Ana serro | Universidade de Lisboa (original) (raw)

Papers by Ana serro

Applied Surface Science, 2011

In the present work we investigate the ageing of acid cleaned femtosecond laser textured 1 0 0 si... more In the present work we investigate the ageing of acid cleaned femtosecond laser textured 1 0 0 silicon surfaces. Changes in the surface structure and chemistry were analysed by Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), in order to explain the variation with time of the water contact angles of the laser textured surfaces. It is shown that highly hydrophobic silicon surfaces are obtained immediately after laser texturing and cleaning with acid solutions (water contact angle > 120 • ). However these surfaces are not stable and ageing leads to a decrease of the water contact angle which reaches a value of 80 • . XPS analysis of the surfaces shows that the growth of the native oxide layer is most probably responsible for this behavior.

Fluid Phase Equilibria, 2006

Our laboratory, along with several others, has been engaged in a project to determine the suitabi... more Our laboratory, along with several others, has been engaged in a project to determine the suitability of diisodecylphthalate (DIDP) as a standard reference material of viscosity for industrial purposes. As a part of that project, we have undertaken a study of the effects of ...

Tribology Letters, 2007

An energetic approach to model the wear of tribological systems in which one of the components of... more An energetic approach to model the wear of tribological systems in which one of the components of the pair is polymeric is presented in this work. Experimental data, obtained in ultra-high molecular weight polyethylene (UHMWPE) pin-on-disk tribological tests, showed that a linear correlation between the wear rate of the polymer and the dissipated energy exists, independently of the lubricant, of the material used as counterbody and of the surface finishing of both polymer and counterbody. This fact strongly suggests that, in UHMWPE-based tribological systems, energy dissipation is mainly caused by the elasto-plastic deformation and wear of the polymer. Based on this assumption, it is developed a mathematical model that yields for a physical interpretation of the parameters of the experimental wear vs. energy correlation. These parameters are intrinsic wear properties of the polymer and can be used for the optimization of polymer-based tribological systems.

Wear, 2007

The tribological behaviour of the prosthetic pair TiN coated stainless steel/ultra high molecular... more The tribological behaviour of the prosthetic pair TiN coated stainless steel/ultra high molecular weight polyethylene (UHMWPE) may be improved by chlorine-implantation of the TiN surface. Friction and wear were determined using a pin-on-disk apparatus and the wear mechanisms were investigated through scanning electron microscopy (SEM) and atomic force microscopy (AFM). Rutherford backscattering spectrometry (RBS) was used to determine the chlorine distribution profiles in the chlorine-implanted TiN coatings before and after the tribological experiments, while X-ray photoelectron spectroscopy (XPS) was used to characterize chemically the same samples. Chlorine-implantation led to a significant polymeric wear reduction when the lubricant was Hanks' balanced salt solution (HBSS). If bovine serum albumin (BSA) was added to HBSS, a strong decrease of both friction and polymeric wear was observed for implanted and non-implanted TiN coatings. The former case was explained by the formation of a titanium oxide layer on the TiN surface, while the latter derived from albumin adsorption.

Fluid Phase Equilibria, 2010

The surface tension and density of almost all members of the 1-alkyl-3-methylimidazolium bis(trif... more The surface tension and density of almost all members of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides family, [C n mim][Ntf 2 ], with 2 ≤ n ≤ 14 were measured for the first time up to 532 K and 473 K, respectively. The large number of ionic liquids studied within this family and the broad interval of working temperature enabled us to show evidence for a thermodynamic anomaly in the thermal expansivity of these ionic liquids (minima in˛p versus T). The minima themselves have never been witnessed before. The data in this extended temperature range also permitted us to discuss the location of hypothetical normal boiling and critical points.

Supported layers of vesicles of dimyristoyl and dipalmitoyl phosphatidylcholine containing choles... more Supported layers of vesicles of dimyristoyl and dipalmitoyl phosphatidylcholine containing cholesterol are adequate models for eukaryotic plasma membranes. In this work it was investigated the adsorption of unilamellar vesicles on the surface of oxidized gold, using a QCM-D, AFM and LSCFM. The results of all techniques indicate that a dense layer of intact liposomes irreversibly adsorbs on the gold surface. It was also investigated the interaction of some examples of local and general anaesthetics with these membrane models, using a QCM-D. Lidocaine, tetracaine, ropivacaine and levobupivacaine were chosen as local anaesthetics examples, and propofol as general anaesthetic. Simultaneously, the interaction of the anaesthetics with the same liposomes in suspension was studied, using DSC. The results showed that all the studied anaesthetics interacted with the used models, causing changes such as raised fluidity and depressed phase transition temperature.

Surface and Interface Analysis, 2008

The importance of protein adsorption on biomaterials is widely recognized, but the dependence of ... more The importance of protein adsorption on biomaterials is widely recognized, but the dependence of the adsorption results on the chosen technique has not been much addressed. The objective of this work is to compare adsorption data obtained using several techniques under experimental conditions as closely as possible. Two case studies were investigated: adsorption of bovine serum albumin (BSA) onto 316L stainless steel (SS) and onto alumina. Both materials were used as powders and plates, whose characterization was done through zeta potential (ZP) measurements. The experimental techniques were depletion, protein radiolabeling, quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). The adsorption isotherms obtained with depletion and QCM-D techniques, although quantitatively different, present some similarities in shape. Both techniques suggest the existence of a compact end-on monolayer of protein on the SS surface, while on the alumina surface a less dense side-on monolayer is formed at lower BSA concentration, followed by a second layer at higher concentration. AFM topographical characterization of the protein films adsorbed on both materials confirms those findings. Further use of AFM in determining the thickness of the film adsorbed on SS yielded values in good agreement with the QDM-D results. Different surface charges measured on powders and plates do not seem to affect adsorption. Protein radiolabeling seems to be the least reliable technique because it yields, for both materials, adsorption values higher than those from the other techniques. In the case of SS, the difference amounts to one order of magnitude. Copyright © 2008 John Wiley & Sons, Ltd.

Journal of Chemical and Engineering Data, 2009

ABSTRACT Room-temperature ionic liquids based on the imidazolium cation are presently widely stud... more ABSTRACT Room-temperature ionic liquids based on the imidazolium cation are presently widely studied, but more data on important properties such as viscosity and surface tension are required. In this work, two imidazolium-based ionic liquids, 1-ethanol-3-methylimidazolium tetrafluoroborate [C2OHMIM][BF4] and 1-methyl-3-octylimidazolium tetrafluoroborate [OMIM][BF4], were investigated. The former is a member of a new series that contains the alcohol (CH2)2OH as a functional group. The latter, already studied by several authors, has a long side chain in the imidazolium ring that is responsible for its surfactant properties. The surface tension was determined using the pendant drop technique in a wide temperature range, (298 to 470) K, which has never been explored before with ILs. The viscosity was measured using a new technique (a quartz crystal microbalance) between (288 and 328) K. The surface tension and the viscosity of [C2OHMIM][BF4] are unexpectedly high when compared with those of [OMIM][BF4]. The experimental values were interpreted in terms of the existing theories.

Surface Science, 2009

In this paper we present a condition for thermodynamic equilibrium of gas bubbles of dimensions l... more In this paper we present a condition for thermodynamic equilibrium of gas bubbles of dimensions larger that a few tens of nanometers trapped at a solid-liquid interface. It is defined an intensive adimensional function that relates bubble stability with wettability and surface corrugation related parameters. From it, we deduce that bubbles can be stable on smooth, highly hydrophobic surfaces and that, on weakly hydrophobic surfaces, bubbles may be stabilized by topographical heterogeneities like pores and grooves.

Wear, 2006

In this work, the most commonly used joint materials for substitution of hip joints, ultra high m... more In this work, the most commonly used joint materials for substitution of hip joints, ultra high molecular weight polyethylene (UHMWPE) for the cup, and alumina, stainless steel or CoCrMo alloy for the head, were submitted to pin-on-disk tribological tests. The interfacial behaviour prosthetic material/lubricant was investigated through wettability measurements. Four lubricants were used: Hanks' balanced salt solution (HBSS) and solutions of bovine serum albumin (BSA), of hyaluronic acid (HA) and of both components (BSA + HA) in HBSS. It was observed that the friction coefficient increased significantly with time, in a random way, when the lubricant did not contain albumin. The addition of HA induced a slight reduction in the initial value of the friction coefficient but did not affect qualitatively its behaviour for longer times. Observation of the worn metallic surfaces revealed parallel grooves typical of abrasive wear and also lumpy transfer film of UHMWPE, while the transfer of polymer to alumina was incipient. Wear was particularly intense on the steel surface where delamination and fatigue wear mechanisms were also found. In the presence of albumin, the friction coefficient remained constant and small for the metallic surfaces and increased in the case of alumina. The observation of the worn surfaces showed that the presence of albumin in the lubricant avoided the transfer of polymer for the metallic surfaces but not for the alumina surfaces. These results, together with the wettability measurements, strongly indicate that the presence of albumin in the lubricant avoids the adhesion and transfer of UHMWPE only for the least hydrophilic surfaces, which are the metallic ones.