Corrosion behaviour and biocompatibility of a novel Ni-free intermetallic coating growth on austenitic steel by hot dipping in an Al-12.6%Si alloy.

Commercial 316 LVM austenitic stainless steel samples have been coated by immersion in a bath of molten Al-12.6%Si alloy for 120 s. The coating consists of the Al(12)(Fe,Cr)(3)Si(2) intermetallic. In vitro corrosion behaviour has been evaluated in the Ringer's solution by means of potentiodynamic curves and electrochemical impedance spectroscopy. The results reveal that the coated specimens exhibit lower susceptibility to localised corrosion with respect to the substrate. XPS analysis suggests that the ennoblement of the pitting potential is due to the formation of a chromium oxyhydroxide containing passive layer. The intermetallic coating shows a good biocompatibility, as demonstrated by culturing human mesenchymal stem cells isolated from bone marrow which attached, grew and differentiated to the osteoblastic lineage to a similar extent on coated and bare steels. In summary, this study proposes a method that generates Ni-free coatings of the stainless steel with useful properties for biomedical applications.

Are the soft, liquid-like structures detected around bacteria by ambient dynamic atomic force microscopy capsules?

High-resolution imaging of bacterial capsules by microscopy is of paramount importance in microbiology due to their role in pathogenesis. This is, however, quite a challenging task due to their delicate nature. In this context, recent reports have claimed successful exploitation of the capacity of atomic force microscopy (AFM) for imaging of extremely deformable (even liquid) surfaces under ambient conditions to detect bacterial capsules in the form of tiny amounts of liquid-like substances around bacteria. In order to further explore this supposed capacity of AFM, in this work, three staphylococcal strains have been scrutinized for the presence of capsules using such an AFM-based approach with a phosphate buffer and water as the suspending liquids. Similar results were obtained with the three strains. AFM showed the presence of liquid-like substances identical to those attributed to bacterial capsules in the previous literature. Extensive imaging and chemical analysis point out the central role of the suspending liquid (buffer) in the formation of these substances. The phenomenon has been reproduced even by using nonliving particles, a finding that refutes the biological origin of the liquid-like substances visualized around the cells. Deliquescence of major components of biological buffers, such as K(2)HPO(4), CaCl(2), or HEPES, is proposed as the fundamental mechanism of the formation of these ultrasmall liquid-like structures. Such an origin could explain the high similarity of our results obtained with three very different strains and also the high similarity of these results to others reported in the literature based on other bacteria and suspending liquids. Finally, possible biological/biomedical implications of the presence of these ultrasmall amounts of liquids wrapping microorganisms are discussed.

Ionic liquid microdroplets as versatile lithographic molds for sculpting curved topographies on soft materials surfaces.

Soft lithography comprises a set of approaches for shaping the surface of soft materials such as PDMS on the microscopic scales. These procedures usually begin with the development of templates/masters normally generated by electron or photolithography techniques. However, the richness in available shapes is limited, usually producing shapes containing sharp parts. Innovation is called for to develop reliable approaches capable of imparting well-defined 3D curved shapes to these solids, a topology that is somehow unnatural for solid surfaces. Here we report on the use of tiny drops of room-temperature ionic liquid, organic liquids that have attracted increasing amounts of attention in recent years because of their unique chemical properties) as a versatile platform for imprinting PDMS with tunable 3D curved geometry, which is out of reach of conventional lithographic techniques and ranges from almost flat depressions to almost closed cavities on the millimeter to micrometer scale. The concept exploits a peculiar combination of physical properties displayed by ionic liquids as their null volatility and their polarity, together with some unique properties of liquid surfaces as their virtually null surface roughness. Proof-of-concept experiments show their application as chemical microreactors and ultrasmooth optical lenses. This all-liquid method is simple, low-cost, versatile, maskless, tension-free, and easily scalable, so we envision a community-wide application in numerous modern physical, chemical, biological, and engineering settings.

An experimental study about the imbibition of aqueous solutions of low concentration of a non-adsorbable surfactant in a hydrophilic porous medium.

The imbibition of aqueous solutions of Triton X-100 in calcium fluoride columns has been studied in order to determine the influence of the interfacial adsorption of the surfactant in the capillary rise of the solutions. This system has been chosen because this surfactant behaves as non-adsorbable at the surface of this solid when it is in aqueous solution. The experiments have consisted of the measurement of the increase in the weight of the porous columns caused by the capillary rise of the solutions. The analysis of the results has been made through a modified expression of Washburn's equation that takes into account that the experimental increase in the weight is caused by the imbibition as well as by the development of a liquid meniscus around the bottom base of the columns. From this analysis, it has been deduced that the surfactant concentration does not influence on the imbibition rate, it being equal to the observed for water. However, it has been also proved that the contact angle depends on the surfactant concentration, taking decreasing values as the surface tension of the solutions decreases. In order to justify these findings, a study about the influence of the interfacial adsorption on the imbibition has been carried out. By means of them, it has been proved that the absence of adsorption at the solid-liquid interface is the reason that explains both the independence of the imbibition rate from the surfactant concentration and the decrease of the contact angle. Moreover, this fact indicates that the depletion of the surfactant molecules from the advancing meniscus, which has been normally adduced as the phenomenon causing the observed behaviour, has to be ruled out as the physical cause that justifies the behaviour found from the analysis of the imbibition experiments. As a corollary, it has been also stated that only if the adsorption at the solid interfaces happened, the imbibition of aqueous solution of surfactant in hydrophilic media could be influenced by the surfactant concentration.

Influence of the interfacial adsorptions on the imbibition of aqueous solutions of low concentration of the non-ionic surfactant Triton X-100 into calcium fluoride porous medium.

The imbibition of aqueous solution of Triton X-100 in porous columns of calcium fluoride has been carried out in order to investigate the surfactant influence on the capillary rise. Experiments have consisted of the measurement of the increase in the weight of the porous columns caused by the imbibition of the solutions. From their analysis, it has been found that the capillary rise velocity does not depend on the surfactant concentration, and that imbibition of these solutions behaves as in the case of water. This unexpected finding can not be attributed to depletion of the surfactant molecules from the advancing meniscus, since this effect can not be caused by the adsorption at the solid. This lack of surfactant adsorption on the solid is precisely the reason that justifies the observed behaviour. So, it has been proved that the driving force for the capillary rise movement will remain unaltered despite the surfactant addition whenever the free energies of the solid interfaces are not modified by the adsorption on the solid. Therefore, it is concluded from this study that only if adsorption on the solid happened, the imbibition could be influenced by the surfactant concentration.

Ionic surfactant adsorption onto activated carbons.

The adsorption of sodium dodecyl sulfate onto a set of activated carbons from aqueous solutions has been studied in the low concentration range. The adsorption isotherms are reasonably well fitted by a double Langmuir equation but the calorimetry of adsorption enthalpies shows a rather wide distribution of energies. This distribution is related to direct adsorbate-adsorbent interactions in pores of different size, without noticeable contributions from the chemical nature of the surface. The adsorbate-adsorbent interaction free energy through water is evaluated using the model proposed by van Oss and co-workers for the interfacial free energy. The obtained results indicate that the calculated free energy is in good agreement with that found from application of the double Langmuir equation to the adsorption isotherms.

On the evaluation of the surface free energy of porous and powdered solids from imbibition experiments: equivalence between height-time and weight-time techniques.

A study of the equivalence between the height-time and weight-time experimental techniques, based on the imbibition of liquids into porous or powdered solids, has been carried out, determining under what conditions the deductions about the surface free energy of these solids made from the analysis of the experimental results obtained as weight versus time are similar to those deduced from the analysis of height versus time. Concretely, we have proved that only if the particular values of the porosity of the solid determined from the proper technique of imbibition, named the effective porosity, are taken into account, the analysis of both type of experimental measurements leads to the same conclusions about the surface free energy of the porous and powdered solids. In order to exemplify this, capillary rise measurements have been carried out by means of these two techniques with different liquids on silica gel layers.

Free energy of interaction of sodium dodecyl sulfate in aqueous solution with carbon black surfaces.

The free energy of the adsorption process of an ionic surfactant from aqueous solutions onto a set of carbon blacks in the range of low concentrations was evaluated using the model proposed by van Oss and co-workers. The obtained results indicated that the free energy of interaction between adsorbent and adsorbate through water results mainly from Lifshitz-van der Waals and electrostatic interactions, and its value showed a good correspondence with that previously found from a combination of the classical measurements of adsorption isotherms and the Langmuir model.

Washburn's equation facing Galileo's transformation: some remarks.

Capillary rise is the basis of some methods employed for the determination of contact angles in porous materials. One of them consists of the measuring of the distance the liquid advances into the pores of the solid. Normally, the results are analyzed by a rectilinear fitting to Washburn's equation. However, if it is taken into account that the distance-time measurements can be made under numerous reference systems and that, therefore, Galileo's transformations must be considered, it can be proved that the analysis of the results has to be carried out by parabolic expression of Washburn's equation. Even more, this work demonstrates theoretically that the traditional expression of Washburn's equation is valid only to describe the distance-time measurements got from that place where the initial contact between the porous solid and the liquid is established.

Analysis of the Silica Surface Free Energy by the Imbibition Technique.

The surface free energy of silica and its components have been evaluated from imbibition experiments performed with liquids of differing surface properties by the distance-time method. Data were analyzed by a parabolic fit to Washburn's equation, because of the uncertainty in the exact position and time at which penetration begins in these kinds of experiments. In addition to the mathematical treatment of the experimental results, the influence of the components and parameters of the surface tension of the liquids used on the values of the solid surface free-energy components has been analyzed. Copyright 2001 Academic Press.

Influence of the Meniscus at the Bottom of the Solid Plate on Imbibition Experiments.

The analysis of the experiments on liquid imbibition into porous solids carried out by using the increase of weight technique proves the influence that meniscus formation has on the experimental increase of weight when the bottom of the porous layer is put in contact with the free surface of the liquid used to perform the penetration experiments. This process (meniscus onset) has a temporary dependence due to the change of the distance between the inferior base of the plate and the free surface of the liquid as a result of the imbibition. We have also found the proper experimental conditions under which such temporary dependence can be minimized after a short time after the start of contact between the porous layer and the free surface of the liquid utilized in the penetration procedure. Thus, the weight increase because of the meniscus formation can be assumed as a constant during the greatest part of the experiments, allowing the use of the proper mathematical form of Washburn's equation in order to describe the imbibition of the liquid into the pores of the solid. Copyright 2001 Academic Press.

Comparison of the Use of Washburn's Equation in the Distance-Time and Weight-Time Imbibition Techniques.

Two experimental methods are usually employed to study liquid penetration in porous media. One of them is based on the measure of the height of the advance liquid front vs time, and the other one is based on the measure of the weight gained by the porous system due to the liquid penetration vs time. Generally, the experimental data obtained from these techniques are analyzed through Washburn's equation. However, depending on which of them is selected, different conditions, coming from the experimental method, are needed to be taken into account in order to get the correct application of Washburn's equation to the experimental data. Although these conditions are different for each method, we prove in this paper that only if these conditions are considered both techniques are equivalent to analyze imbibition experiments using Washburn's equation. Copyright 2001 Academic Press.

On the Use of Washburn's Equation in the Analysis of Weight-Time Measurements Obtained from Imbibition Experiments.

A careful analysis of the experimental results from imbibition experiments, expressed as the increased weight of a porous layer once it was in contact with a liquid, vs time, has been done on the basis of Washburn's equation, which is usually the main tool for that analysis. It has been found that the experimental results reflect other physical phenomena more than imbibition, among them the effect of the initial contact between plate and liquid and the evaporation of liquid from the wetted porous layer. The unavoidable disturbance introduced by the first of these phenomena leads to the need for rescaling of the experimental data. Once this is done, Washburn's equation has to be applied accordingly to the new reference system chosen. Also, it has been shown that if evaporation is present, experimental data deviate from Washburn's predictions. This effect disappears if the porous layer is covered. Copyright 1999 Academic Press.

Distance-Time Measurements in Capillary Penetration: Choice of the Coordinate System.

Washburn's equation is a useful tool for studying the penetration of liquids in porous media because it predicts a relationship between the distance penetrated by the liquid in the porous media, x, and the time spent in this process, t. Despite its extended applicability, some problems in the interpretation of the results obtained arise from its use in the linearized form of x2 vs t. In this paper a careful mathematical analysis of Washburn's equation shows that most of the discordance found from its application comes from inadequate use of Washburn's equation. To confirm this assessment, the penetration of different liquids in plates used in thin-layer chromatography was investigated. From that study it has been proved that the alien behavior appearing when Washburn's equation is used in a linearized form disappears when it is used in its polynomial expression. Copyright 1999 Academic Press.