Tension Gradient Research Articles

Effects of combined conventional and modified ultrafiltration in adult patients

Background. Modified ultrafiltration (MUF) improves hemodynamics and postoperative recovery in children. Ultrafiltration (UF) may have similar benefits in adults. The purpose of this study was to investigate the effects of UF in adult patients. Methods. A total of 40 adult patients undergoing cardiac surgery were randomized into a study group of conventional UF during bypass + venovenous MUF after bypass and a control group with no UF. Perioperative clinical variables, cytokines, and endothelin-1 levels were compared between groups. Results. There was no mortality in either group. The patients in the study group had a greater rise in hematocrit (5.7% ± 2.4% vs 1.2% ± 1.9%, p < 0.001), hemoglobin (1.7 ± 0.8 mg/mL vs 0.5 ± 0.6 mg/mL, p < 0.0005), and platelet levels (27,800 ± 29,200 vs −9,000 ± 30970, p < 0.001). Mean arterial blood pressure and CI increased after MUF (from 64.2 ± 16.9 mm Hg to 72.3 ± 14.1 mm Hg, p = 0.05, and from 2.4 ± 0.7 to 2.8 ± 0.6, p < 0.03, respectively). Postoperative oxygenation was better in the study group (alveolo-arterial P O 2 tension gradient 74.6 ± 43.9 mm Hg vs 107.2 ± 27.8 mm Hg, p = 0.03). Ultrafiltration reduced postoperative bleeding (522.2 ± 233.4 mL vs 740 ± 198.4 mL, p < 0.003). Conclusions. A combination of conventional and modified UF is effective and safe in adult patients undergoing cardiac surgery. Ultrafiltration improved hemodynamics, hemostatic, and pulmonary functions. We recommend the use of combined UF in high-risk adult patients.

Influence of Agarose Gel on Electrophoretic Stretch, on Trapping, and on Relaxation of DNA

The long-range hydrodynamic interaction between DNA and agarose gel is treated by modeling the gel as a uniformly porous medium, characterized by Debye's hydrodynamic shielding length L. We derive the liquid flow field around a solid sphere moving through the gel and the hydrodynamic friction of the sphere. These results are used in the hydrodynamic interaction between DNA and the gel during electrophoretic stretch and relaxation of stretched DNA. Experiments by Gurrieri, Smith, and Bustamante (Proc. Natl. Acad. Sci. USA 1999, 96, 453) lead to an estimate of L = 4 × 10 -8 m for the shielding length of 1.2% agarose gel. The critical trapping force in such a gel is found to be 29 pN, consistent with results reported by Gurrieri et al. (1999). If the unwinding of the double helix at the gel contact is essential for trapping, addition of salt should cause a significant increase in the critical field strength for trapping DNA. Relaxation of the length and of the tether force of initially stretched DNA is described by a balance between the elastic forces (tension gradients) of DNA and hydrodynamic friction. Brownian motion is introduced in an approximate way. Its effect on the relaxation is moderate. The influence on relaxation of several factors, including gel porosity and DNA size, is studied. The scaling with DNA size is better for relaxation of the tether force than of the DNA extension. The relaxation of DNA predicted in pure solvent agrees fairly well with experiments by Perkins, Quake, Smith, and Chu (Science 1994, 264, 822).

Drying of alkyd emulsion paints

The drying and film formation of long oil alkyd emulsions was investigated with emphasis on loss of dry upon storage. The main reason for loss of dry was adsorption of the cobalt (drier) on pigment surfaces as a precipitated hydroxide. Titanium dioxides with alumina surface treatment and organic pigments were most detrimental to drying. Acrylate- and phosphate-based dispersants also deactivated the cobalt, presumably due to complexation and precipitation of the cobalt. Emulsions prepared with an emulsifiable cobalt drier containing 2,2′-bipyridyl (complexing agent for cobalt) showed the best resistance to loss of dry. Atomic force microscopy of films formed from the emulsions showed that the emulsifier migrated to the film surface which, when washed with water, leaves holes or pits in the film. The pits were arranged in a hexagonal pattern characteristic of Benard cells, which form due to surface tension and/or temperature gradients generated in the film during evaporation of water.

Tension gradients and Marangoni flows in nematic interfaces.

This brief report (i) presents equations that govern the balance of tangential forces in interfaces between isotropic viscous fluids and nematic liquid crystals, and (ii) establishes the physical origin of nematic Marangoni flows. It is shown that surface gradients in the orientation dependent surface free energy gives rise to tangential nematic Marangoni forces. Tangential nematic Marangoni forces are caused by surface gradients of the nematic tensor order parameter, and the kinetic coefficient characterizing this interfacial phenomenon is proportional to nematic elastic storage. Expressions of the Marangoni forces using a classical constitutive equation for the surface free energy are given for general and uniaxial nematic ordering states. Nematic Marangoni flows or nematocapillarity augments the class of Marangoni flows present in electrocapillarity, diffusocapillarity, and thermocapillarity.

Diurnal fluctuations of water and heat flows in a bare soil

The complexity of coupled transport of heat and moisture at the soil surface necessitates a combination of field and numerical experiments to evaluate the interactions between liquid and vapor phase flow. The near‐surface moisture and temperature conditions of a bare soil were investigated experimentally and by using the SOIL model to assess the importance of water vapor flow. During a 1‐month period in early fall, intensive measurements of water content, water tension, and temperature were made in a bare soil plot. Soil thermal conductivity, measured on soil cores extracted for laboratory analysis, was found to agree with estimates based on the Kersten equation. Simulated water content and soil temperature agreed well with observations. Modeled soil vapor flow was significant compared to liquid flow only during the initial dry days when the inclusion of vapor flow improved the predicted diurnal variation in water tension. Model predictions were sensitive to an accurate representation of the soil surface energy balance, including the consideration of steep gradients in tension near the soil surface, and to the enhancement of vapor flow.

Effects of water stress on vessel size and xylem hydraulic conductivity in Vitis vinifera L.

Modifications of vessel size and of shoot hydraulic conductivity induced by different water availability levels (Ψ leaf - 0.35 MPa, -0.6 MPa and - 0.8 MPa, respectively) were investigated in container-grown grapevine plants. Plant water loss, measured as xylem sap flow and as leaf transpiration, was lower in water-stressed plants. Morphometric measurements on xylem showed that vessels of water-stressed plants had lower transectional areas. Shoot hydraulic conductivity (k h ), shoot specific conductivity (k s ) and leaf specific conductivity (k l ) were lower in water-stressed plants. When conductivities were measured on shoot portions, differences between treatments were particularly high at the basal internodes. At the lower stress level no embolism was detected, and reduced conductivity could be explained by the reduction of vessel diameter, according to the Poiseuille equation. At the higher stress level k h was further reduced by formation of vessel embolisms. The tension gradient along the shoot increased only at the higher stress level. Stomatal conductance was linearly correlated with k l at low stress levels, suggesting a concerted regulation of water flow, while at higher stress levels stomatal conductance decreased with no changes of k l Reduced development of xylem vessels in grapevines subjected to moderate water stress may contribute to the control of water flow and to a reduction in vulnerability to xylem embolism.

Convection Generated by a Gradient of Interfacial Tension in a Phase Separated Binary Liquid Mixture with a Miscibility Gap

Convection Generated by a Gradient of Interfacial Tension in a Phase Separated Binary Liquid Mixture with a Miscibility Gap

Hydrodynamical Effects of Chemical Waves in Quasi-Two-Dimensional Solution in Belousov–Zhabotinsky Reaction

The evidence was studied that the chemical wave accompanied by hydrodynamic instability showed some noteworthy characteristics in a quasi-two-dimensional shallow layer of the unstirred excitable Belousov–Zhabotinsky (BZ) reaction. This chemical wave has been known as the "Big Wave" or the "Hydrochemical Soliton", which showed soliton-like properties. We clarified properties of the wave using an optical technique (Mach–Zehnder interferometry). The Big Wave acceleratingly propagated with large velocity, and simultaneously caused flow in the bulk of the solution as well as large surface deformation (~ 5μm). We proposed that the main mechanism of this wave was chemically coupled Marangoni instability, which was induced by the gradient of surface tension due to the thermal and/or material inhomogeneity in the BZ solution.

Effect of minor chemistry elements on GTA weld fusion zone characteristics of a commercial grade titanium

Gas Tungsten Arc Welding (GTAW) is the most common technique employed in the fabrication of rolled thin tubes. One of the major manufacturing problems concerns the stability of weld fusion zone on materials from different casts, notwithstanding stringent monitoring of the process parameters -- current, voltage and travel speed. These parameters determine the theoretical weld heat and are expected to control the instantaneous mass of melt. According to the data compiled by Sahoo et al., oxygen is known to reduce the surface tension of most of the metals. However, investigations on the role of minor changes in concentrations of elements like sulphur, oxygen, selenium, bismuth, aluminium, and titanium in steels have very often attributed the cast to cast variations to different temperature gradients of surface tension over the weldpool. To the author`s knowledge, no reported work so far has revealed changing weld profiles in autogeneous mechanized GTA welds on titanium due to minor composition changes.

Surface tension effects in tunneling microscopy

Induced electric charge lowers the surface tension of the substrate below the tunneling tip and creates a sharp surface tension gradient in its vicinity. For mercury as the substrate, the random waving is amplified by the gradient of the surface tension, resulting in observable mechanical ‘‘waving’’ of the surface that increases with the applied bias voltage. Its amplitude reaches a minimum at the potential of zero charge. In agreement with the Gibbs–Lippmann equation these effects are mitigated by the presence of adsorbed films but may lead to translocation of adsorbates by the biased tunneling tip.

Observation of Cell Structures Developed by Marangoni Convection in Glass Thick Films

Cell structures developed by convection in glass thick films have been studied with special emphasis on the effects of material parameters and heating conditions on the cell structure. Borosilicate glass films from 10 to 30 μm thick were prepared on alumina substrates by printing and firing at temperatures between 750° and 950°C up to 2 h, and surface flow patterns were observed with an optical microscope under dark‐field illumination. In most cases a flow pattern developed and changed with time, finally reaching a steady state. Cell structures with regular polygons of four to seven sides from 15 to 150 μm across developed, depending on the glass composition. The effects of heating time, temperature, film thickness, and glass chemistry on the cell structure have been examined. From observations of cell structures, the driving force for the convection in glass thick films has been identified to be the gradient in surface tension due to small temperature fluctuations on the surface. Hence, it is concluded that the convection in glass thick films is of Marangoni type.

Long-term xylem pressure measurements in the liana Tetrastigma voinierianum by means of the xylem pressure probe

Diurnal changes of xylem pressure in the liana Tetrastigma voinierianum have been measured under greenhouse conditions by means of the recently developed xylem pressure probe. During the early morning hours, tensions in the vessels developed more or less rapidly with time, depending on light intensity. On sunny days, absolute negative pressures down to about -0.4 MPa (atmospheric = 0.1 MPa) were recorded around noon in petiolar or stem xylem vessels, whereas on rainy or cloudy days the xylem pressure remained in the positive sub-atmospheric or slightly negative pressure range. Towards the evening the tension in the vessels always decreased, i.e. the xylem pressure shifted to about atmospheric, or even above-atmospheric, values during the night. Simultaneous xylem pressure recordings at heights of 1 and 5 m frequently yielded either no gradient in tension at all, or far less than expected from the Cohesion Theory. Occasionally, tension gradients were even opposite to those predicted by this theory. Stem-toleaves pressure gradients in accord with the Cohesion Theory were recorded only when tension had been developed during sunny days in the upper branches of the liana, because increases in tension were not immediately propagated to the xylem of the leaves at ground level, as would be expected from a strictly coupled hydraulic system. Parallel recordings of the “xylem tension” using the pressure chamber yielded rather variable values ranging from 0.1 to 1 MPa; diurnal pressure changes could not be detected at all. The data are discussed on the basis of the equation for the chemical activity of water. They strongly suggest that the xylem tension induced by transpiration is not the sole force for water ascent. Other forces, such as osmotic pressure or convectional and interfacial forces, which to a remarkable extent have already been postulated for decades, seem to be equally important.

Mechanical properties of aponeurosis and tendon of the cat soleus muscle during whole-muscle isometric contractions.

Recent studies have suggested that the mechanical properties of aponeurosis are not similar to the properties of external tendon. In the present study, the lengths of aponeurosis, tendon, and muscle fascicles were recorded individually, using piezoelectric crystals attached to the surface of each structure during isometric contractions in the cat soleus muscle. We used a surgical microscope to observe the surface of the aponeurosis, which revealed a confounding effect on measures of aponeurosis length due to sliding of a thin layer of epimysium over the proximal aponeurosis. After correcting for this artifact, the stiffness computed for aponeurosis was similar to tendon, with both increasing from around 8 F0/Lc (F0 is maximum isometric force and Lc is tissue length) at 0.1 F0 to 30 F0/Lc at forces greater than 0.4 F0. At low force levels only (0.1 F0), aponeurotic stiffness increased somewhat as fascicle length increased. There was a gradient in the thickness of the aponeurosis along its length: its thickness was minimal at the proximal end and maximal at the distal end, where it converged to form the external tendon. This gradient in thickness appeared to match the gradient in tension transmitted along this structure. We conclude that the specific mechanical properties of aponeurosis are similar to those of tendon.

A New Theory for the Ascent of Sap—Cohesion Supported by Tissue Pressure

Recent work contradicting both the assumptions of the Cohesion Theory, and the tensions measured in the xylem sap by the pressure-chamber, is reviewed. Measurements with the xylem-pressure probe reveal pressures in vessels around 0 bar absolute, and no detectable gradients of pressure with tree height. Under high water stress, pressures down to -6 bar were found, but then cavitations occurred very readily. Also, measurements of the cavitation thresholds of water show an average threshold of about -2 bar. The uncertain foundations of the Cohesion Theory are recalled from the years before 1965. Soon after that date, Scholander's measurements with the pressure chamber were agreed to have confirmed the theory and the existence of high tensions in the xylem. Before 1965, many experiments over many years pointed to the conclusions now rediscovered, viz., no high tensions, and no gradients of tension. A resolution of these paradoxes is offered in the form of a new theory. This proposes that the driving force and the transmission of the force are the same as in the Cohesion Theory, but the operating pressure of the xylem is raised into a stable range by compensating tissue pressures pressing upon the tracheary elements. The tissue pressure does not propel the transpiration stream, which is still driven by evaporation, but protects the stream from cavitation. Evidence is presented for the existence of positive pressures in roots, wood, and leaves. It is shown that the anatomy of roots, wood, and monocotyledon and cryptogam vascular bundles is organized so that pressure is confined by mechanical barriers, and exerted upon the tracheary elements by the living cells of the phloem and the xylem parenchyma. The Compensating-Pressure Theory also explains, among other things, root pressure, the function of the endodermis, the structure of wood, the constant association of xylem and phloem, the absence of gas spaces in vascular tissue, the absence of a gravitational gradient in the xylem, bleeding from cut palm inflorescences, how insects are able to withdraw sap from the xylem, and the variable that is measured by the pressure chamber. This instrument measures the water potential, but this is the potential not of xylem in tension, but of the compensating pressure applied to the xylem. The requirements of the Theory are explained, and a number of predictions are made which are open to experimental testing.

Influence of liquid bridge volume on the onset of oscillation in floating zone convection II. Numerical simulation

The onset of oscillation in the floating zone convection driven by the gradient of surface tension was studied numerically for an unsteady and two-dimensional model, and studies were concentrated on the influence of liquid bridge volume on the onset of oscillation in comparison with the experimental results in the Paper I. The numerical results agree with the experimental ones presented in the previous paper, in which the distributions of critical applied temperature difference depending on the volume of liquid bridge and a gap range of liquid volume in marginal stability curve were obtained.

Climate of the Tanana River floodplain near Fairbanks, Alaska

This paper describes the general environmental conditions on the Tanana River floodplain in the vicinity of the Bonanza Creek Experimental Forest. It describes and discusses environmental measurements taken in three successional stages (stage III, an open willow stand; stage V, a closed balsam poplar–alder stand; and stage VIII, a mature white spruce stand) and compares these measurements with those taken in adjacent artificial clearings. The impact of vegetation-mediated changes on soil temperatures, ground surface evaporation, and precipitation regimes is substantial, but the magnitude of these changes declines with advancing succession. Removing vegetation in the three successional stages resulted in warming of the mineral soil and increased surface evaporation, even though there were only slight changes in air temperature. Vertical soil moisture movement occurs along a tension gradient from the water table for distances of up to 50 cm in the mineral soil and can be correlated with fluctuations in the river level. Both fluvial and biotic controls interact to influence the course of vegetation and environmental changes in the successional sequence on the floodplain.

Measurement of redox states of mitochondrial cytochrome aa3 in regions of liver lobule by reflectance microspectroscopy.

To evaluate the sublobular distribution of oxygen tension, the redox states of mitochondrial cytochrome aa3 were assessed in local regions of the liver lobule by reflectance microspectroscopy. The reflected light from two focused microspots (20 microns in diam) on the surface of the perfused liver, placed on the stage of microscope, was conducted to the spectrophotometer with two separate light guides, giving the reflectance spectra. The redox state of cytochrome aa3 was calculated from the difference in reflectance between 603 and 630 nm. The degree of cytochrome reduction increased with the stepwise decline of the influent or effluent oxygen concentrations. The local oxygen concentration in periportal region for the half-maximal reduction of cytochrome aa3 was higher than that in pericentral region. In addition, the degree of cytochrome aa3 reduction increased with the distance from the periportal region, reflecting a physiological gradient of oxygen tension along the sinusoidal flow, although with an intersinusoidal heterogeneity within liver lobule.

Direct Measurements of the Pressure and Flow in the Xylem Vessels of Nicotiana tabacum and their Dependence on Flow Resistance and Transpiration Rate

AbstractThe relationships between xylem tension, velocity of water ascending and transpiration in tobacco plants were measured by means of the “xylem pressure probe technique” (Balling, A. and Zimmermann, U., Planta 182, 325–338, 1990). The flow velocity was determined by suction or injection of fluorescein (or FITC‐labelled dextrans of various molecular weights) from the microcapillary of the pressure probe into the punctured xylem vessel, followed by serial‐sectioning of the stem after a given propagation time. The distance travelled was defined as the distance from the injection point to the uppermost xylem section in which the dye could be detected. For a transpiration rate of 0.52 ± 0.12 ml . h−1, a linear dependence between the flow velocity and the tension gradients was found as expected from the Hagen‐Poiseuille law. The slope of the straight lines decreased with increasing molecular weight of the fluorescent labelled compound, presumably because of (partial) plugging of the pit membranes. The average value of the flow velocity (2.5 . 10−4 ± 0.9 . 10−4 m . s−1) was one magnitude smaller than the value estimated from the geometric dimensions of the xylem vessels, but agreed well with the literature value of 2.8 . 10−4 m . s−1 for herbs (determined by the heat pulse technique; Huber, B. Ber. deutsch. bot. Ges. 50, 89–109, 1932). The average pressure gradient was determined to be 0.39 ± 0.23 MPa . m−1, in agreement with the literature (Begg, J. E. and Turner, N. C. Plant Physiol. 46, 343–346, 1970).The first response of xylem pressure (or tension) and of flow velocity to a reduction of the transpiration rate (0.14 ± 0.06 ml . h−1) occurred after about 24 h, when an increase of the xylem pressure towards higher values associated with a decrease in flow velocity was observed. In contrast, re‐establishment of the normal transpiration rate brought the pressure (or tension) and the flow velocity back to normal values within half an hour.Similary, introduction of a transverse cross‐sectional cut into the stem did not lead during the first 10 h to a change in xylem tension (or velocity). However, during the following day the pressure fell to relatively low values (about −0.13 MPa). The velocity increased 10‐fold. In the next two days the xylem pressure increased again to normal values (average +0.03 MPa), whereas the flow velocity assumed higher values than normal.The data are discussed in terms of the water status and storage of the adjacent tissue cells.

FUNCTION OF THE PULMONARY SURFACTANT IN THE CLEARANCE OF RESPIRATORY BRONCHIOLES: A MATHEMATICAL MODEL

Abstract An idea of clearance of respiratory bronchioles as a result of continuous flow of extracellular liquid layer generated by the gradient of the surface tension is presented. This gradient is due to nonhomogeneities in the distribution of the surface-active substances produced by type II pneumocytes in the alveoli. The mathematical model which describes the flow of the hypophase has been formulated and numerically examined for some values of viscosity and density of the lining layer and of differences in the surface concentration of surfactant between the bronchiole edges. The results show a strong dependence of the clearance rate on these differences and hypophase viscosity and very weak effect of the gravity forces.

Two-phase kinetics of the solubilization in reverse micelles

The rates of the solubilization of methylene blue, water, and different electrolytes were measured in reversed AOT micelles in a liquid/liquid system using a two-phase stirred cell. Variation in the forced convection in the cell allowed the diffusional and chemical steps of the process to be differentiated. Applying the usual concept of mass transfer resistances it is possible to prove unequivocally that the rates of solubilization of methylene blue and water are controlled by the convective transport in the aqueous and organic phase, respectively. The transfer of metal cations, however, is a very slow process that is independent of the stirring rate and dependent on the interfacial area, which indicates that an interfacial step is rate controlling. With these results, particularly related to the kinetic behavior of mixed electrolytes, a mechanism for the interfacial solubilization is proposed, and some conclusions concerning the kind of aggregation are drawn. The values of the interfacial tension and tension gradients caused by convection suggest that appearance of a locally negative interfacial tension could be the reason for the encapsulation process.