The size of the unstirred layer as a function of the solute diffusion coefficient.

By monitoring the concentration distribution of several solutes that are diffusing at the same time under given mixing conditions, it was established that the unstirred layer (USL) has no clearly defined boundary. For the cases of solute permeation and water movement across planar bilayer lipid membranes, respectively, experiments carried out with double-barreled microelectrodes have shown that the thickness of the USL depends on which species is diffusing. Small molecules with a larger diffusion coefficient encounter an apparently thicker USL than larger molecules with a smaller diffusion coefficient. The ratio of the USL thicknesses of two different substances is equal to the third root of the ratio of the respective diffusion coefficients. This experimental finding is in good agreement with theoretical predictions from the theory of physicochemical hydrodynamics.     

Measurement of the effective thickness of the mucosal unstirred layer in Necturus gallbladder epithelium

The effective thickness of the unstirred fluid layer (USL) adjacent to an epithelial barrier can be estimated from the time course for the accumulation or depletion of a solute at the membrane surface. In 1985 we reported an unstirred layer thickness of approximately 70 microns for Necturus gallbladder epithelium. In our earlier studies the delay caused by noninstantaneous bulk solution mixing was not taken into account and thus the USL thickness was systematically overestimated. In the present studies we describe an analysis of the time course of solute arrival at the membrane surface that takes into account noninstantaneous bulk solution mixing. We also describe a simple technique to monitor the accumulation or depletion of a solute at the membrane surface. The time course for the change in the concentration of either tetramethylammonium (TMA+) or tetrabutylammonium (TBA+) upon elevation of bulk solution concentration is sensed at the membrane surface with an ion-sensitive microelectrode. Because of the high selectivity of the ion-sensitive resin for TMA+ or TBA+ over other monovalent cations in the solution (Na+ and K+), a low concentration (1-2 mM) of the probe can be used. By measuring the time course of the arrival of first one probe and then the other, under identical superfusion conditions, sufficient information is obtained to eliminate multiple fits to the data, obtained when only one probe is used. Neglecting bulk solution mixing caused an error greater than 50% in estimated apparent USL thickness. The effective thickness of the USL depends critically upon chamber geometry, flow rate, and the position of superfusion and suction pipettes. Under our experimental conditions the effective USL at the mucosal surface of Necturus gallbladder epithelium was approximately 40 microns.     

Use of a membrane-bound fluorophore to characterize diffusion boundary layers around human erythrocytes.

A novel method is used to demonstrate the presence of diffusion boundary layers around erythrocytes following rapid mixing in a stopped-flow spectrophotometer and to estimate the apparent dimensions of the diffusion boundary layers. Pink erythrocyte ghosts labeled on their external surfaces with tetramethyl rhodamine isothiocyanate (TRITC) were mixed in a stopped-flow apparatus with 50 mM NaI in Ringer's solutions. I- is an effective collisional quencher of TRITC fluorescence. TRITC fluorescence after flow stopped decreased monoexponentially with time. The concentration of I- at the cell surface as a function of time was estimated from the dependence of TRITC fluorescence on I- concentration in steady-state experiments. The kinetics of the increase in I- concentration at the cell surface was fit to two diffusional models: a planar erythrocyte ghost bounded by planar diffusion boundary layer and a spherical erythrocyte surrounded by a spherical shell diffusion boundary layer. The planar model best fits the experimental data with a diffusion boundary layer 4.68 microns thick. Using the spherical model the experimental data is best fit by a 6.9 microns diffusion boundary layer.     

森林生态系统与大气边界层相互作用的数值模拟

基于大气边界层和植被冠层微气象学基本原理,建立了一个森林生态系统与大气边界层相互作用的数值模式．应用该模式模拟了森林生态系统的热量平衡、植被温度、植被冠层内空气温度、地表温度日变化特征,及森林生态系统下垫面大气边界层风速、位温、比湿、湍流交换系数的时空分布和廓线的日变化特征．该模式还可应用于不同下垫面,模拟陆面物理过程与大气边界层相互作用机制及其区域气候效应的研究,这将为气候模式与生物圈的耦合研究奠定一个良好的基础．     

Boundary layer budgets for regional estimates of scalar fluxes

The paper considers the theory and application of budget techniques for regional scalar flux estimation using the daytime convective boundary layer (CBL) and the nocturnal boundary layer (NBL). CBL techniques treat the well mixed layer of air between heights of, say, 100 m and 1000 m as an integrator of surface fluxes along the path of a column of air moving over the landscape. They calculate the average surface flux from the scalar concentration in and above the mixed layer, and the CBL height. The flux estimates are averaged over regions of 10–10⁴ km² extending 10 to 100 km upwind. An integral form of the CBL budget is used to estimate daily regional rates of CO₂ uptake and evaporation from three data sets. There was plausible agreement between the estimates and locally measured fluxes. CBL budgets have great potential for estimating regional scalar fluxes, but there is an urgent need for validation through direct measurements of fluxes and budget parameters. NBL budgets are useful when low-level, radiative inversions inhibit vertical mixing. Surface scalar fluxes can then be estimated from the rate of concentration change below the inversion. An example application for estimating the nocturnal CO₂ flux is given. While simple in concept, NBL budgets are more difficult to apply in practice because of the unpredictability of the depth of the layer and sometimes, its absence altogether. On the other hand, the depth of the atmospheric mixing chamber is better defined, few assumptions are required and the concentration changes usually will be larger and hence more easily detectable than in CBL budgetting.     

Bioturbational disturbance of the Cretaceous-Palaeogene (K-Pg) boundary layer: Implications for the interpretation of the K-Pg boundary impact event

Detailed field observations across and along the Cretaceous-Palaeogene (K-Pg) boundary interval in the Caravaca section, SE Spain, together with laboratory analyses reveal a well-developed lowermost Danian dark-colored trace fossil assemblage. The trace fossils range continuously from the bioturbated horizons in the dark boundary layer (lowermost Danian), to the uppermost Maastrichtian sediments. The rusty boundary layer at the base of the dark boundary layer, usually related to the K-Pg boundary impact, is traditionally considered as undisturbed. However, ichnological analysis at the Caravaca section shows that this rusty boundary layer is cross-cut vertically by Zoophycos and Chondrites, but also penetrated laterally by Chondrites, revealing an important colonization of the substrate. Stereomicroscope analysis shows sharp burrow margins of dark-colored Chondrites directly against the surrounding red sediment of the rusty layer. Colonization of unfavorable substrates by Zoophycos and Chondrites tracemakers, as that represented by the rusty boundary layer, was possible because of constructing of open, probably of actively ventilated burrows that facilitate colonization of sediments poor in oxygen and food. Significant bioturbational disturbance of the rusty layer can cause vertical and horizontal redistribution of the components related to the K-Pg boundary impact and, in consequence, to induce erroneous interpretations. A detailed ichnological analysis of the K-Pg boundary interval, with special attention to the rusty layer, reveals an essential tool to avoid misinterpretations.     

Computational fluid dynamics of flow regime and hydrodynamic forces generated by a gliding North Atlantic right whale (Eubalaena glacialis)

Accurate estimates of drag on marine animals are required to investigate the locomotive cost, propulsive efficiency, and the impacts of entanglement if the animal is carrying fishing gear. In this study, we performed computational fluid dynamics analysis of a 10 m (length over all) right whale to obtain baseline measurements of drag on the animal. Swimming speeds covering known right whale speed range (0.125 m/s to 8 m/s) were tested. We found a weak dependence between drag coefficient and Reynolds number. At a swimming speed of 2 m/s, we analyzed the boundary layer thicknesses, the flow regimes, and drag components. We found the thickest boundary layer at the lateral sides of the peduncle, whereas the boundary layer thickness over the outer part of the flukes was less than 1.7 cm. Laminar flow occurred over the anterior ~0.6 LoA and turbulent flow from ~0.8 LoA to the fluke notch. On the surfaces of the flukes outside of the body wake region, flow was laminar. Our most significant finding is that the drag coefficient (0.0071–0.0059) of a right whale for swimming speeds ranging from 0.25 m/s to 2 m/s is approximately twice that of many previous estimates for cetaceans.     

Effects of oxygen supply on the production of nikkomycin with immobilized cells of Streptomyces tendae

Summary For continuous production of the antibiotic nikkomycin immobilized cells have been used in a fluidized bed bioreactor. Cells of Streptomyces tendae were immobilized on sintered glass particles. Different biomass concentrations on the particles correspond to different thicknesses of mycelial layers because growth occurs only on the outer surface of the particles. The antibiotic productivity decreased with increasing layer thickness. In fermentations with higher concentrations of both biomass on the particles and dissolved oxygen levels of about 70% the productivity was also limited because of limited oxygen diffusion in the layers. Offprint requests to: H. U. Trück     

Buoyancy effect on forced convection in the leaf boundary layer

Abstract. Mixed convection (forced convection plus free convection) in the leaf boundary layer was examined by air flow visualization and by evaluation of the boundary layer conductance at different leaf-air temperature differences ( T _L- T _A) under low wind velocities. The visualized air flow was found to become more unstable and buoyant at higher T _L- T _A. An ascending longitudinal plume was induced along the upper surface, and the air flow along the lower surface ascended after passing the trailing leaf edge. The air flow modified by buoyancy was considered to result in an increase in boundary layer conductance ( G _A) for mixed convection, which became higher with higher T _L- T _A as compared with the conductance for pure forced convection without buoyancy. This increase in G _A appeared larger at larger Grashof number (Gr) and at smaller Reynolds number (Re). The dependences of buoyancy effect on Gr and Re were related to 'edge-effects'.     

Inferring nocturnal surface fluxes from vertical profiles of scalars in an Amazon pasture

Ecosystem carbon budgets depend on there being good representative surface flux observations for all land use types during the entire diurnal cycle. In calm conditions that often occur at night, especially in areas of small roughness (such as pastures), ecosystem respiration rate is poorly measured using the eddy covariance (EC) technique. Nocturnal vertical profiles of temperature, humidity and winds were observed using tethered balloon soundings in a pasture in the eastern Amazon during two campaigns in 2001. The site is characterized by very weak winds at night, so that there is insufficient turbulence for the EC technique to determine fluxes accurately. To compensate, the time evolution of the profiles is used to determine surface fluxes at early morning and these are compared with those observed by EC at a nearby micrometeorological tower. The nocturnal boundary layer thickness h is determined as the height to which the surface fluxes must converge so that energy budget closure is achieved. The estimated values range from 30 m, around 22:00 hours LST, to more than 100 m just before dawn. These are in good agreement with the observed thickness of a frequently observed fog layer during the middle of the night. During the early portion of the night, when the accumulation layer is shallow, there is appreciable decrease of dCO₂/dt with height. On calm nights, CO₂ accumulation rate is larger near the surface than at higher levels. On windier nights, this accumulation rate is vertically uniform. Hence, extrapolation of tower profiles for estimating fluxes must be done carefully. Although uncertainties remain large, an alternate approach to the EC method is described for measuring nighttime surface CO₂ fluxes under stable atmospheric conditions.     

Direct force measurements between cellulose surfaces and colloidal silica particles

The interaction of cellulose layers with colloidal silica particles was investigated by direct force measurements with the atomic force microscope (AFM). Upon approach, repulsive forces were found between the negatively charged silica particles and the cellulose surface. The forces were interpreted quantitatively in terms of electrostatic interactions due to overlap of diffuse layers originating from negatively charged carboxylic groups on the cellulose surface. The diffuse layer charge density of cellulose was estimated to be 0.80 mC/m2 at pH 9.5 and 0.21 mC/m2 at pH 4. The forces upon retraction are characterized by molecular adhesion events, whereby individual cellulose chains desorb from the probe surface. The retraction profiles are dominated by well-defined force plateaus, which correspond to single-chain desorption forces of 35-42 pN. We surmise that adsorption of cellulose to probe surfaces is dominated by nonelectrostatic forces, probably originating from hydrogen bonding. Electrostatic contributions to desorption force could be detected only at high pH, where the silica surface is highly charged.     

A simple method for determining the boundary layer resistance in leaf cuvettes

Abstract. A new method is described for determining the boundary layer resistance over wet filter paper exposed within a leaf cuvette, based on the energy balance of the filler paper. The boundary layer resistance is calculated by an iterative procedure from measurements of the relative humidity and temperature of the air in the cuvette. Comparisons between the new and the conventional method, involving measurement of the filter paper temperature, show close agreement.
To simplify the method further, a graph has been constructed for the relationship between boundary layer resistance and cuvette relative humidity at temperatures from 15 to 35°C, determined at one value of the ratio of the flow rate through the cuvette to the filter paper area.
An analysis of errors suggests that the new method is less sensitive than the conventional to errors in temperature and humidity.     

An assessment of the ship drag penalty arising from light calcareous tubeworm fouling

A test coupon coated with light calcareous tubeworm fouling was scanned, scaled and reproduced for wind-tunnel testing to determine the equivalent sand grain roughness k_s. It was found that this surface had a k_s = 0.325 mm, substantially less than the previously reported values for light calcareous fouling. This result was used to predict the drag on a fouled full scale ship. To achieve this, a modified method for predicting the total drag of a spatially developing turbulent boundary layer (TBL), such as that on the hull of a ship, is presented. The method numerically integrates the skin friction over the length of the boundary layer, assuming an analytical form for the mean velocity profile of the TBL. The velocity profile contains the roughness (fouling) information, such that the prediction requires only an input of k_s, the free-stream velocity (ship speed), the kinematic viscosity and the length of the boundary layer (the hull length). Using the equivalent sandgrain roughness height determined from experiments, a FFG-7 Oliver Perry class frigate is predicted to experience a 23% increase in total resistance at cruise, if its hull is coated in light calcareous tubeworm fouling. A similarly fouled very large crude carrier would experience a 34% increase in total resistance at cruise.     

Hard shell gas-filled contrast enhancement particles for colour Doppler ultrasound imaging of tumors

Hollow hard shell particles of 200 nm and 2 micron diameter with a 10 nm thick porous silica shell have been synthesized using polystyrene templates and a sol-gel process. The template ensures than the hollow particles are monodispersed, while the charged silica surface ensures that they remain suspended in solution for weeks. When filled with perfluorocarbon gas, the particles behave as an efficient contrast agent for colour Doppler ultrasound imaging in human breast tissue. The silica shell provides unique properties compared to conventional soft shell particles employed as ultrasound contrast agents: uniform size control, strong adsorption to tissue and cells immobilizing particles at the tissue injection site, a long imaging lifetime, and a silica surface that can be easily modified with biotargeting ligands or small molecules to adjust the surface charge and polarity.     

Nutrient diffusion and simple nth-order consumption in regenerative tissue and biocatalytic sensors

This contribution addresses intra-tissue molar density profiles for nutrients, oxygen, growth factors, and other essential ingredients that anchorage-dependent cells require for successful proliferation on biocompatible surfaces. One-dimensional transient and steady state models of the reaction-diffusion equation are solved to correct a few deficiencies in the first illustrative example of diffusion and zeroth-order rates of consumption in tissues with rectangular geometry, as discussed in Ref. [(Griffith and Swartz, 2006) 1]. The functional form of the molar density profile for each species depends on geometry and the magnitude of the species-specific intra-tissue Damk?hler number. The tissue's central core is reactant starved at high consumption rates and low rates of intra-tissue diffusion when the Damk?hler number exceeds its geometry-sensitive critical value. Ideal tissue engineering designs avoid the diffusion-limited regime such that attached cells are exposed to all of the ingredients required for proliferation everywhere within a regenerative matrix. Analytical and numerical molar density profiles that satisfy the unsteady state modified diffusion equation with pseudo-homogeneous n(th)-order rates of intra-tissue consumption (i.e., n=0,1,2) allow one to (i) predict von Kármán-Pohlhausen mass transfer boundary layer thicknesses, measured inward from the external biomaterial surface toward its central core, and, most importantly, (ii) estimate the time required to achieve steady state conditions for regenerative tissue growth and biocatalytic sensing.     

农田林网化地区近地面层廓线特征研究

大面积林网化地区近地面层大气的系留气球观测发现,近地面层大气的风、温、湿分为两层,即下边界层和扰动边界层,其厚度和廓线分布与理查逊数有关,在中性天气条件下均满足对数分布规律,边界层（特别是扰动边界层）内的摩擦速度Ｕ．＂与旷野对比点的摩擦速度Ｕ０增大１个数量级,而粗糙度Ｚ＂０增大２个数量级,甚至更大。     

PARTICULATE ORGANIC MATTER AS AN ALTERNATIVE NUTRIENT SOURCE FOR TROPICAL SARGASSUM SPECIES (FUCALES, PHAEOPHYCEAE)

Large Fucales are abundant on coastal coral reefs of the Great Barrier Reef, but are often limited by the availability of inorganic nutrients. Particle loads in these reef waters are high, which is generally perceived as detrimental for aquatic plants due to a reduction of light. Here, I provide evidence that several abundant Sargassum species supplement their nutrient supply with nutrients derived from the layer of particulate matter (PM) deposited on their thalli. In experiments involving removal or addition of PM, growth rates of Sargassum spp. were up to 180% higher when particles were present on the thalli. Tissue nitrogen and phosphorus levels of thalli with a surface PM layer were 10%–30% higher than those of thalli without PM. The amount of PM deposited in situ on thalli of five species of brown algae ranged from 0.6 to 0.9 g C_org·g⁻¹ dry weight alga, depending on the species' morphology. I suggest that a nutrient-rich diffusive boundary layer is created on the thallus surface by an epiphytic microbial community that remineralizes the particulate nutrients. When algal growth is nutrient limited, the use of particle-derived nutrients as a source alternative to nutrients in the water column may outweigh any potential adverse effects of the thallus particle layer.     

Osmotic gradient dependence of osmotic water permeability in rabbit proximal convoluted tubule

Summary To assess steady-state transepithelial osmotic water permeability (P _f ), rabbit proximal convoluted tubules were perfused in vitro with the impermeant salt, sodium isethionate at 26°C. Osmotic gradients () were established by varying the bath concentration of the impermeant solute, raffinose. When lumen osmolality was 300 mOsm and bath osmolality was 320, 360 and 400 mOsm, apparentP _f decreased from 0.5 to 0.10 to 0.08 cm/sec, respectively. Similar data were obtained when lumen osmolality was 400 mOsm. Five possible causes of the dependence of apparentP _f were considered experimentally and/or theoretically: (1) external unstirred layer (USL); (2) cytoplasmic USL; (3) change in surface area; (4) saturation of water transport; (5) down-regulation ofP _f . ApparentP _f was inhibited 83% byp-chloromercuribenzene sulfonate (pCMBS) at 20 mOsm, but not at 60 mOsm , suggesting presence of a serial barrier resistance to water transport. Increases in perfusate or bath solution flow rate and viscosity did not alter apparentP _f , ruling out an external USL. A simple cytoplasmic USL, described by a constant USL thickness and solute diffusion coefficient, could not account for the dependence of apparentP _f according to a mathematical model. The activation energy (E _a ) for apparentP _f increased from 7.0 to 12.5 kcal/mol when was increased from 20 to 60 mOsm, not consistent with a simple USL or a change in membrane surface area with transepithelial water flow. These findings are most consistent with a complex cytoplasmic USL, where the average solute diffusion coefficient and/or the area available for osmosis decrease with increasing . These results (1) indicate that trueP _f (at physiologically low ) is very high (>0.5 cm/sec) in the rabbit proximal tubule; (2) provide an explanation for the wide variation inP _f values reported in the literature using different , and (3) suggest the presence of a flow-dependent cytoplasmic barrier to water flow.     

Slow‐flow habitats as refugia for coastal calcifiers from ocean acidification

The pH of the oceans’ surface water is dropping, termed ocean acidification (OA), and the 0.4 unit reduction in pH by 2100 is projected to negatively impact benthic coastal organisms that produce calcium carbonate “skeletons.” Research has focussed on identifying species that are susceptible to OA, but there is an urgent need to discover refuge habitats that will afford protection to vulnerable species. The susceptibility of calcium carbonate skeletons to dissolution by OA depends on the pH at their surface, and this is controlled by the interaction between seawater velocity and organismal metabolism. This perspective considers how seawater velocity modifies the responses of calcifying organisms (seaweed, shellfish, and tropical corals) to OA through its action on controlling diffusion boundary layer thickness and thereby the pH and calcium carbonate saturation state (Ω) at the organisms’ surface. Evidence is presented to support the idea that slow‐flow habitats, such as wave‐sheltered bays or the within canopies of seaweed/seagrass beds, might provide inexpensive refugia from OA for vulnerable coastal calcifiers.