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Results for “Thermal Diffusion”

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PubMed2005

An oscillating electric field with thermal noise increases the rotational diffusion and drives rotation in a dipole.

Here we consider a dipole in a viscous medium under the influence of an oscillating electric field and thermal noise. Because of the very low Reynolds numbers involved in molecular processes, we considered overdamped Langevin dynamics. As a consequence the inertia term becomes negligible. We observed a great increase in the rotational diffusion and also net rotation for some values of the parameters.

Journal Article↗

PubMed2005

Improved magnetic resonance thermal imaging by combining proton resonance frequency shift (PRFS) and apparent diffusion coefficient (ADC) data.

Effective thermal therapy for cancer is dependent on adequate tumor heating. Adequate heating, in turn, is dependent on reliable volumetric measurement of temperature to guide heating. Proton resonance frequency shift (PRFS) magnetic resonance imaging is frequently used for the purpose of three-dimensional temperature imaging. However, this method is susceptible to drift in the imaging magnetic field, leading to a corresponding drift in the measured temperature. A new approach to correcting for this drift is presented. It combines PRFS imaging alternated with water apparent diffusion coefficient (ADC) imaging to yield resulting thermal images that are drift corrected and possess the complementary strengths of the two methods. The drift-corrected PRFS images retain the high resolution and relatively noise-free characteristic of PRFS imaging while adding on the drift-free stability of ADC imaging. This technique is successfully demonstrated in a phantom experiment. This technique is also applicable to correcting motion-induced sudden large discontinuities in PRFS imaging, although not explicitly demonstrated in this work.

Body Temperature↗

PubMed1993

Optimization of electric field strength for DNA sequencing in capillary gel electrophoresis.

Capillary gel electrophoresis (CGE) has demonstrated the ability to separate DNA sequencing reactions at speeds up to 25 times as great as conventional slab gel electrophoresis. These increased speeds are made possible by the efficient heat dissipation of capillaries, which permits higher electric fields to be employed without deleterious thermal effects. The high electric fields, however, also lead to a reduction in the spacing between bands with a concomitant loss of resolution. The resulting tradeoff between speed and resolution is a very important practical aspect of these high-field separations. This work addresses this question by investigating the band broadening and resolution of DNA fragments as they are separated through a fixed distance of gel at field strengths ranging from 50 to 400 V/cm. It is found that the bandwidths of DNA fragments do decrease with the higher field strengths due to a reduction in the diffusional broadening of bands. However, at sufficiently high electric field strengths, the bands begin to broaden again due to the thermal gradient across the gel. This behavior causes the optimum electric field strength for maximum fragment resolution to depend upon the length of fragments being separated. The relative contributions of diffusion and thermal gradients are discussed and used to predict the ultimate performance of constant field capillary gel electrophoresis.

Diffusion↗

PubMed2004

Centroid molecular dynamics approach to the transport properties of liquid para-hydrogen over the wide temperature range.

Fundamental transport properties of liquid para-hydrogen (p-H(2)), i.e., diffusion coefficients, thermal conductivity, shear viscosity, and bulk viscosity, have been evaluated by means of the path integral centroid molecular dynamics (CMD) calculations. These transport properties have been obtained over the wide temperature range, 14-32 K. Calculated values of the diffusion coefficients and the shear viscosity are in good agreement with the experimental values at all the investigated temperatures. Although a relatively large deviation is found for the thermal conductivity, the calculated values are less than three times the amount of the experimental values at any temperature. On the other hand, the classical molecular dynamics has led all the transport properties to much larger deviation. For the bulk viscosity of liquid p-H(2), which was never known from experiments, the present CMD has given a clear temperature dependence. In addition, from the comparison based on the principle of corresponding states, it has been shown that the marked deviation of the transport properties of liquid p-H(2) from the feature which is expected from the molecular parameters is due to the quantum effect.

Journal Article↗

PubMed2002

Importance of cutaneous cooling during photothermal epilation: theoretical and practical considerations.

BACKGROUND AND OBJECTIVES: Sapphire contact cooling is widely used to prevent non-specific epidermal injury from occurring during selective laser treatment of unwanted hair and vascular lesions. This small study was performed to examine the clinical response of the skin to 800-nm laser irradiation with varying extent of cutaneous sapphire contact cooling. Observed clinical responses are compared to those predicted by theoretical analysis in an attempt to construct a more complete picture of sapphire contact cooling and its role in preventing non-targeted tissue devitalization during laser treatment of the skin using a wavelength, pulse duration, and radiant exposure commonly used for laser hair removal. STUDY DESIGN/MATERIALS AND METHODS: Three subjects each received a total of three pulses of laser light (800 nm) of equal radiant exposure (60 J/cm(2)), pulse duration (30 msec), and spot size (9 mm x 9 mm), but with varying extent of cutaneous cooling. One site was pre-cooled and heat-sinked with a chilled (5 degrees C) sapphire window in contact with the skin; another site was heat-sinked only with a room-temperature (20 degrees C) sapphire window in contact with the skin; and a third site received no pre-cooling or heat-sinking. Each site was examined immediately after treatment and at intervals throughout a 3-month period. The thermal response of the tissue was calculated in each case using a Monte Carlo model for light transport in multi-layered tissues coupled with an axisymmetric finite-difference heat diffusion model. Thermal injury was modeled as a first-order kinetic rate process using an Arrhenius expression. RESULTS: In all three subjects, the sites that were pre-cooled and heat-sinked showed no evidence of epidermal or dermal devitalization. The three sites that were heat-sinked only had a few patches of perifollicular epidermal devitalization and subsequent desquamation without any permanent epidermal or dermal injury, as would be evidenced by pigmentary alteration or textural change. In each subject, the site that received no pre-cooling or heat-sinking sustained epidermal and dermal devitalization, appearing as ulceration and resulting in sustained erythema and textural alteration. Clinical responses predicted by theoretical analysis agree with the clinical observations and show that the dominant effect of sapphire contact cooling for pulse durations of 30 msec or less is the reduction of fluence within the epidermis resulting from index matching at the skin surface. CONCLUSIONS: The results of this small study suggest that by judiciously selecting the laser pulse duration and pre-cooling and heat-sinking the epidermis in a manner that provides index matching and compression of the skin, epidermal damage can be avoided while administering the highest, most effective radiant exposures.

Aluminum Oxide↗

PubMed2004

Pulse laser acoustics for the characterization of inhomogeneities at interfaces of microstructures.

Interfaces between neighbouring materials are often subjected to diffusion processes which cause layers having gradually varying mechanical properties--like densities, Young's moduli or shear moduli--perpendicular to the surface or interface. In this investigation particular interest is drawn on the question how the propagation characteristics of bulk acoustic waves are affected by diffusion layers. The reflection and transmission behavior of bulk acoustic waves encountering a continuum having a spatially dependent sound velocity is discussed based on numerical simulations as well as on experimental verifications. The simulated results are part of an on-going project in which material properties of MEMS devices are investigated by short pulse laser acoustic methods. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. For metals this leads to wavelengths of 10-20 nm and the corresponding frequencies amount to 0.3-0.6 THz. In contrast to previous work done in this field in which diffusion effects are generally considered as undesirable phenomena, the deliberate realization of microstructures having well defined gradually varying material properties in one or more dimensions represents a goal of this investigation. For metallic thin film multilayers thermally induced diffusion processes have shown to be an easy and reliable technique for the realization of layered structures having continuously varying mechanical properties within several 10 nm. Among the experimental methods suitable for the in-depth profiling of submicron metallic thin films providing resolutions of several nanometers, are short pulse laser acoustic methods, Rutherford backscattering spectroscopy (RBS), and glow discharge optical emission spectroscopy (GDOES). Short pulse laser acoustic methods and RBS have the advantage to be nondestructive. The short pulse laser acoustic method is described in detail and RBS measurements are presented for verification purposes. Finally potential engineering applications like micro-machined spectrum analyzers, acoustic isolation layers, and band pass filters, operating at very high frequencies are presented.

Journal Article↗

PubMed2005

Diffuse-reflection boundary conditions for a thermal lattice Boltzmann model in two dimensions: evidence of temperature jump and slip velocity in microchannels.

We discuss the implementation of diffuse reflection boundary conditions in a thermal lattice Boltzmann model for which the upwind finite difference scheme is used to solve the set of evolution equations recovered after discretization of the velocity space. Simulation of heat transport between two parallel walls at rest shows evidence of temperature jumps at the walls that increase with Knudsen number. When the walls move in opposite directions with speeds +/- u(W), fluid velocity slip is observed at the walls, together with temperature jumps.

Journal Article↗

PubMed1994

Experimental calculations of droplet diffusion in a low-pressure cloud chamber.

A low-pressure cloud chamber was used for several years to display the tracks created by the passage of ionizing particles through vapors of interest. The spatial distributions of the ions that were formed were of special interest, but the accuracy with which these distributions could be determined was reduced by the presence of diffusion. This meant that the droplets, when photographed, had moved significantly away from the point of creation of the parent ion. In the present investigation photographs obtained by previous workers have been analyzed in an attempt to quantify the extent to which the droplets had diffused. The results suggest that the diffusion, when converted to standard density (1000 kg/m3), was independent of the pressure inside the cloud chamber and the mixture used. It could be represented by a one-dimensional root-mean-square diffusion distance whose value was calculated to be 2.42 +/- 0.04 nm. Values for the diffusion of thermalized electrons (< approximately 4 eV) before capture to form negative ions were also calculated. They appeared to lie in the range 3.5-5.0 nm, and were again independent of the pressure and nature of the mixture. The magnitude of the diffusion was large enough to mask any measurable prediffusion structure for a distance in the region of 10 nm radially around the track path of the alpha-particle and proton tracks analyzed.

Alpha Particles↗

PubMed2006

Irreversible thermal denaturation of beta-lactoglobulin retards adsorption of carrageenan onto beta-lactoglobulin-coated droplets.

The influence of isothermal heat treatments on the adsorption of anionic carrageenan molecules to the surfaces of anionic beta-lactoglobulin-coated droplets has been investigated. The zeta-potential, mean particle diameter, microstructure, and creaming stability of emulsions containing beta-lactoglobulin-coated droplets and/or carrageenan molecules that had previously been heat treated at temperatures ranging from 30 to 90 degrees C for 20 min were measured (pH 6.0, 150 mM NaCl). Three different heat treatments were used to establish the physicochemical origin of the influence of thermal history on the adsorption of carrageenan molecules to the protein coated droplets: (i) droplets and carrageenan were mixed at room temperature, then heated together; (ii) droplets were heated, cooled to room temperature, then mixed with carrageenan; (iii) carrageenan was heated, cooled to room temperature, then mixed with droplets. For treatments i and ii appreciably more carrageenan adsorbed to the protein-coated droplet surfaces at temperatures < or = 60 degrees C than at higher temperatures. For treatment iii, carrageenan adsorbed to the droplet surfaces across the whole temperature range. These results suggest that an irreversible thermal denaturation of the adsorbed beta-lactoglobulin molecules inhibited the adsorption of carrageenan molecules to the droplet surfaces. We postulate that there is a patch of positive charge on the surface of the native globular protein molecules which becomes more diffuse upon thermal denaturation. We found that the carrageenan molecules were unable to protect the beta-lactoglobulin-coated droplets at high temperatures (T > 60 degrees C) because they desorbed from the droplet surfaces. Nevertheless, adsorption of iota-carrageenan was capable of protecting the droplets against flocculation caused by surface denaturation of the adsorbed proteins at lower temperatures (T < or = 50 degrees C).

Adsorption↗

PubMed2003

Universal non-diffusive slow dynamics in aging soft matter.

We use conventional and multispeckle dynamic light scattering to investigate the dynamics of a wide variety of jammed soft materials, including colloidal gels, concentrated emulsions, and concentrated surfactant phases. For all systems, the dynamic structure factor f(q,t) exhibits a two-step decay. The initial decay is due to the thermally activated diffusive motion of the scatterers, as indicated by the q(-2) dependence of the characteristic relaxation time, where q is the scattering vector. However, due to the constrained motion of the scatterers in jammed systems. the dynamics are arrested and the initial decay terminates in a plateau. Surprisingly, we find that a final, ultraslow decay leads to the complete relaxation of f(q,t), indicative of rearrangements on length scales as large as several microns or tens of microns. Remarkably, for all systems the same very peculiar form is found for the final relaxation of the dynamic structure factor: f(q,t) approximately exp[-(t/tau s)p], with p approximately equal to 1.5 and tau s approximately q(-1), thus suggesting the generality of this behavior. Additionally, for all samples the final relaxation slows down with age. although the aging behavior is found to be sample dependent. We propose that the unusual ultraslow dynamics are due to the relaxation of internal stresses, built into the sample at the jamming transition, and present simple scaling arguments that support this hypothesis.

Algorithms↗

PubMed1993

Determination method for methyl iodide using diffusive sampler and capillary GC with thermal desorption injector and FID.

A determination method for methyl iodide in the work place has been developed, using a diffusive sampler and a capillary gas chromatograph with a thermal desorption cold trap injector and an FID. The sampler consists of a glass tube (6 mm o.d., 4 mm i.d., 16 cm length) packed with porous polymer beads. The diffusion path of the sampler is made of a 3 mm i.d., 52 mm length poly (tetrafluoroethylene) tube inserted in the glass tube. The sampling rate of the sampler for methyl iodide was 4.6 ml/h, and variations caused by temperature, relative humidity, and wind speed were found to be small and negligible. The sample could be stored for at least two weeks without significant loss. A time-weighted average concentration of methyl iodide in the range of sub ppm to ppm for two to six hours can be measured by this method. The average concentration of methyl iodide for a period of a few seconds can also be measured by grab sampling, i.e., injecting 20 ml of air directly into the sampler.

Air Pollutants, Occupational↗

PubMed1989

One-dimensional diffusion of microtubules bound to flagellar dynein.

Dynein is a multisubunit ATPase that powers microtubule-based motility. We find that a dissociated dynein particle containing the beta heavy chain subunit translocates microtubules unidirectionally over a glass surface in the presence of ATP. However, after nucleotide hydrolysis is inhibited by vanadate, unidirectional translocation ceases, and microtubules instead undergo irregular back-and-forth motion along their longitudinal axes. Quantitative analysis reveals that this motion is due to thermal-driven diffusion, but, unlike a particle undergoing Brownian motion, the diffusion is restricted to one dimension. The properties of the diffusional movement indicate that dynein can interact with microtubules in a way that permits the latter to diffuse only along their longitudinal axes. This weak binding interaction may constitute an important intermediate state in dynein's force-generating cycle.

Adenosine Triphosphatases↗

PubMed2000

Renormalization group analysis for thermal turbulent transport.

In this study, we continue with our previous renormalization group analysis of incompressible turbulence, aiming at determination of various thermal transport properties. In particular, the temperature field T is considered a passive scalar. The quasinormal approximation is assumed for the statistical correlation between the velocity and temperature fields. A differential argument leads to derivation of the turbulent Prandtl number Pr(t) as a function of the turbulent Peclet Pe(t) number, which in turn depends on the turbulent eddy viscosity nu(t). The functional relationship between Pr(t) and Pe(t) is comparable to that of Yakhot et al. [Int. J. Heat Mass Transf. 30, 15 (1987)] and is in close consistency with direct-numerical-simulation results as well as measured data from experiments. The study proceeds further with limiting the operation of renormalization group analysis, yielding an inhomogeneous ordinary differential equation for an invariant thermal eddy diffusivity sigma. Simplicity of the equation renders itself a closed-form solution of sigma as a function of the wave number k, which, when combined with a modified Batchelor's energy spectrum for the passive temperature T, facilitates determination of the Batchelor constant C(B) and a parallel Smagorinsky model and the model constant C(P) for thermal turbulent energy transport.

Journal Article↗

PubMedDate not supplied

Temperature distribution in selective laser-tissue interaction.

Selective photothermal interaction using dye enhancement has proven to be effective in minimizing surrounding tissue damage and delivering energy to target tissue. During laser irradiation, the process of photon absorption and thermal energy diffusion in the target tissue and its surrounding tissue are crucial. Such information allows the selection of proper operating parameters such as dye concentrations, laser power, and exposure time for optimal therapeutic effect. Combining the Monte Carlo method for energy absorption and the finite difference method for heat diffusion, the temperature distributions in target tissue and surrounding tissue in dye enhanced laser photothermal interaction are obtained. Different tissue configurations and dye enhancement are used in the simulation, and different incident beam sizes are also used to determine optimum beam sizes for various tissue configurations. Our results show that the algorithm developed in this study could predict the thermal outcome of laser irradiation. Our simulation indicates that with appropriate absorption enhancement of the target tissue, the temperature in the target tissue and in the surrounding tissue can be effectively controlled. This method can be used for optimization of lesion treatment using laser photothermal interactions. It may also provide guidance for laser immunotherapy in cancer treatment, since the immunological responses are believed to be related to tissue temperature changes.

Animals↗

PubMed2006

Direct thermal desorption of semivolatile organic compounds from diffusion denuders and gas chromatographic analysis for trace concentration measurement.

A novel method for collection and analysis of vapor-phase semivolatile organic compounds (SOCs) in ambient air is presented. The method utilizes thermal desorption of SOCs trapped in diffusion denuders coupled with cryogenic preconcentration on Tenax-TA and analysis by high resolution gas chromatography (GC)-electron-capture detection (ECD). The sampling and analysis methods employ custom-fabricated multicapillary diffusion denuders, a hot gas spike (HGS) apparatus to load known quantities of thermally stable standards into diffusion denuders prior to sample collection, a custom-fabricated oven to thermally desorb SOCs from the diffusion denuder, and a programmable temperature vaporization (PTV) inlet containing a liner packed with Tenax-TA for effective preconcentration of the analytes and water management. High flow rates into the PTV inlet of 750mLmin(-1)during thermal desorption are ca. a factor of ten greater than typically used. To improve resolution and retention time stability, the thermal desorption and PTV inlet programming procedure includes three steps to prevent water from entering the analytic column while effectively transferring the analytes into the GC system. The instrumentation and procedures provide virtually complete and consistent transfer of analytes collected from ambient air into the GC evidenced by recovery of seven replicates of four internal standards of 90.7+/-4.0-120+/-23% (mean+/-95% confidence interval, CI). Retention time based compound identification is facilitated by low retention time variability with an average 95% CI of 0.024min for sixteen replicates of eight standards. Procedure details and performance metrics as well as ambient sampling results are presented.

Air↗

PubMed2003

Thermal convection in binary fluid mixtures with a weak concentration diffusivity, but strong solutal buoyancy forces.

Thermal convection in binary liquid mixtures is investigated in the limit where the solutal diffusivity is weak, but the separation ratio is large. Representative examples are colloidal suspensions such as ferrofluids. With a grain size being large on molecular length scales, the particle mobility is extremely small, allowing to disregard the concentration dynamics in most cases. However, this simplification does not hold for thermal convection: Due to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces are dominant. Indeed, convective motion is found to set in at Rayleigh numbers well below the critical threshold for single-component liquids. A nonlinear analysis demonstrates that the amplitude quickly saturates in a state of stationary convective motion.

Journal Article↗

PubMed1993

Thermal drift is enough to drive a single microtubule along its axis even in the absence of motor proteins.

One-dimensional diffusion of microtubules (MTs), a back-and-forth motion of MTs due to thermal diffusion, was reported in dynein motility assay. The interaction between MTs and dynein that allows such motion was implicated in its importance in the force generating cycle of dynein ATPase cycle. However, it was not known whether the phenomenon is special to motor proteins. Here we show two independent examples of one-dimensional diffusion of MTs in the absence of motor proteins. Dynamin, a MT-activated GTPase, causes a nucleotide dependent back-and-forth movement of single MT up to 1 micron along the longitudinal axes, although the MT never showed unidirectional consistent movement. Quantitative analysis of the motion and its nucleotide condition indicates that the motion is due to a thermal driven diffusion, restricted to one dimension, under the weak interaction between MT and dynamin. However, specific protein-protein interaction is not essential for the motion, because similar back-and-forth movement of MT was achieved on coverslips coated with only 0.8% methylcellulose. Both cases demonstrate that thermal diffusion could provide a considerable sliding of MTs only if MTs are restricted on the surface appropriately.

Animals↗

PubMed2005

Monte Carlo assessment of soil moisture effect on high-energy thermal neutron capture gamma-ray by 14N.

Among many conventional techniques, nuclear techniques have shown to be faster, more reliable, and more effective in detecting explosives. In the present work, neutrons from a 5 Ci Am-Be neutron source being in water tank are captured by elements of soil and landmine (TNT), namely (14)N, H, C, and O. The prompt capture gamma-ray spectrum taken by a NaI (Tl) scintillation detector indicates the characteristic photo peaks of the elements in soil and landmine. In the high-energy region of the gamma-ray spectrum, besides 10.829 MeV of (15)N, single escape (SE) and double escape (DE) peaks are unmistakable photo peaks, which make the detection of concealed explosive possible. The soil has the property of moderating neutrons as well as diffusing the thermal neutron flux. Among many elements in soil, silicon is more abundant and (29)Si emits 10.607 MeV prompt capture gamma-ray, which makes 10.829 MeV detection difficult. The Monte Carlo simulation was used to adjust source-target-detector distances and soil moisture content to yield the best result. Therefore, we applied MCNP4C for configuration very close to reality of a hidden landmine in soil.

Journal Article↗