Biomedical subjects
Publications and source records attributed to M Salmeron.
We present an analytical model that explains how, in humid environments, the electric field near a sharp tip enhances the formation of water menisci and bridges between the tip and a sample. The predictions of the model are compared with experimental measurements of the critical distance where the field strength causes bridge formation.
The influence of chain length and ripening time on the self-assembly of tetradecyl (C(14)), hexadecyl (C(16)), and octadecylamine (C(16)) on mica has been studied by atomic force microscopy. The overall process can be described in three stages characterized by different time scales. First, alkylamine molecules adsorb in a process controlled by water mediated interactions of the NH(2) head groups and mica. Second, surface diffusion and aggregation into islands takes place, driven by energy interactions between alkyl chains. The third stage consists of a progressive tilt of the self-assembled molecules towards the surface, driven by relaxation of the electrostatic repulsion between protonated amino groups due to water uptake from atmosphere.
A remarkable dependence of the friction force on carrier concentration was found on doped silicon substrates. The sample was a nearly intrinsic n-type Si(100) wafer patterned with 2-micrometer-wide stripes of highly B-doped p-type material. The counter surface was the tip of an atomic force microscope coated with conductive titanium nitride. The local carrier concentration was controlled through application of forward or reverse bias voltages between the tip and the sample in the p and the n regions. Charge depletion or accumulation resulted in substantial differences in friction force. The results demonstrate the capability to electronically control friction in semiconductor devices, with potential applications in nanoscale machines containing moving parts.
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The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smoluchowski effect were measured from the electrostatic force exerted on the biased tip of a scanning tunneling microscope. By varying the tip-sample bias the contribution of the step dipole was separated from changes in the force due to van der Waals and polarization forces. Combined with electrostatic calculations, the method was used to determine the local dipole moment in steps of different heights on Au(111) and on the twofold surface of an Al-Ni-Co decagonal quasicrystal.
Atomic force microscopy operating in noncontact electrostatic force mode was used to study the interaction of water with films of alkylamines and alkylsilanes on mica. The films efficiently block water adsorption except in exposed mica areas, where it strongly modifies the mobility of surface ions. We also studied the molecular orientation of octadecylamine molecules forming monolayers and multilayer islands. In monolayer films the molecules bind to mica through the amino group, producing a positive contact potential relative to mica (dipole pointing up). In multilayer films the methyl and amino group terminations are exposed in alternating layers that give rise to alternating values of the contact potential. These findings correlate with low and high friction forces measured in the methyl termination and amino terminations.
Strong friction anisotropy is found when the twofold surface of an atomically clean aluminum-nickel-cobalt quasicrystal slides against a thiol-passivated titanium-nitride tip. Friction along the aperiodic direction is one-eighth as much as that along the periodic direction. This anisotropy, which is about three times as large as the highest value observed in anisotropic crystalline surfaces, disappears after the surface is oxidized in air. These results reveal a strong connection between interface atomic structure and the mechanisms by which energy is dissipated, which likely include electronic or phononic contributions, or both.
The diffusion and interactions of CO molecules on Pd(111) were studied by scanning tunneling microscopy. By following the random walk motion of individual molecules as a function of temperature, an activation energy barrier for diffusion of 118 +/- 5 meV was determined. The interaction between CO molecules was found to be repulsive for pairs separated by one or two Pd(111) lattice distances, and weakly attractive at a separation of sqrt[3].
Scanning tunneling microscopy (STM) images of water submonolayers on Pd(111) reveal quasiperiodic and isolated adclusters with internal structure that would ordinarily be ascribed to icelike puckered hexagonal units. However, density functional theory and STM simulations contradict this conventional picture, showing instead that the water adlayers are composed mainly of flat-lying molecules arranged in planar water hexagons. A new rule for two dimensional (2D) water growth is offered that generates the structures observed experimentally from planar hexamer units.
During reaction, a catalyst surface usually interacts with a constantly fluctuating mix of reactants, products, 'spectators' that do not participate in the reaction, and species that either promote or inhibit the activity of the catalyst. How molecules adsorb and dissociate under such dynamic conditions is often poorly understood. For example, the dissociative adsorption of the diatomic molecule H2--a central step in many industrially important catalytic processes--is generally assumed to require at least two adjacent and empty atomic adsorption sites (or vacancies). The creation of active sites for H2 dissociation will thus involve the formation of individual vacancies and their subsequent diffusion and aggregation, with the coupling between these events determining the activity of the catalyst surface. But even though active sites are the central component of most reaction models, the processes controlling their formation, and hence the activity of a catalyst surface, have never been captured experimentally. Here we report scanning tunnelling microscopy observations of the transient formation of active sites for the dissociative adsorption of H2 molecules on a palladium (111) surface. We find, contrary to conventional thinking, that two-vacancy sites seem inactive, and that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation.
The adsorption, diffusion, and clustering of water molecules on a Pd(111) surface were studied by scanning tunneling microscopy. At 40 kelvin, low-coverage water adsorbs in the form of isolated molecules, which diffuse by hopping to nearest neighbor sites. Upon collision, they form first dimers, then trimers, tetramers, and so on. The mobility of these species increased by several orders of magnitude when dimers, trimers, and tetramers formed, and decreased again when the cluster contained five or more molecules. Cyclic hexamers were found to be particularly stable. They grow with further exposure to form a commensurate hexagonal honeycomb structure relative to the Pd(111) substrate. These observations illustrate the change in relative strength between intermolecular hydrogen bonds and molecule-substrate bonds as a function of water cluster size, the key property that determines the wetting properties of materials.
Using scanning tunneling microscopy, we show the phase transition between new structures of NO on Rh(111) in equilibrium with the gas phase near 300 K, in the Torr pressure range. Two phases with (2 x 2) and (3 x 3) periodicity transform into each other as the pressure and temperature change around the equilibrium P-T line. By measuring P and T at coexistence, we determined the heat of adsorption in the (3 x 3) structure. From the phase boundary dynamics, the activation energy barrier between phases were estimated. The results demonstrate that unique information can be obtained from high-pressure and high-temperature studies.
The dynamics of the expulsion of the last liquid monolayer of molecules confined between two surfaces (measured recently for the first time) has been analyzed by solving the two-dimensional Navier-Stokes equation combined with kinetic Monte Carlo simulations. Instabilities in the boundary line of the expelled film were observed. We show that the instabilities produce a rough boundary for all length scales above a critical value and a smooth boundary for shorter lengths. The squeezing out of all but a few trapped islands of liquid is shown to be the result of the pressure gradient in the contact area.
One quarter of Meckel's diverticula are covered by ectopic gastric mucosa and many may present histological inflammatory changes. Helicobacter pylori grows preferentially on an acid ground but has been found in different parts of the gastrointestinal tract. A prior hypothesis of a pathogenic role for this bacterium in some of Meckel's diverticula with ectopic mucosa is not confirmed by this study covering 21 cases.
Multiple beam interferometry and video microscopy were used to investigate the layering transition of thin liquid films of 1-undecanol confined between atomically smooth mica surfaces. The expulsion of a molecularly thin lubricant layer was followed directly in two dimensions. Overall, the dynamics of the transition follows theoretical predictions based on two-dimensional hydrodynamics. Frequently, pockets of liquid remain trapped inside the contact area at the end of the transition. The trapped pockets undergo shape transformations to minimize elastic and interfacial energy.
The wetting properties of 8CB ( 4(')-n-octyl-4-cyanobiphenyl) on silicon wafers have been studied with scanning polarization force microscopy (SPFM). Layer-by-layer spreading of 8CB droplets is observed. With the help of the surface potential mapping capability of SPFM, we found that the molecular dipole of the first monolayer of 8CB is parallel to the surface. A layer of nearly vertical molecular dimers on top of the monolayer has an associated surface potential of 40 mV, which is attributed to a distortion of the dimer. The dimer distortion propagates to the subsequent smectic bilayers, producing an additional 7 mV potential increase in the second layer, 2 mV on the third, and approximately 1 mV on the fourth.
The clinicopathologic and immunohistochemical features of three metastasizing fibrous histiocytomas of the skin are presented. The first patient had a 1.3-cm nodule in the right thigh, with right inguinal lymph node metastases 19 years later. The second patient, who had a 3-cm nodule excised from his left thigh and inguinal lymph node metastasis after 4 months, had a favorable outcome 14 years after local radiotherapy and chemotherapy. The third had a 2-cm nodule in his neck, which recurred 16 months later. Four months later, cervical lymph node metastases were found. The patient was alive and well 26 months after initial surgery. All three primary skin tumors involved the dermis and subcutis, appeared well-delineated but nonencapsulated, were associated with some degree of epidermal hyperplasia, and showed features of aneurysmal/atypical or cellular fibrous histiocytoma. The number of mitoses ranged from 6 to 11 per 10 high-power fields. Recurrences and metastases showed morphologic features similar to primary lesions. Tumor cells were positive, at least focally, for CD 68, Ki-M1p, and Factor XIIIa, and occasionally for smooth muscle actin. Desmin, CD 34, S-100 protein, and cytokeratin stainings were negative. Primary neoplasms, recurrences, and metastases showed a Mib-1 labeling index of 10% or less. Cellular, aneurysmal, and atypical (pseudosarcomatous) fibrous histiocytomas of the skin can metastasize, yet they often show a protracted clinical course. Risk factors for metastatic dissemination include large size, high cellularity, aneurysmal changes, marked cellular pleomorphism, high mitotic activity, tumor necrosis, and repeated local recurrences.
The variation of contact area with load between adhesive elastic spheres depends upon the effective range of attractive surface forces. Relatively simple forms to describe limiting cases exist, but the general intermediate case requires a more complex analysis. Maugis, using a Dugdale model [D. Maugis, J. Colloid. Interf. Sci. 150, 243 (1992)], provides an analytic solution, but the resulting equations are cumbersome if one wishes to compare with experimental data such as atomic force microscope measurements. In this paper we present a simpler general equation that approximates Maugis' solution extremely closely. The general equation is amenable to conventional curve fitting software routines and provides a rapid method of determining the value of the "transition parameter" which describes the range of surface forces. Copyright 1999 Academic Press.