Experimental examination of the phase transition of water on silica at 298 K.

The objective of this study was to investigate the prediction of the wetting characteristics obtained from the equilibrium adsorption analysis using the Zeta adsorption isotherm approach with an experimental study. Water vapor's adsorption and wetting characteristics on a hydroxylated and nano-polished silica substrate were studied in near-equilibrium conditions at temperatures near 298 K. Using a UV-visible interferometer, water vapor adsorbate film thicknesses were measured and converted into amount adsorbed per unit area. The current results show that the wetting transition occurred at an average subcooling value of 0.39 K, less than the predicted value of 0.49 K. All the different experimental observations showed growth of film thickness as a function of subcooling value with a maximum film thickness of 12.6 nm. The analysis of the results further showed that the maximum stable film was in a metastable state that then condensed in a dropwise manner, if perturbed by increasing the subcooling. The study further revealed that the adsorbate is unstable after transitioning. The solid surface energy calculated by including the near-equilibrium observations was comparable and close to that of the equilibrium studies, thus supporting solid surface energy as a material property.

Initiation of wetting, filmwise condensation and condensate drainage from a surface in a gravity field.

The zeta adsorption isotherm is based on the hypothesis that a vapour adsorbed on a solid surface consists of a collection of molecular clusters. We use this isotherm to propose a method for determining the wetting condition on a vertically oriented silicon surface exposed to heptane in a gravity field. Measurements indicate the amount adsorbed is larger at positions of smaller potential energy. The wetting condition is taken to be reached when the adsorbed vapour is transformed into the adsorbed liquid phase: adsorption lowers the surface tension of Si from the value in the absence of adsorption to that of liquid heptane at wetting, and then as the Si-heptane is cooled further it is reduced to zero, at a subcooling of 3.7 K. The expectation is that when this subcooling is reached, gravity would cause the larger molecular clusters to drain down the surface. This prediction is supported by experimental observations.

Vapour adsorption kinetics: statistical rate theory and zeta adsorption isotherm approach.

The equilibrium zeta adsorption isotherm for vapours indicates the amount adsorbed is finite for vapour-phase pressures approaching the saturation value, and is strongly supported by experimental measurements for a number of different vapour-solid surface systems. This isotherm assumes the adsorbate consists of differently sized molecular clusters in local equilibrium rather than the adsorbate being in layers. We use the local-equilibrium approximation and develop a method to determine the expression for chemical potential of the adsorbate in terms of the amount adsorbed, nA(t). This allows us to apply statistical rate theory to calculate nA(t) at five different vapour-phase pressures, xV (≡PV/Psat), in terms of a parameter, re. Statistical rate theory indicates that re describes the dynamics of a given isolated system under equilibrium conditions. We consider two methods for determining the value of re that give the best agreement with the measurements performed at each of five values of xV. In one method, we assume, re, is a function of both temperature, T, and pressure, xV, and determine the five best-fit values of re. In the experiments, xV changes by a factor of more than two, but the standard deviation in the re values is 6%. In the second method, we assume re is a function only of T; and find that the value of re is not changed significantly. In all cases, the calculated nA(t) agree with the measurements.

From adsorption to condensation: the role of adsorbed molecular clusters.

The adsorption of heptane vapour on a smooth silicon substrate with a lower temperature than the vapour is examined analytically and experimentally. An expression for the amount adsorbed under steady state conditions is derived from the molecular cluster model of the adsorbate that is similar to the one used to derive the equilibrium Zeta adsorption isotherm. The amount adsorbed in each of a series of steady experiments is measured using a UV-vis interferometer, and gives strong support to the amount predicted to be adsorbed. The cluster distribution is used to predict the subcooling temperature required for the adsorbed vapour to make a disorder-order phase transition to become an adsorbed liquid, and the subcooling temperature is found to be 2.7 ± 0.4 K. The continuum approach for predicting the thickness of the adsorbed liquid film originally developed by Nusselt is compared with that measured and is found to over-predict the thickness by three-orders of magnitude.

Nucleation and growth of condensate in nanoporous materials.

We consider the adsorption-desorption cycles of water and of three hydrocarbons on MCM-41 and on SBA-15. We show that during the desorption portion of a cycle, when the condensate is still at the mouth of the pores, in equilibrium, and the pressure, P, is the minimum value reached before pore-emptying begins, the contact angle is zero. This value of the contact angle is used with the Kelvin equation to calculate the pore radius of each of the mesoporous silicas considered. The standard deviations in the values are found to differ by only a few percent. We propose a method for predicting the size of adsorbed-molecular clusters that must be formed in the pores to initiate condensate formation there. Once formed, the condensate grows spontaneously to the pore mouth. If the vapour-phase pressure when this condition is reached is also P, the adsorption-desorption cycle is reversible. Three of the eight systems considered meet this condition and their adsorption-desorption cycles are experimentally reversible.

Prediction of the wetting condition from the Zeta adsorption isotherm.

We use the Zeta adsorption isotherm and propose a method for determining the conditions at which an adsorbed vapour becomes an adsorbed liquid. This isotherm does not have a singularity when vapour phase pressure, P(V), is equal to the saturation-vapour pressure, Ps, and is empirically supported by earlier studies for P(V) < Ps. We illustrate the method using water and three hydrocarbon vapours adsorbing on silica. When the Zeta isotherm is combined with Gibbsian thermodynamics, an expression for γ(SV), the surface tension of the solid-vapour interface as a function of x(V)(≡P(V)/Ps) is obtained, and it is predicted that adsorption lowers γ(SV) from the surface tension of the substrate in the absence of adsorption, γ(S0), to that at the wetting condition. The wetting hypothesis indicates that γ(SV) at wetting, x, is equal γ(LV), the surface tension of the liquid-vapour interface. For water vapour adsorbing on silica, adsorption lowers γ(SV) to γ(LV) at xVW equal unity, but for the hydrocarbons heptane, octane and toluene adsorbing on silica xVW is found to be 1.40, 1.30 and 1.32 respectively.

Energy transport by thermocapillary convection during Sessile-Water-droplet evaporation.

The energy transport mechanisms of a sessile-water droplet evaporating steadily while maintained on a Cu substrate are compared. Buoyancy-driven convection is eliminated, but thermal conduction and thermocapillary convection are active. The dominant mode varies along the interface. Although neglected in previous studies, near the three-phase line, thermocapillary convection is by far the larger mode of energy transport, and this is the region where most of the droplet evaporation occurs.

alpha(4)-integrin mediates neutrophil-induced free radical injury to cardiac myocytes.

Previous work has demonstrated that circulating neutrophils (polymorphonuclear leukocytes [PMNs]) adhere to cardiac myocytes via beta(2)-integrins and cause cellular injury via the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase enzyme system. Since PMNs induced to leave the vasculature (emigrated PMNs) express the alpha(4)-integrin, we asked whether (a) these PMNs also induce myocyte injury via NADPH oxidase; (b) beta(2)-integrins (CD18) still signal oxidant production, or if this process is now coupled to the alpha(4)-integrin; and (c) dysfunction is superoxide dependent within the myocyte or at the myocyte-PMN interface. Emigrated PMNs exposed to cardiac myocytes quickly induced significant changes in myocyte function. Myocyte shortening was decreased by 30-50% and rates of contraction and relaxation were reduced by 30% within the first 10 min. Both alpha(4)-integrin antibody (Ab)-treated PMNs and NADPH oxidase-deficient PMNs were unable to reduce myocyte shortening. An increased level of oxidative stress was detected in myocytes within 5 min of PMN adhesion. Addition of an anti-alpha(4)-integrin Ab, but not an anti-CD18 Ab, prevented oxidant production, suggesting that in emigrated PMNs the NADPH oxidase system is uncoupled from CD18 and can be activated via the alpha(4)-integrin. Addition of exogenous superoxide dismutase (SOD) inhibited all parameters of dysfunction measured, whereas overexpression of intracellular SOD within the myocytes did not inhibit the oxidative stress or the myocyte dysfunction caused by the emigrated PMNs. These findings demonstrate that profound molecular changes occur within PMNs as they emigrate, such that CD18 and associated intracellular signaling pathways leading to oxidant production are uncoupled and newly expressed alpha(4)-integrin functions as the ligand that signals oxidant production. The results also provide pathological relevance as the emigrated PMNs have the capacity to injure cardiac myocytes through the alpha(4)-integrin-coupled NADPH oxidase pathway that can be inhibited by extracellular, but not intracellular SOD.

Surface tension of solids in the absence of adsorption.

A method has been recently proposed for determining the value of the surface tension of a solid in the absence of adsorption, gammaS0, using material properties determined from vapor adsorption experiments. If valid, the value obtained for gammaS0 must be independent of the vapor used. We apply the proposed method to determine the value of gammaS0 for four solids using at least two vapors for each solid and find results that support the proposed method for determining gammaS0.

Statistical rate theory determination of water properties below the triple point.

We report a new method for determining the saturation vapor pressure, Ps(T), of water at conditions below its triple point. Ps(T) appears as a parameter in the statistical rate theory (SRT) expression for the net evaporation flux. We use measurements of the interfacial conditions during steady-state evaporation and condensation experiments and SRT to determine the values of Ps(T) from 50 different experiments over a range of interfacial conditions. From these values of Ps(T), we develop an analytical expression and from it calculate the liquid-vapor latent heat, Llv(T), and the constant pressure specific heat, cp(L)(T). The calculated values of these properties are compared with those obtained from independent measurements. This comparison indicates the SRT expressions for Llv(T) and cp(L)(T) are consistent with the measurements over a range of temperatures.

Measurement of the adsorption at solid-liquid interfaces from the pressure dependence of contact angles.

Earlier studies have indicated that in an isothermal three-phase system, the liquid-phase pressure at the three-phase line, xL3, may be viewed as the independent variable of the contact angle, theta, and that adsorption at the solid-liquid interface is the mechanism relating them. When the liquid-vapor interface is axi-symmetric, we show that theta can be predicted as a function of xL3 and that by measuring theta(xL3), the amount adsorbed at the solid-liquid interface can be determined. We consider water in differently sized borosilicate glass cylinders. For progressively larger cylinders, xL3 increases with cylinder radius, but when a particularly sized cylinder is rotated about it longitudinal axis, xL3 is decreased. The observed value of theta in each case is found to be in close agreement with that predicted. A Gibbs model of the interphase is used, and the Gibbs adsorption at the solid-liquid interface is found to be negative. As xL3 increases above its value at wetting, the amount adsorbed at the solid-liquid interface becomes progressively more negative. Negative adsorption is shown to mean that the concentration of the fluid component is greater in the bulk liquid than in the interphase and that the difference in concentration increases as xL3 is increased. The data is used to investigate the hypothesis that the curvature of the three-phase line affects theta through line tension, but we find no relation between line tension and theta. There is an apparent relation between the curvature of the liquid-vapor interface, CLV and theta, but this is shown to be because CLV affects xL3.

Role of molecular phonons and interfacial-temperature discontinuities in water evaporation.

During steady-state water evaporation, when the vapor phase is heated electrically, the temperature on the vapor side of the interface has been reported to be as much as 27.83 degrees C greater than that on the liquid side. The reported interfacial temperatures were measured with thermocouple beads that were less than 50 microm in diameter and centered 35 microm from the interface in each phase. We examine the reliability of these measurements by using them with a theory of kinetics to predict the interfacial-liquid temperature. The predicted temperature discontinuities are found to be in agreement with those measured up to a temperature discontinuity of 15.69 degrees C , but larger discontinuities cannot be confirmed because of uncertainties in the vapor-phase pressure measurements. The theory of kinetics used in the analysis includes molecular phonons in the expression for the evaporation flux. We show it is essential to include these terms if the theory is to be used to predict the temperature discontinuities.

Actions of emigrated neutrophils on Na(+) and K(+) currents in rat ventricular myocytes.

Interactions between neutrophils and the ventricular myocardium can contribute to tissue injury, contractile dysfunction and generation of arrhythmias in acute cardiac inflammation. Many of the molecular events responsible for neutrophil adhesion to ventricular myocytes are well defined; in contrast, the resulting electrophysiological effects and changes in excitation-contraction coupling have not been studied in detail. In the present experiments, rat ventricular myocytes were superfused with either circulating or emigrated neutrophils and whole-cell currents and action potential waveforms were recorded using the nystatin-perforated patch method. Almost immediately after adhering to ventricular myocytes, emigrated neutrophils caused a depolarization of the resting membrane potential and a marked prolongation of myocyte action potential. Voltage clamp experiments demonstrated that following neutrophil adhesion, there was (i) a slowing of the inactivation of a TTX-sensitive Na(+) current, and (ii) a decrease in an inwardly rectifying K(+) current. One cytotoxic effect of neutrophils appears to be initiated by enhanced Na(+) entry into the myocytes. Thus, manoeuvres that precluded activation of Na(+) channels, for example holding the membrane potential at -80 mV, significantly increased the time to cell death or prevented contracture entirely. A mathematical model for the action potential of rat ventricular myocytes has been modified and then utilized to integrate these findings. These simulations demonstrate the marked effects of (50-fold) slowing of the inactivation of 2-4% of the available Na(+) channels on action potential duration and the corresponding intracellular Ca(2+) transient. In ongoing studies using this combination of approaches, are providing significant new insights into some of the fundamental processes that modulate myocyte damage in acute inflammation.

Effect of adsorption on the surface tensions of solid-fluid interfaces.

A method is proposed for determining the surface tensions of a solid in contact with either a liquid or a vapor. Only an equilibrium adsorption isotherm at the solid-vapor interface needs to be added to Gibbsian thermodynamics to obtain the expressions for the solid-vapor and the solid-liquid surface tensions, gamma[1](SV) and gamma[1](SL), respectively. An equilibrium adsorption isotherm relation is formulated that has the essential property of not predicting an infinite amount adsorbed when the pressure is equal to the saturation-vapor pressure. Five different solid-vapor systems from the literature are examined, and found to be well described by the new isotherm relation. The surface-tension expressions obtained from the isotherm relation are examined by determining the surface tension of the solid in the absence of adsorption, gamma[1](S0), a material property of a solid surface. The value of gamma[1](S0) can be determined by adsorbing different vapors on the same solid, determining the isotherm parameters in each case, and then from the expression for gamma[1](SV) taking the limit of the pressure vanishing to determine gamma[1](S0). From previously reported measurements of benzene and of n-hexane adsorbing on graphitized carbon, the same value of gamma[1](S0) is obtained.

Pressure dependence of the contact angle.

When a liquid and its vapor contact a smooth, homogeneous surface, Gibbsian thermodynamics indicates that the contact angle depends on the pressure at the three-phase line of an isothermal system. When a recently proposed adsorption isotherm for a solid-vapor interface is combined with the equilibrium conditions and the system is assumed to be in a cylinder where the liquid-vapor interface can be approximated as spherical, the contact-angle-pressure relation can be made explicit. It indicates that a range of contact angles can be observed on a smooth homogeneous surface by changing the pressure at the three-phase line, but it also indicates that the adsorption at the solid-liquid interface is negative, and leads to the prediction that the contact angle increases with pressure. The predicted dependence of the contact angle on pressure is investigated experimentally in a system that has an independent mechanism for determining when thermodynamic equilibrium is reached. The predictions are in agreement with the measurements. The results provide a possible explanation for contact angle hysteresis.

Primary DNA sequence determines sites of maintenance and de novo methylation by mammalian DNA methyltransferases.

Analysis of the enzymatic methylation of oligodeoxynucleotides containing multiple C-G groups showed that hemimethylated sites in duplex oligomers are not significantly methylated by human or murine DNA methyltransferase unless those sites are capable of being methylated de novo in the single- or double-stranded oligomers. Thus, the primary sequence of the target strand, rather than the methylation pattern of the complementary strand, determines maintenance methylation. This suggests that de novo and maintenance methylation are the same process catalyzed by the same enzyme. In addition, the study revealed that complementary strands of oligodeoxynucleotides are methylated at different rates and in different patterns. Both primary DNA sequence and the spacing between C-G groups seem important since in one case studied, maximal methylation required a specific spacing of 13 to 17 nucleotides between C-G pairs.

Surface thermal capacity and its effects on the boundary conditions at fluid-fluid interfaces.

We have formulated a generalization of the energy boundary condition for fluid-fluid interfaces that includes the transport of the Gibbs excess internal energy. A newly measured surface property - the surface thermal capacity c(sigma) - appears in the result, and couples the temperature and velocity fields. If this term is not included in the energy boundary condition at liquid-vapor interfaces, the energy-conservation principle cannot be satisfied during steady-state evaporation of H(2)O(l) or D(2)O(l) . The c(sigma) term is possibly important in a number of other circumstances, and its importance can be determined from the magnitude of two nondimensional numbers.

Surface excess properties from energy transport measurements during water evaporation.

When water evaporates at high rates, recent studies indicate thermal conduction to the interface does not provide enough energy to evaporate water at the observed rate and that it is perhaps thermocapillary convection that transports the remaining energy. This possibility is examined by applying the Gibbs dividing-surface approximation to develop an expression for the energy transported along the interface. When this energy transport rate is compared with that required to evaporate the liquid at the observed rate, it is found that a Gibbs excess property, the "surface-thermal capacity," can be evaluated. A series of 19 evaporation experiments has been conducted under conditions for which there was no buoyancy-driven convection and for which the evaporation rate was progressively increased. For Marangoni numbers, (Ma) less than approximately 100, the interface was quiescent and thermal conduction (the Stefan condition) correctly predicted the energy transport rate to the surface. For experiments with 10022,000, the interfacial flow was turbulent and viscous dissipation became important.

Surface-thermal capacity of from measurements made during steady-state evaporation.

When D2O(l) evaporates into its vapor under steady-state conditions with the temperature field in the liquid arranged so that there is no buoyancy-driven convection and the Marangoni number is less than approximately 100, it is found that the interface is quiescent and thermal conduction to the interface supplies energy at a sufficient rate to evaporate the liquid. However, if the evaporation rate is raised so that the Marangoni number goes above approximately 100, the interface is transformed: a fluctuating thermocapillary flow occurs, and thermal conduction no longer supplies energy at a sufficient rate to evaporate the liquid. An energy analysis indicates conservation of energy can be satisfied only if thermocapillary convection is taken into account, and the surface-thermal capacity csigma is assigned a value of 32.5+/-0.8 kJ/(m2 K) when the temperature is in the range -10 degrees C< or =TLV< or =3.7 degrees C. This value is consistent with that found previously for H2O, and application of the Gibbs model gives a qualitative explanation for the value. Once the value of the surface-thermal capacity is known, the local heat flux along the interface can be calculated and statistical rate theory can be used to predict the local vapor-phase pressure on the interface. Since this theory introduces no adjustable parameters, the predicted pressure can be compared directly with that measured: this comparison indicates the mean of the pressures predicted to exist on the interface is in close agreement with those measured approximately 20 cm above the interface, and the small pressure gradient along the interface is consistent with the thermocapillary convection predicted from the interfacial temperature gradient.