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At least 991 records · Page 55Linked to original sources
PubMed2006

Enhanced mass transport in nanofluids.

Thermal conductivity enhancement in nanofluids, which are liquids containing suspended nanoparticles, has been attributed to localized convection arising from the nanoparticles' Brownian motion. Because convection and mass transfer are similar processes, the objective here is to visualize dye diffusion in nanofluids. It is observed that dye diffuses faster in nanofluids compared to that in water, with a peak enhancement at a nanoparticle volume fraction, phi, of 0.5%. A possible change in the slope of thermal conductivity enhancement at that same phi signifies that convection becomes less important at higher phi. The enhanced mass transfer in nanofluids can be utilized to improve diffusion in microfluidic devices.

Journal Article↗
PubMed2005

Fluid structure and transport properties of water inside carbon nanotubes.

The fluid structure and transport properties of water confined in single-walled carbon nanotubes (CNTs) with different diameters have been investigated by molecular-dynamics simulation. The effects of CNT diameter, density of water, and temperature on the molecular distributions and transport behaviors of water were analyzed. It is interesting that the water molecules ordered in helix inside the (10, 10) CNT, and the layered distribution was clearly observed. It was found that the axial and radial diffusivities in CNTs were much lower than that of the bulk, and it ever decreased as the diameter of CNT decreases. The axial thermal conductivity and shear viscosity in CNTs are obviously larger than that of the bulk and those in the radial direction, they increase sharply as the diameter of CNT decreases, which is clearly in contrast to the diffusivity. The inner space of CNT and the interactions between water molecules and the confining walls play a key role in the structure and transport properties of water confined in the CNTs.

Journal Article↗
PubMed2006

Systematic modeling study of channel waveguide fabrication by thermal silver ion exchange.

A systematic study of thermal silver ion exchange used for the fabrication of optical channel waveguides is reported in a single-alkali glass. The diffusion equilibrium and diffusion dynamics are experimentally studied, and the concentration-dependent diffusion coefficients are determined. The relationship between the fabrication conditions, i.e., time, temperature, and melt concentration, and the induced waveguide refractive index profile is established. It is demonstrated that the diffusion equation can be solved, without use of any free parameters, to predict the refractive index profiles of both planar and channel waveguides. A 1.6 cm diameter integrated optic ring resonator, with a propagation loss of 0.1 dB/cm, is fabricated in a glass by thermal silver ion exchange. The induced refractive index profile is related to the optical characteristics of the functional device.

Journal Article↗
PubMed1990

Acupuncture and diffuse noxious inhibitory controls: naloxone-reversible depression of activities of trigeminal convergent neurons.

Recordings were made from convergent neurons in trigeminal nucleus caudalis of the rat. These neurons could be activated by both innocuous and noxious mechanical stimuli applied to their excitatory receptive fields on the ipsilateral part of the muzzle. Percutaneous application of suprathreshold, 2 ms square-wave electrical stimuli to the centre of the excitatory field resulted in responses to A- and C-fibres being observed. The effects on these responses of manual acupuncture performed by a traditional Chinese acupuncturist at the "Zusanli" point on the right hindlimb were compared with the effects induced by acupuncture applied at a non-acupoint, next to "Zusanli". In addition, the effects of acupuncture were compared with the inhibitory effects evoked by noxious thermal stimulation of the left hindlimb on the responses of the same neurons. This last type of inhibition has been described previously by our group and termed diffuse noxious inhibitory controls. Acupuncture, either applied at "Zusanli" or at a non-acupoint and noxious thermal stimulation induced similar strong inhibitory effects on the C-fibre-evoked responses of trigeminal convergent neurons (77.9 +/- 4.4%; 72.5 +/- 4.6% and 78.5 +/- 3.6% inhibition, respectively) and these inhibitions were followed by long-lasting aftereffects. In addition, both the acupuncture- and noxious thermal stimulation-evoked inhibitions were significantly reduced by systemic naloxone (0.4 mg/kg, i.v.). Since the antinociceptive effects elicited by acupuncture (i) had a similar magnitude and time-course to those evoked by noxious thermal stimulation, (ii) exhibited a lack of topographical specificity and (iii) involved an opioidergic link, we would suggest that, at least in our experimental conditions, acupuncture manoeuvres trigger the neuronal mechanisms involved in diffuse noxious inhibitory controls.

Acupuncture Points↗
PubMed2004

Non-thermal effects of power-line magnetic fields (50 Hz) on gene expression levels of pluripotent embryonic stem cells-the role of tumour suppressor p53.

The diffusion of extremely low-frequency (50 Hz) electromagnetic fields (ELF-EMF) in the human environment raises the question of the induction of biological effects of EMF on mammalian cells. We used the model of mouse pluripotent embryonic stem (ES) cells, which have the capacity to develop in vitro into cells of all lineages, to analyse non-thermal effects of ELF-EMF. Wild type (wt) and p53-deficient ES cells were exposed under controlled conditions to ELF-EMF signals simulating power-line (50 Hz) magnetic field (PL-MF) exposure. Different flux densities of 0.1 mT, 1.0 mT or 2.3 mT and intermittency schemes with various ON/OFF cycles were applied for 6 h or 48 h during the first stages of cell differentiation. Transcript levels of regulatory genes, such as egr-1, p21, c-jun, c-myc, hsp70 and bcl-2, were analysed by semi-quantitative RT-PCR immediately after exposure or after a recovery time of 18 h. Intermittent PL-MF exposure to 5 min ON/30 min OFF cycles at a flux density of 2.3 mT for 6 h resulted in a significant up-regulation of c-jun, p21 and egr-1 mRNA levels in p53-deficient, but not in wild-type cells. No significant effects were observed in both cell systems by PL-MF at lower flux densities, longer exposure time or after 18 h recovery time. Our data indicate that 5 min ON/30 min OFF intermittent PL-MF exposure is capable of evoking non-thermal responses in ES cells, dependent on the cellular p53 function. The nature of the biological responses triggered by PL-MF is discussed.

Animals↗
PubMed2001

Vacancies in solids and the stability of surface morphology.

Determining how thermal vacancies are created and destroyed in solids is crucial for understanding many of their physical properties, such as solid-state diffusion. Surfaces are known to be good sources and sinks for bulk vacancies, but directly determining where the exchange between the surface and the bulk occurs is difficult. Here we show that vacancy generation (and annihilation) on the (110) surface of an ordered nickel-aluminium intermetallic alloy does not occur over the entire surface, but only near atomic step edges. This has been determined by oscillating the sample's temperature and observing in real time the response of the surface structure as a function of frequency (a version of Angström's method of measuring thermal conductivity) using low-energy electron microscopy. Although the surface-exchange process is slow compared with bulk diffusion, the vacancy-generation rate nevertheless controls the dynamics of the alloy surface morphology. These observations, demonstrating that surface smoothing can occur through bulk vacancy transport rather than surface diffusion, should have important implications for the stability of fabricated nanoscale structures.

Journal Article↗
PubMed2006

Intracellular transport dynamics of endosomes containing DNA polyplexes along the microtubule network.

We have explored the transport of DNA polyplexes enclosed in endosomes within the cellular environment by multiple particle tracking (MPT). The polyplex-loaded endosomes demonstrate enhanced diffusion at short timescales (t<7 s) with their mean-square displacement (MSD) Deltax(t)2 scaling as t1.25. For longer time intervals they exhibit subdiffusive transport and have an MSD scaling as t0.7. This crossover from an enhanced diffusion to a subdiffusive regime can be explained by considering the action of motor proteins that actively transport these endosomes along the cellular microtubule network and the thermal bending modes of the microtubule network itself.

Biological Transport, Active↗
PubMed2006

Conditions for extreme sensitivity of protein diffusion in membranes to cell environments.

We study protein diffusion in multicomponent lipid membranes close to a rigid substrate separated by a layer of viscous fluid. The large-distance, long-time asymptotics for Brownian motion are calculated by using a nonlinear stochastic Navier-Stokes equation including the effect of friction with the substrate. The advective nonlinearity, neglected in previous treatments, gives only a small correction to the renormalized viscosity and diffusion coefficient at room temperature. We find, however, that in realistic multicomponent lipid mixtures, close to a critical point for phase separation, protein diffusion acquires a strong power-law dependence on temperature and the distance to the substrate H, making it much more sensitive to cell environment, unlike the logarithmic dependence on H and very small thermal correction away from the critical point.

Animals↗
PubMed2006

Anomalous diffusion in periodic potentials under self-similar colored noise.

We present numerical studies of anomalous diffusion in periodic potentials by simulating a generalized Langevin equation. It is proved that the particle driven by a thermal colored noise with the spectral density vanishing at zero frequency allows superdiffusive motion. It is found that the system subjected to sub- or superohmic damping exhibits two motion modes in a corrugated plane: running oscillated state and mixed running and oscillating states, respectively. Induced, the anomalous power can be enhanced up twice for the latter case and thus a wide range of diffusive regimes is observed with changing titled force.

Journal Article↗
PubMed2004

Thermal stability of oxygen-modulated chemical-composition gratings in standard telecommunication fiber.

Experimental results and a discussion on the formation and decay of oxygen-modulated chemical-composition gratings in a standard telecommunication fiber are presented. Comparison between the decay experiment and the model provides a diffusion coefficient with an activation energy of 490 kJ/mol, which is in close agreement with reported values of oxygen self-diffusion in silica. The gratings have a diffusion-controlled decay behavior, with more than 50% of the reflectivity remaining after 7.5 h at a temperature of 1230 degrees C. The gratings show higher thermal stability when heated in air than in an inert argon atmosphere.

Journal Article↗
PubMed1992

Sonographic changes during hepatic interstitial laser photocoagulation. An investigation of three optical fiber tips.

RATIONALE AND OBJECTIVES: Interstitial laser photocoagulation (ILP) destroys tumors thermally, using laser energy delivered from implanted optical fibers. The objectives of the study are to identify a fiber tip/delivered energy combination which produces lesions of useful size, visible on ultrasound (US) during ILP, and to compare ILP lesions and their US images. METHODS: Hepatic ILP was performed at laparotomy in six pigs, using three different fiber tips (cylindrical diffusing, spherical diffusing, plane-cut). US images were obtained during ILP, immediately after ("early" images), and before the animals were killed (2-2.5 hours, "late" images). Actual lesions were assessed histopathologically. RESULTS: Few US changes were seen around cylindrical diffusing and spherical diffusing tips until tip destruction. Plane-cut tips, at 1.5 to 2.0 W, produced prominent US images of the 1- to 2-cm thermal lesions. Early images tended to overestimate necrosis. Late images approximated necrosis. CONCLUSION: For US-controlled ILP, plane-cut tips are better than currently available cylindrical diffusing or spherical diffusing tips. Lesion image growth periods might enable control of lesion size. Further studies are needed to determine the consistency of the described relationship between lesion images and actual lesions.

Animals↗
PubMed1984

A personal sampling method for the determination of nitrous oxide exposure.

A diffusive sampler is described which is suitable for the determination of time weighted average personal and static exposures to nitrous oxide. The sampler is based on a commercially available tube design and is analyzed by thermal desorption techniques. The performance of the sampler when exposed to a wide range of nitrous oxide concentrations is described. The effects of humidity changes on diffusive uptake have been tested and shown to be negligible. The precision of the sampler has been determined and the effects of back-diffusion and storage have been investigated. The results of these experiments and those of field comparisons between the diffusive sampler and conventional pump and tube techniques suggest that the precisions of both methods are similar. The diffusive method is preferred because of its simplicity and convenience.

Air Pollutants, Occupational↗
PubMed2002

Reaction and concentration dependent diffusion model.

We study the formation of patterns in the genuinely nonlinear reaction diffusion model equation u(t)+2a(u(2))(x) = (u(2))(xx)+F(x,u), where u may be viewed as an amplitude of a thermal wave in plasma or density of a biological species and F = u(1-u) or F = q(x)u(l), l = 0,2. We provide a transformation which maps the model into a purely diffusive process free of its interacting part and its intrinsic temporal and spatial scales. The well known attractors of the diffusive process enable us to completely characterize the emerging patterns which, depending on F and initialization, may be a semicompact, or a compact, traveling wave or a nontrivial equilibrium.

Diffusion↗
PubMed2001

b matrix errors in echo planar diffusion tensor imaging.

Diffusion-weighted magnetic resonance imaging (DW-MRI) is a recognized tool for early detection of infarction of the human brain. DW-MRI uses the signal loss associated with the random thermal motion of water molecules in the presence of magnetic field gradients to derive parameters that reflect the translational mobility of the water molecules in tissues. If diffusion-weighted images with different values of b matrix are acquired during one individual investigation, it is possible to calculate apparent diffusion coefficient maps that are the elements of the diffusion tensor. The diffusion tensor elements represent the apparent diffusion coefficient of protons of water molecules in each pixel in the corresponding sample. The relation between signal intensity in the diffusion-weighted images, diffusion tensor, and b matrix is derived from the Bloch equations. Our goal is to establish the magnitude of the error made in the calculation of the elements of the diffusion tensor when the imaging gradients are ignored.

Brain Infarction↗
PubMed2005

A theoretical study of the effects of RF fields in the vicinity of membranes.

In this article the forces associated with the gradients of a radio frequency (RF) field at the boundary between fluids and cell membranes are calculated, and it is shown that they can be large enough to affect the particle motion by amounts that are on the same order of magnitude as the random diffusion motion when the energy imparted to the particles is a reasonable fraction of the thermal energy. The induced dipole moment is assumed to track the alternating RF so that the force exerted by the gradient is in a constant direction; and this in turn leads to a modification of the particle distribution, even when the energy added to the particle is very small. For RF fields of 45 V/m the energy acquired by an induced dipole moment is expected to be on the order of a micro electron volt and small compared to the average thermal energy.

Animals↗
PubMed2001

Fluorescence microscopy of single viral capsids.

To build a foundation for the single-molecule fluorescence microscopy of protein complexes, the present study achieved fluorescence microscopy of single, nucleic acid-free protein capsids of bacteriophage T7. The capsids were stained with Alexa 488 (green emission). Manipulation of the capsids' thermal motion was achieved in three dimensions. The procedure for manipulation included embedding the capsids in an agarose gel. The data indicate that the thermal motion of capsids is reduced by the sieving of the gel. The thermal motion can be reduced to any desired level. A semilogarithmic plot of an effective diffusion constant as a function of gel concentration is linear. Single, diffusing T7 capsids were also visualized in the presence of single DNA molecules that had been both stretched and immobilized by gel-embedding. The DNA molecules were stained with ethidium (orange emission). This study shows that single-molecule (protein and DNA) analysis is possible for both packaging of DNA in a bacteriophage capsid and other events of DNA metabolism. The major problem is the maintenance of biochemical activity.

Bacteriophage T7↗