Thin Water Layer Research Articles

Semi-analytic solutions for freezing induced by evaporative cooling

Freezing in a one-dimensional region induced by evaporative cooling is investigated. Semi-analytic solutions in the limit of low mass transfer rates and Stefan numbers are presented. An nth order polynomial is used to express the vapor pressure function in terms of the evaporating fluid layer temperature. The freezing of a slab of calcium chloride hexahydrate subject to evaporative cooling of a thin layer of water into air is used to illustrate the utility of the model. Results show the existence of a transition region, depending upon the relative humidity and Biot number, where the fluid layer dry-out time competes with the freeze time of the slab.

Cell surface electrochemical heterogeneity of the Fe(III)-reducing bacteria Shewanella putrefaciens.

Acid-base titration experiments and electrostatic force microscopy (EFM) were used to investigate the cell surface electrochemical heterogeneity of the Fe(III)-reducing bacteria, Shewanella putrefaciens. The acid-base titrations extended from pH 4 to 10, and the titration data were fit using a linear programming pKa spectrum approach. Overall, a five-site model accounted for the observed titration behavior with the most acidic sites corresponding to carboxylic groups and phosphodiester groups, intermediate sites phosphoryl groups, and two basic sites equivalent to amine or hydroxyl groups. The pH for the point of zero charge on the bacteria was 5.4. In EFM images of cells rinsed in solutions at pH 4.0, 7.0, and 8.0, a pronounced increase in small (< or = 100 nm diameter) high contrast patches was observed on the cells with increasing pH. The pH dependence of EFM image contrast paralleled the pattern of cell surface charge development inferred from the titration experiments; however, quantitative analysis of high contrast regions in the EFM images yielded lower surface charge values than those anticipated from the titration data. For example at pH 7, the calculated surface charge of high contrast regions in EFM images of the bacterial cells was -0.23 microC/cm2 versus -20.0 microC/cm2 based on the titration curve. The differences in surface charge estimates between the EFM images and titration data are consistent not only with charge development throughout the entire volume of the bacterial cell wall (i.e., in association with functional groups that are not directly exposed at the cell surface) but also with the presence of a thin structural layer of water containing charge-compensating counterions. In combination, the pKa spectra and EFM data demonstrate that a particularly high degree of electrochemical heterogeneity exists within the cell wall and at the cell surface of S. putrefaciens.

Conductivity Measurements of Sand-Clay Mixtures: A Modified Experimental Method

We present experimental results of electrical conductivity measurements on sand-clay mixtures saturated with brines and on sand saturated with trichloroethylene (TCE). These experiments were conducted to aid in the study of detecting common groundwater contaminants and to expand the database of conductivity measurements made on unconsolidated sediments. Our experimental apparatus is based on the standard four-point electrode measurement technique utilizing removable unconsolidated sample “cores.” We can change saturating fluids and make seismic measurements in the same apparatus without disturbing the sample. Our brine data show little change in magnitude with frequency and follow Archie’s law with Archie’s cementation exponents ranging from approximately 1.5 to 2.5. Samples with higher clay content show a lower bulk conductivity. This trend may be due to both incomplete saturation and∕or the clay packing. This finding may affect the interpretation of common field methods, as it is contrary to results on consolidated shaley sandstones. Measurements made on a sand sample saturated with TCE do not follow Archie’s law. This deviation may be due to a thin conductive water layer and∕or incomplete flushing of the sample.

Metabolic activity of permafrost bacteria below the freezing point.

Metabolic activity was measured in the laboratory at temperatures between 5 and -20 degrees C on the basis of incorporation of (14)C-labeled acetate into lipids by samples of a natural population of bacteria from Siberian permafrost (permanently frozen soil). Incorporation followed a sigmoidal pattern similar to growth curves. At all temperatures, the log phase was followed, within 200 to 350 days, by a stationary phase, which was monitored until the 550th day of activity. The minimum doubling times ranged from 1 day (5 degrees C) to 20 days (-10 degrees C) to ca. 160 days (-20 degrees C). The curves reached the stationary phase at different levels, depending on the incubation temperature. We suggest that the stationary phase, which is generally considered to be reached when the availability of nutrients becomes limiting, was brought on under our conditions by the formation of diffusion barriers in the thin layers of unfrozen water known to be present in permafrost soils, the thickness of which depends on temperature.

Modelling a bottom diurnal boundary layer and its control of massive alga blooms in an estuary

Harvey Estuary, which lies in the mediterranean climate of southwest Australia, experienced massive blooms of the blue-green alga Nodularia spumigena during the 1980s. Bloom size showed great inter-annual variability which was linked to minimum winter salinity. The blooms were initiated by phosphorus released from sediments due to a thin bottom boundary layer, which formed at night after cessation of a daily seabreeze, and was depleted of oxygen by decay of detritus from the winter diatom bloom. Two models of this bottom layer are analysed. One is a molecular diffusive boundary layer, whose thickness is estimated from the Batchelor length scale for oxygen and turbulent dissipation by the very weak nocturnal tide. The other model envisages bottom stratification due to nocturnal intrusion, and convection, of a thin layer of higher salinity water. Agreement is obtained with the observed inter-annual variability of the size of the Nodularia bloom. Limited 1980s data confirm the existence of a thin anoxic, and highly stratified, bottom layer whose thickness is similar to the present predictions. The blooms seem to have been a direct result of the diurnal cycle in vertical mixing cycle and the small tidal range.

Phase transitions in vapor-deposited water under the influence of high surface electric fields

Field ionization of vapor-deposited water on a platinum field emitter tip was studied over a temperature range of 103–150 K. Water adlayers 50–3600 Å thick were grown under field-free conditions by exposure of a cryogenically cooled emitter tip to water vapor in ultrahigh vacuum. Field ionization was probed by ramped field desorption, in which desorption of ionic species (hydrated protons) is measured while increasing the applied electric field linearly in time. The dependence of the field required for ionization onset as a function of temperature and water thickness is presented and discussed. Experiments were performed for equal deposition and desorption temperatures over the range of 105 to 150 K and also for a mixed temperature case (145 K deposition, 110 K desorption) to investigate the nature of the deposited layer on ionization properties. The onset field for ionization increased linearly with adlayer thickness, but with different slopes for thin and thick layers of water. The thin layers were more sensitive to the deposition conditions of the adlayer than were the thick layers, with the difference attributed to different ionization locations in the two cases: at the exposed water–vacuum interface for thin layers and at the buried tip–water interface for thick layers. In the limit of zero adlayer thickness the onset field decreased from 0.5 to 0.2 V Å −1 as temperature increased from 105 to 150 K. An activation barrier of 0.7 eV (16 kcal mol −1) for ionization of amorphous and crystalline water was estimated from the temperature dependence of the onset field. This is in excellent agreement with the 0.74 eV (17 kcal mol −1) required to produce a pair of ions from a pair of solvated water molecules.

Surface of active polarons: A semiexplicit solvation method for biomolecular dynamics

We present a strategy for solvating biomolecules in molecular dynamics or Monte Carlo simulations. The method employs a thin layer (often monomolecular) of explicit water with additional external forces representing the electrostatics, pressure, fluctuations, and dissipations caused by the neglected bulk. Long-range electrostatic corrections are supplied through a set of variable surface charges (polarons) that recreates the mean reaction field (or dielectric properties) of an infinite solvent. We refer to this “fictitious” boundary layer as a “surface of active polarons” (or SOAP). Test simulations of the solvation free energies of 15 amino acid analogs and nine ions are in good agreement with experiment (correlation coefficients: 0.995 and 1.000, respectively) despite the use of unaltered published force-fields with only one adjustable parameter. Dynamical capabilities of SOAP are illustrated by application to a six residue peptide with a stable conformation (SYPFDV), as well as a flexible nine residue HIV-1 gp120 peptide (TLTSCNTSV from PDB 1hhg). Future extensions, calibrations, and applications are discussed briefly.

Interaction of the Third Helix of Antennapedia Homeodomain with a Deposited Phospholipid Bilayer: A Neutron Reflectivity Structural Study

Neutron reflectivity has been used to study the structure of deposited phospholipid bilayers in the gel phase and the modifications induced by the presence of a 16 amino acid peptide, the third helix of the Antennapedia homeodomain, p-Antp43-58. The phospholipids were dipalmitoyl phosphatidylcholine (DPPC) and mixtures of DPPC with 10% mol/mol of the negatively charged dipalmitoyl phosphatidylserine (DPPS). They were deposited on silicon single crystals by using the Langmuir−Schaeffer technique. By pushing the resolution of neutron reflectivity measurements, the bilayer structure was determined at 1 A precision. The thickness of the thin water layer between the substrate and the phospholipids was found to be 5 ± 1 A, that of the headgroups 9 ± 1 A and that of the chains 36 ± 1 A. The deposited bilayer had the same roughness, 5 ± 1 A, as the substrate itself. After peptide insertion, the thickness and roughness of DPPC bilayers did not change while the peptide appeared uniformly distributed in the interfac...

Protein insertion within a biological freestanding film

Free-standing films are constituted by two monolayers of surfactant molecules surrounding an internal thin water layer, reduced to the ultimate hydration water molecules in the so-called newton black films (NBF). We report in this paper, the insertion of a soluble protein within an NBF made from soluble surfactants, leading to the formation of a dense 2D monolayer of proteins. In a second step, we show that this insertion method could also be used for phospholipidic films, for which the internal structures are close to that of biological membranes

The effect of humidity on probe-sample interactions in near-field scanning optical microscopy

The shear force damping between a tapered near-field probe and a well-cleaned glass sample depends on the humidity. A slow decay region, up to few hundred Å, appeared in the onset of the damping curves as the humidity increased. This humidity-dependent interaction force is of viscous type and ∼0.1 nN. The frictional coefficient increases faster than the estimation from Newton’s law of viscosity as the probe and sample distance decreases. This is attributed to the enhancement of viscosity in thin water layer.

Bed properties of Siple Dome and adjacent ice streams, West Antarctica, inferred from radio-echo sounding measurements

AbstractWe have used ground-based radio-echo sounding (RES) profiles to reveal the spatial distribution of basal and internal ice properties across Siple Dome, West Antarctica, and under the dormant ice streams on its flanks. The RES-detected bed-reflection power, corrected for the effects of instrumentation and ice-thickness variation, is nearly constant across Siple Dome at a value suggesting spatially homogeneous basal properties of ice frozen to bedrock. Till, if present under the dome, must be thin (&lt;0.1 m). The high basal reflectivity measured under now dormant “Siple Ice Stream” (SIS) and Ice Stream C suggests that they are underlain by either a thin (&lt;0.05 m) water layer or a thick (&gt;1 m) thawed or frozen till layer. The evidence that the dormant SIS is not frozen directly to underlying bedrock (but is separated by a water or till layer) is a further indication that it was once an active ice stream, and suggests that streaming motion may have ceased before the basal layer was frozen. The absence of a thick till layer beneath Siple Dome is consistent with its apparent stability as an inter-ice-stream ridge in the past and may suggest that it will remain as a stable limitation of ice-stream width in the future.

Shock waves in basalt rock generated with high-powered lasers in a confined geometry

An investigation was made into the interaction of Q-switched Nd: glass laser pulses, with energies of 5–100 J and intensities in the range of 1–15 GW cm−2, with thin slab samples of basalt, in a geometry in which the laser–matter interaction was enhanced by confining the laser-induced plasma and vapor against the rock surface with a thin water layer. The high pressure generated during expansion of the ablated material produced a shock wave which traveled into the rock. On basalt samples of thicknesses of up to 18 mm, reflection of the shock wave at the rear surface of the sample caused a thin rock layer to be spalled. Examination of the structure within the interior of the rock after laser exposure revealed extensive grain disruption near to the laser spot, due to the shock-wave passage. Rock specimens of thickness 50 mm were tested via Vickers hardness microindenter. The laser exposure was shown to produce a hardness decrease within the rock which was up to 50%, and extended to depths of 25 mm beneath the target position. An empirical model accounting for shock-wave formation and propagation was developed and compared to the experimental results.

Drag Reduction in Heavy Oil

Transporting heavy crude oil by pipeline requires special facilities because the viscosity is so high at normal field temperatures. In some cases the oil is heated with special heaters along the way, while in others the oil may be diluted by as much as 30 percent with kerosene. Commercial drag reducers have not been found to be effective because the single-phase flow is usually laminar to only slightly turbulent. In this work we show the effective viscosity of heavy oils in pipeline flow can be reduced by a factor of 3–4. It is hypothesized that a liquid crystal microstructure can be formed so that thick oil layers slip on thin water layers in the stress field generated by pipeline flow. Experiments in a 1 1/4-in. flow loop with Kern River crude oil and a Venezuela crude oil BCF13 are consistent with this hypothesis. The effect has also been demonstrated under field conditions in a 6-in. flow loop using a mixture of North Sea and Mississippi heavy crude oils containing 10 percent brine.

The Effect of Thin Water Films on Force Microscopy Measurements

Abstract The atomic force microscope (AFM) is a powerful tool for probing surface topography, in addition to chemical and mechanical properties of surfaces. To investigate the chemical properties of a sample, force measurements are made between the AFM's “molecular finger,” or probe, and the sample. Force measurements can be made in many environmental conditions, which makes the AFM a versatile technique. However, in ambient conditions most samples are covered by a thin layer of water. When the AFM probe comes into contact with the thin water layer, a capillary bridge forms between the AFM probe and sample. Because of this water layer, it is believed that in ambient conditions a non-surface specific capillary force dominates the adhesive force between the AFM probe and sample. The non-surface specific capillary force should obscure any information about the surface. Currently, we are studying to what extent the capillary forces are non-surface specific and what influence this has on pull-off-force measurements.

Ice and water film growth from incoming supercooled droplets

A one-dimensional model for ice growth in the presence of an incoming supercooled fluid is developed by specifying a Stefan problem. A number of assumptions are made, relevant to relatively thin ice and water layers, which allow the thermal problem to be solved exactly in terms of the layer thicknesses and ambient conditions. The ice thickness is then determined by combining the mass balance with the phase change condition to reduce the problem to a single first-order ordinary differential equation, which requires a numerical solution. Quantities such as the water film thickness and temperature profiles in the ice and water can be determined easily once the ice thickness is known. Results are presented showing ice and water growth, subject to various ambient conditions. Qualitative agreement with experiment is also shown and the implications of the results to the physical process discussed.

Experiments on the relative mobility of muddy subaqueous and subaerial debris flows, and their capacity to remobilize antecedent deposits

Subaqueous debris flows share many similarities with their subaerial counterparts, but also differ in striking ways. The present treatment is devoted to an experimental comparison of paired subaqueous and subaerial debris flows. The debris slurry consisted of 39% water, 25% kaolin, 24% silt and 12% sand by weight. The debris flows ran down a rectangular channel with a length of 10 m and a width of 0.20 m. In some of the runs, the channel bottom was `hard' (i.e., a fixed, rough inerodible surface) and in others it was `soft' (i.e., an antecedent debris-flow deposit over which the debris flow was allowed to run). On the `hard' bottom, the subaqueous debris flows ran farther downslope than their subaerial counterparts with the same rheology. Their deposits also displayed extensional features not seen in the subaerial case, and were much thinner than that which would be expected based on yield strength. On the `soft' bottom, the subaerial debris flows extensively remobilized antecedent debris-flow deposits, whereas the corresponding subaqueous debris flows ran over antecedent debris-flow deposits with no detectable remobilization. The reason for these differences is hypothesized to be the incorporation of a thin layer of ambient water underneath a subaqueous debris flow as its head hydroplanes slightly above the bed. The lubricating layer appears to prevent the transmission of shear stress between the two layers.

New method to determine the true transmissibilities and permeabilities of oxygen in hydrogel membranes

In the present paper, an electrochemical method to obtain the true transmissibilities and permeabilities of hydrogels to oxygen is proposed. The method involves the measurement of the electric current arising from the flow of oxygen through a potentiostat cell integrated by a top water layer, the hydrogel membrane, and a thin water layer between the membrane and the cathode, where the oxygen is reduced. Varying the thickness of the top water layer, the true permeation characteristics of the hydrogels are obtained. A parallel study was carried out in which the transmissibilities and permeation coefficients were obtained from the variation of the steady state current with the thickness of the membranes. The permeation results obtained by the two methods agree very satisfactorily. The method also permits to determine the values of the transmissibility of the water layers of the experimental set-up.

Electric Field Effects in Ionization of Water−Ice Layers on Platinum

Field ionization of water-ice adsorbed onto a platinum field emitter tip of radius 350 A was studied as a function of temperature over the range of temperature 80-145 K and of water layer thickness 100-3000 A. The water adlayer was grown under field-free conditions by exposure to water vapor in ultrahigh vacuum. Field ionization was probed by ramped field desorption (RFD), in which desorption of ionic species (hydrated protons) is measured while increasing the applied electric field linearly in time. The dependence of the field required for onset of ionization as a function temperature and thickness is presented and discussed. In the limit of thin water layers, the onset field of ionization decreased from 0.6 to 0.3 V/A with temperature increasing from 80 to 145 K An activation barrier of 0.75 eV for ionization of water to produce hydrated protons and hydroxide ions was estimated from the temperature dependence of the onset field. The onset field increased with water layer thickness, and a break point in the slope of onset field versus thickness was interpreted as a transition in the ionization location from the water-vacuum interface to the tip-water interface. The relevance ofthese experiments in simulating electrode-electrolyte interfaces is discussed.

Application of 1H NMR spectroscopy method for determination of characteristics of thin layers of water adsorbed on the surface of dispersed and porous adsorbents

The paper presents 1H NMR spectroscopy as a perspective method of the studies of the characteristics of water boundary layers in the hydrated powders and aqueous dispergated suspensions of the adsorbents. The method involves measurements of temperature dependence proton signals intensity in the adsorbed water at temperatures lower than 273 K. Free energy of water molecules at the adsorbent/water interface is diminished due to the adsorption interactions causing the water dosed to the adsorbent surface freezes at T<273 K. Thickness of a non-freezing layer of water can be determined from the intensity of the water signal of 1H NMR during the freezing–thawing process. Due to a disturbing action of the adsorbent surface, water occurs in the quasi-liquid state. As a result, it is observed in the 1H NMR spectra as a relatively narrow signal. The signal of ice is not registered due to great differences in the transverse relaxation times of the adsorbed water and ice. The method of measuring the free surface energy of the adsorbents from the temperature dependence of the signal intensity of non-freezing water is based on the fact that the temperature of water freezing decreases by the quantity which depends on the surface energy and the distance of the adsorbed molecules from the solid surface. The water at the interface freezes when the free energies of the adsorbed water and ice are equal. To illustrate the applicability of the method under consideration the series of adsorption systems in which the absorbents used differed in the surface chemistry and porous structure. In particular, the behaviour of water on the surface of the following adsorbents is discussed: non-porous and porous silica (aerosils, silica gels); chemically and physically modified non-porous and porous silica (silanization, carbonization, biopolymer deposition); and pyrogeneous Al 2O 3 and aluminasilicas. The effect of preliminary treatment of the adsorbent (thermal, high pressure, wetting with polar and non-polar solvents) on the characteristics of the structurized water layers was discussed. The influence of the adsorbent porous structure on the free energy of the adsorbed water was also studied. The discussion of the obtained results was made.

Seasonal deepening of the pycnocline in a shallow shelf ecosystem and its influence on near-bottom dissolved oxygen

A 3 yr record (1992 to 1994) of dissolved oxygen (DO) concentrations from an intensive grid of 21 sampling stations in a ∼100 km 2 study area of western Massachusetts Bay (∼25 to 50 m water depth) showed a regular seasonal decline in bottom waters during stratification, but considerable spatial and temporal variability within and across years. Mean near-bottom, subpycnocline DO concentrations for the area reached 7.57, 7.85, and 6.2 mg l -1 in mid October 1992, late September 1993, and late September 1994, respectively; individual station readings were as low as 4.8 mg l -1 in 1994. Overall stratified-season rates of subpycnocline DO decline were ∼0.025 to 0.031 mg l -1 d -1 , but rates increased late in the season as the bottom layer sharply warmed to its annual temperature maximum. Concurrent with relatively lower DO concentrations in 1994, field measurements indicated high bottom-water temperatures >12°C (≥4°C above 1992 to 1993) and a deepened pycnocline just prior to overturn. To address how factors like temperature and vertical structure of the water column interact with metabolic processes to shape observed trends in DO decline and spatio-temporal variability, we used a simple model with physical and biological measurements from field monitoring as inputs. From field and model sensitivity results, we conclude that temperature and stratification strongly influence DO minima and rates of decline, and these factors interact with the bathymetric slope, as well as the topographic and depositional heterogeneity of the study area, to create subpycnocline variability in DO. With respect to lower DO in 1994, temperature contributed by accelerating both water and sediment metabolism, but a major effect was the late-season deepening of the pycnocline that enhanced the contribution of sediment respiration to DO decline by isolating a thin near-bottom water layer. In addition, dynamics of seasonal pycnocline deepening are a principal influence on interannual variability in bottom-water DO because, in contrast to the late-season effect, early in the stratified season a shallow pycnocline depth may moderate DO decline by allowing mid-water primary production to add DO to subpycnocline water.