced decrease in DBF seemed to be due to mechanical compression of the vessels. The negative correlation between DBF and endothelin during vibration exposure suggests that a reduction in release of endothelin from smooth muscle into the vessel cavity during vibration leads to vasodilatation, possibly attributable to the local axon reflex.

Blood Flow Velocity↗
PubMed2006

The Soret effect in dilute polymer solutions: influence of chain length, chain stiffness, and solvent quality.

Thermal diffusion in dilute polymer solutions is studied by reverse nonequilibrium molecular dynamics. The polymers are represented by a generic bead-spring model. The influence of the solvent quality on the Soret coefficient is investigated. At constant temperature and monomer fraction, a better solvent quality causes a higher affinity for the polymer to the cold region. This may even go to thermal-diffusion-induced phase separation. The sign of the Soret coefficient changes in a symmetric nonideal binary Lennard-Jones solution when the solvent quality switches from good to poor. The known independence of the thermal diffusion coefficients of the molecular weight is reproduced for three groups of polymers with different chain stiffnesses. The thermal diffusion coefficients reach constant values at chain lengths of around two to three times the persistence length. Moreover, rigid polymers have higher Soret coefficients and thermal diffusion coefficients than more flexible polymers.

Journal Article↗
PubMed2003

Methodology for characterizing heat removal mechanism in human skin during cryogen spray cooling.

Cryogen spray cooling (CSC) reduces epidermal damage during laser treatment of various dermatoses. The goal of this study was to determine the heat removal mechanism in skin and quantify the amount in response to CSC. Thermocouples were imbedded in four model substrates with a range of thermal diffusivities, greater than three orders of magnitude in difference, to measure the temperature profiles in response to CSC and sapphire contact cooling, which removes heat completely by conduction. An algorithm solving an inverse heat conduction problem was subsequently used to quantify the amount of heat removal from the substrates using the measured temperatures. The interface thermal conductance and internal temperatures within the substrates were computed by a finite difference algorithm that solved the heat conduction equation. Results verify a marked increase in heat removal and interface thermal conductance with increasing thermal diffusivity. By estimation from the model substrate results, heat removal and interface thermal conductance values for skin were obtained. Data demonstrate that during CSC, evaporation is the dominant heat transfer mechanism in materials with higher thermal diffusivities; however, conductive cooling dominates in substrates with lower thermal diffusivities such as skin.

Administration, Cutaneous↗
PubMed1996

Evaluation of colour images printed by a thermal dye diffusion process.

Dry printing of medical images, without photochemicals, may be based on the principle of thermal dye diffusion. The Drystar (Agfa Gevaert, Mortsel, Belgium) system was evaluated in colour-coded Doppler examinations of the carotid arteries. A total of 25 consecutive patients were examined for the colour test and 37 patients for the black-and-white test. Colour and black-and-white data were available on the same view. The performances of the Drystar were tested against against laser films, matrix films and two types of glossy paper using the following criteria: grey scale, colour scale, quality of reproduction of vascular colouring, registration, endothelial layer, intraluminal echogenicity, spectral resolution, and artefacts. The overall handiness of the document was graded. A significant difference (p = 0.00005) was noted in favour of the Drystar concerning registration, grey scale, spectral resolution and vascular colouring. The global score for handiness of the colour film was 96%. Radiologists preferred the colour hard copy as compared with glossy paper prints. As in the black-and-white test, blue-base and clear-base films obtained with the Drystar were rated inferior to laser films, a double line print mode was included in the printer, giving a density of 2.3 OD.

Angiography↗
PubMed1990

[Development of an improved system for monitoring the peripheral circulation and its application. Part 1. Fundamental study].

In order to assess tissue blood flow quantitatively under various temperature conditions, we have developed a new blood-flow measurement apparatus based on the thermal diffusion method. This new method is hardly affected by environmental temperature, by means of using the thermal clearance curve with its baseline corrected before each measurement. Fundamental studies concerning the measurement of human skin blood flow were carried out in the finger. The results obtained were as follows. 1) The gradient of the thermal clearance curve did not show significant changes under various temperature conditions. This result indicated that the effects of environmental temperature on the thermal clearance curve were almost removed. 2) A highly positive correlation was obtained between the thermal conductivity of various materials similar to human skin in thermal conductivity and the gradient of the thermal clearance curve from a probe placed on a plate of each material. 3) There was linear relationship between the water flow and the gradient of the thermal clearance curve in various water flows of a micro-circulation model. The gradient of the thermal clearance curve obtained for each water flow had good reproducibility. 4) The gradient of the thermal clearance curve changed in parallel with the finger ischemia and hyperemia induced by inflation of a tourniquet. There was good reproducibility in continuous measurements. 5) A highly significant positive correlation was obtained between finger blood flow measured by the inhaled hydrogen clearance method and the gradient of the thermal clearance curve, indicating that it was possible to assess finger blood flow quantitatively by the gradient of the thermal clearance curve. The weak point in the former thermal diffusion method, where the baseline was unstable as a result of environmental temperature, was removed. It was concluded that this new thermal diffusion method using the thermal clearance curve, which was calibrated previously by the hydrogen clearance method, enabled us to measure finger blood flow noninvasively at various temperatures. This method will be useful for diagnosis and research in various medical fields.

Blood Circulation↗
PubMed2001

Effect of intra-arterial papaverine on regional cerebral blood flow in hemodynamically relevant cerebral vasospasm.

BACKGROUND AND PURPOSE: It remains controversial whether the intra-arterial administration of papaverine (IAP) is effective in reversing vasospasm-associated cerebral hypoperfusion after aneurysmal subarachnoid hemorrhage. The aim of the present study was to continuously assess regional cerebral blood flow (rCBF) during and after IAP with the use of quantitative, bedside thermal diffusion flowmetry. METHODS: Eight patients with cerebral vasospasm after subarachnoid hemorrhage (mean flow velocity >120 cm/s; angiographic vessel constriction >33%; hemispheric cerebral blood flow [CBF] <32 mL/100 g per minute) were prospectively entered into the study. Before IAP, thermal diffusion microprobes were implanted into the white matter of each affected vascular territory (n=10) for rCBF monitoring. During and after IAP (300 mg papaverine/50 mL saline over 1 hour), mean arterial blood pressure, intracranial pressure, cerebral perfusion pressure, thermal diffusion rCBF (TD-rCBF), and cerebrovascular resistance (CVR) were recorded continuously. RESULTS: IAP significantly increased TD-rCBF from 7.3+/-1.6 to 37.9+/-6.6 mL/100 g per minute (mean+/-SEM), indicating reversal of cerebral hypoperfusion. This TD-rCBF response was dependent on the degree of cerebral vasospasm and reduced perfusion within the vascular territory. Long-term analysis of TD-rCBF, however, demonstrated that this beneficial effect of IAP on cerebral hypoperfusion was only transient: within 3 hours after treatment, TD-rCBF and CVR returned to baseline values. Furthermore, a lack of correlation between transcranial Doppler sonography and thermal diffusion flowmetry suggested that transcranial Doppler sonography is not suited for CBF-based neuromonitoring after IAP. CONCLUSIONS: IAP is not effective in permanently reversing cerebral hypoperfusion in patients with cerebral vasospasm. The need to validate alternative therapeutic strategies that seek to improve cerebral perfusion in vasospasm warrants continued development of CBF-based neuromonitoring strategies.

Adult↗
PubMed2006

Transport in amorphous solid water films: implications for self-diffusivity.

Thermal desorption spectroscopy is employed to examine transport mechanisms in structured, nanoscale films consisting of labeled amorphous solid water (ASW, H(2)(18)O, H(2)(16)O) and organic spacer layers (CCl(4), CHCl(3)) prior to ASW crystallization (T approximately 150-160 K). Self-transport is studied as a function of both the ASW layer and the organic spacer layer film thickness, and the effectiveness of these spacer layers as a bulk diffusion "barrier" is also investigated. Isothermal desorption measurements of structured films are combined with gas uptake measurements (CClF(2)H) to investigate water self-transport and changes in ASW film morphology during crystallization and annealing. CCl(4) desorption is employed as a means to investigate the effects of ASW film thickness and heating schedule on vapor-phase transport. Combined, these results demonstrate that the interlayer mixing observed near T approximately 150-160 K is inconsistent with a mechanism involving diffusion through a dense phase; rather, we propose that intermixing occurs via vapor-phase transport through an interconnected network of cracks/fractures created within the ASW film during crystallization. Consequently, the self-diffusivity of ASW prior to crystallization (T approximately 150-160 K) is significantly smaller than that expected for a "fragile" liquid, indicating that water undergoes either a glass transition or a fragile-to-strong transition at a temperature above 160 K.

Journal Article↗
PubMed2005

The role of heat-conducting walls in the measurement of heat and mass transport in transient grating experiments.

We present a two-dimensional model to account for the role of heat-conducting walls in the measurement of heat transport and Soret-effect--driven mass transport in transient holographic grating experiments. Heat diffusion into the walls leads to non-exponential decay of the temperature grating. Under certain experimental conditions it can be approximated by an exponential function and assigned an apparent thermal diffusivity D(th, app)<D(th, s), where D(th,s) is the true thermal diffusivity of the sample. The ratio D(th, app)/D(th, s) depends on only three dimensionless parameters, d/l(s), kappa(s)/kappa(w), and D(th, s)/D(th, w). d is the grating period, l(s) the sample thickness, kappa(s) and kappa(w) the thermal conductivities of sample and wall, respectively, and D(th,w) the thermal diffusivity of the wall. If at least two measurements are performed at different d /l(s), both D(th,s) and kappa(s) can be determined. Instead of costly solving PDEs, D(th,s) can be obtained by finding the zero of an analytic function. For thin samples and large grating periods, heat conduction into the walls plays a predominant role and the concentration grating in binary mixtures is no longer one-dimensional. Nevertheless, the normalized heterodyne diffraction efficiency of the concentration grating remains unaffected and the true thermal and collective diffusion coefficient and the correct Soret coefficient are still obtained from a simple one-dimensional model.

Journal Article↗
PubMed2005

Variation of the thermal neutron diffusion cooling properties of wet rock material (Monte Carlo simulations of the pulsed neutron experiments).

The water content in a rock material can significantly change the thermal neutron diffusion parameters with respect to those of the dry medium. The effect has been studied for dolomite, CaMg(CO3)2, by Monte Carlo simulations of the variable buckling experiments for 10 series of samples. The density-removed diffusion cooling coefficient C(M) varies hyperbolically by two orders of magnitude with water content in the range of 0-20%.

Journal Article↗
PubMed1994

Laser-induced photoacoustic signal phase study of stratum corneum and epidermis.

Laser-induced photoacoustic signal phase was employed for measurements of optothermal parameters of pig abdomen stratum corneum and epidermis. The non-radiative relaxation time was found to be 28 ms and the thermal diffusion time was 370 ms, yielding a thermal diffusivity value of (4.1 +/- 0.8) x 10(-4) cm2 s-1. This value compares well with other literature values for skin obtained using photothermal techniques.

Abdomen↗
PubMed2003

Regional cerebral blood flow monitoring in the diagnosis of delayed ischemia following aneurysmal subarachnoid hemorrhage.

OBJECT: The goal of this study was to evaluate regional cerebral blood flow (rCBF) monitoring, performed using thermal-diffusion (TD) flowmetry, as a novel means for the bedside diagnosis of symptomatic vasospasm. METHODS: Fourteen patients with high-grade subarachnoid hemorrhage (SAH) who underwent early clip placement for anterior circulation aneurysms were prospectively entered into the study. Thermal-diffusion microprobes were implanted into the white matter of vascular territories that were deemed at risk for developing symptomatic vasospasm. Data on arterial blood pressure, intracranial pressure, cerebral perfusion pressure, rCBF measurement obtained using a TD probe (TD-rCBF), cerebrovascular resistance (CVR), and blood flow velocities were collected at the patient's bedside. The diagnosis of symptomatic vasospasm was based on the manifestation of a delayed ischemic neurological deficit and/or a reduced territorial level of CBF as assessed using stable Xe-enhanced computerized tomography (CT) scanning in combination with vasospasm demonstrated by angiography. Bedside monitoring of TD-rCBF and CVR allowed the detection of symptomatic vasospasm. In the 10 patients with vasospasm the TD-rCBF decreased from 21 +/- 4 to 9 +/- 1 ml/100 g/min (mean +/- standard error of the mean), whereas in the four other patients the TD-rCBF value remained unchanged (mean TD-rCBF = 25 +/- 4 compared with 21 +/- 4 m/100 g/min). A comparison of the results of TD-rCBF and Xe-enhanced CT studies, as well as the calculation of sensitivities, specificities, predictive values, and likelihood ratios, identified a TD-rCBF value of 15 ml/100 g/min as a reliable cutoff for the diagnosis of symptomatic vasospasm. In addition, TD flowmetry was characterized by a more favorable diagnostic reliability than transcranial Doppler ultrasonography. CONCLUSIONS: Thermal-diffusion flowmetry represents a promising method for the bedside monitoring of patients with SAH to detect symptomatic vasospasm. This is of major clinical interest for patients with high-grade SAH, who often cannot be assessed neurologically.

Adolescent↗
PubMedDate not supplied

[Evaluation of the thermal effects of the plasma microtorch by infrared thermography].

This study presents a detailed example of the examination of the tooth treated by thermal therapy, by infrared thermography and the different manners to show the results of the examination. The results of the work shows: the thermal diffusion into the tooth is similar to the thermal diffusion into an isotropic environment, the fusion heat of the dentine is reached without any damage to the pulp. The study of the tooth treated by the thermal action of the MICRO PLASMA SYSTEM confirms the thérapeutical effects of the thermal treatment without any damage to the pulp.

Dental Instruments↗