María J. Nuevo

Experimental analysis of the influence of surface topography on the adhesion force as measured by an AFM

Force curves have been acquired using an atomic force microscope (AFM) on homogeneous microspheres of three different materials (latex, glass and yttria), in order to study the possible influence of the surface topography/geometry on the adhesion force as measured by an AFM. Forces were measured in regions at the top of the spheres ( ≈ 90°), at half-heights ( ≈ 0°) and in an intermediate region between these two ( ≈ 45°), where the angle is measured from the equatorial plane of the sphere to its polar axis. A very irregular and non-reproducible behaviour was found at ≈ 0°, so only the other two regions were quantitatively analysed. For all the three materials, a much smaller adhesion force was obtained in the region corresponding to ≈ 45° as compared to ≈ 90°. Moreover, a quite similar adhesion decrease ratio of about 1.60 ± 0.5 was obtained for all the three materials, which may suggest that the observed behavior might be due to geometrical factors. This observed influence could, in part, explain the observed heterogeneity in adhesion maps of microbial cells reported in the literature. The influence of the surface roughness is also discussed and it seems to result in a poor reproducibility of force curves.

Optical interference artifacts in contact atomic force microscopy images.

Atomic force microscopy images are usually affected by different kinds of artifacts due to either the microscope design and operation mode or external environmental factors. Optical interferences between the laser light reflected off the top of the cantilever and the light scattered by the surface in the same direction is one of the most frequent sources of height artifact in contact (and occasionally non-contact) images. They are present when imaging highly reflective surfaces, or even when imaging non-reflective materials deposited onto reflective ones. In this study interference patterns have been obtained with a highly polished stainless steel planchet. The influence of these artifacts in surface roughness measurements is discussed, and a semi-quantitative method based on the fast Fourier transform technique is proposed to remove the artifacts from the images. This method improves the results obtained by applying the usual flattening routines.

Comparison of link-cell and neighbourhood tables on a range of computers

Abstract The recent improvement to the link-cell method (Comput. Phys. Commun. 60 (1990) 195), making it faster on a scalar computer than the traditional neighbour table (NT) method, was tested on different vectorial computers. The efficiency of both methods depends greatly on the particular processor architecture. In contrast to scalar computer results, the NT method was found to be better on the vectorial computers.

Application of XRF spectrometry to the study of pigments in glazed ceramic pots.

Non-destructive techniques for analysis of components of a sample are very useful, and indeed essential, when the samples are unique, such as works of art, including not only pieces exhibited and preserved in museums, but also archaeological structures belonging to a historical and cultural heritage. X-ray fluorescence analysis is a suitable technique for these kinds of pieces and objects that must neither be damaged nor moved. In this work, an X-ray fluorescence spectrometer (XRFS), assembled at the University of Extremadura (Badajoz, Spain), has been applied to the study of pigments in the decorative paints of modern ceramics of known and unknown origins.

Path integral molecular dynamics methods: Application to neon

Feynmans path integral formulation of quantum statistical mechanics, which has commonly been applied be Monte Carlo methods, is now also implemented by traditional molecular dynamics simulations of the microcanonical ensemble and in the Nosé‐Hoover method simulating the isothermal‐isobaric ensemble. In this article these two methods are applied to solid and liquid neon, in which quantum effects are not negligible. The validity of the procedure is shown by comparison with Monte Carlo and Brownian Dynamics computer simulations and with experiment.

Statistical error method in computer simulations

Abstract Two methods of studying the statistical error in the sequences of data obtained by computer simulation are compared. The first method divides the sequence into blocks whose length is selected graphically by means of the “statistical inefficiency.” The second method uses the autocorrelation function of all the values obtained and analytically calculates its convergence by means of the “correlation length.” The general relationship between the two parameters is found mathematically and is in good agreement with the experimental data obtained by molecular dynamics simulation in the melting zone when a very accurate algorithm is used. As a consequence, the analytical method is more accurate than the graphical method.

Translational and rotational diffusion of model nanocolloidal dispersions by molecular dynamics simulations

Molecular dynamics, MD simulations have been applied to model nanocolloids in solution at infinite dilution. Simulations were carried out with atomistically rough clusters of atoms with variable dimensions compared to the solvent molecule, using the Weeks–Chandler–Andersen (WCA) interaction between solute and solvent. Nanocolloidal particles containing between 20 and 256 atoms held together with strong Lennard-Jones interactions and with up to eight times the diameter of the solvent molecule were modelled. At liquid-like densities the translational and rotational self-diffusion coefficients for the nanocolloids of all sizes were statistically independent of the ratio of solute to solvent particle densities. The clusters induced a ‘layering’ of solvent molecules around them. The translational and rotational diffusion coefficients decreased with increasing cluster size and solvent density. The simulations reveal that there is a clear separation of timescales between angular velocity and orientation relaxation, consistent with the classical small-step diffusion picture encapsulated in Hubbards relationship which is obeyed well by the simulation data. Application of the Stokes–Einstein (translation) and Stokes–Einstein–Debye (rotation) equations to these data indicate that the translational degrees of freedom experience a local viscosity in excess of the bulk value, whereas rotational relaxation generally experiences a smaller viscosity than the bulk, dependent on cluster size and solvent density and reasonably in accord with the Gierer–Wirtz model. Both of these observations are consistent with the observed layering of the solvent molecules around the cluster, whose effects appear to be significant for clusters on the nanoscale.

HYDRODYNAMIC BEHAVIOUR OF A SOLUTE PARTICLE BY MOLECULAR DYNAMICS

The hydrodynamic behaviour of a single solute particle immersed in a solvent system of (N-1 particles, interacting through a repulsive Lennard-Jones potential, has been studied for several system sizes, ranging from N = 108 to 2048 particles using isothermal-isochoric molecular dynamics computer simulation. The solute particle was projected at a fixed relative velocity with respect to the host fluid. The computations show that the linear resistance force versus velocity behaviour, postulated by the Stokes law, is obeyed quite well even up to relatively large (i.e. near thermal) solute velocities. However, two finite size effects have been found for large solute particles (cα σB/σs > 1, where σBand σs the solute and solvent particles, respectively) when they are confined in such small periodic systems. At high drift velocity, there is a breakdown in this linear relationship for the smaller systems leading to a maximum in the opposing force on the solute particle. Als o there is a slow system size convergen...

A technique for improving the link-cell method

Abstract A method for speeding up the link-cell (LC) method for very large systems is described. The size of the subcells into which the systems are divided is less than the cutoff distance of the potential. One case among all those studied can be chosen, called the favourable case, which most efficiently improves the conventional LC and the traditional neighbour table methods.

Application of atomic and nuclear techniques to the study of inhomogeneities in electrodeposited α-particle sources

Three α-particle sources made by different methods of electrodeposition were analysed using α-particle spectrometry, Rutherford backscattering (RBS), and atomic force microscopy (AFM) on several surface zones. The thickness and homogeneity of these sources was studied using RBS, and the results were analysed jointly with those obtained with α-particle spectrometry and AFM techniques. The comparison of the electrodeposition methods showed that the most homogeneous electrodeposited zones corresponded to the source made with a stirring cathode.