365 research outputs found
Force measurements with the atomic force microscope: Technique, interpretation and applications
No full textThe atomic force microscope (AFM) is not only a tool to image the topography of solid surfaces at high resolution. It can also be used to measure force-versus-distance curves. Such curves, briefly called force curves, provide valuable information on local material properties such as elasticity, hardness, Hamaker constant, adhesion and surface charge densities. For this reason the measurement of force curves has become essential in different fields of research such as surface science, materials engineering, and biology.
Another application is the analysis of surface forces per se. Some of the most fundamental questions in colloid and surface science can be addressed directly with the AFM: What are the interactions between particles in a liquid? How can a dispersion be stabilized? How do surfaces in general and particles in particular adhere to each other? Particles and surfaces interactions have major implications for friction and lubrication. Force measurements on single molecules involving the rupture of single chemical bonds and the stretching of polymer chains have almost become routine. The structure and properties of confined liquids can be addressed since force measurements provide information on the energy of a confined liquid film.
After the review of Cappella [B. Cappella, G. Dietler, Surf. Sci. Rep. 34 (1999) 1–104] 6 years of intense development have occurred. In 1999, the AFM was used only by experts to do force measurements. Now, force curves are used by many AFM researchers to characterize materials and single molecules. The technique and our understanding of surface forces has reached a new level of maturity. In this review we describe the technique of AFM force measurements. Important experimental issues such as the determination of the spring constant and of the tip radius are discussed. Current state of the art in analyzing force curves obtained under different conditions is presented. Possibilities, perspectives but also open questions and limitations are discussed
The influence of laccase on the chemo-enzymatic synthesis of lignin graft-copolymers
No full textThe mechanism unterlying the chemo-enzymatic graft copolymerization of lignin with acrylic compounds was studied. When lignin sulfonate (LS) was incubated with the phenoloxidase laccase in the presence of a peroxide species (t-butylhydroperoxide, i.e. t-BHP), the decrease of the phenolic groups was significantly higher than when LS was incubated with laccase or t-BHP, separately. The enzymatic oxidation of the phenolic groups did lead to the formation of quinones however, the number of quinone groups formed in LS in the presence of t-BHP? was not significantly higher than in its absence. Still, a certain proportion of the quinones formed in the presence of t-BHP appeared to be further oxidized resulting in an opening of the aromatic rings. In addition, the rate of LS polymerization, i.e. its increase in (M) over bar (w) instigated by laccase, was significantly reduced in the presence of t-BHP even though the activity of laccase was only slightly affected by the peroxide. Incubation of LS with laccase prior to the grafting reaction resulted in a significant increase of die spin concentration as detected in the EPR spectra. In spite of a high steady state concentration of radicals, the generation rate of these radicals in pre-incubated LS was lower than in unpretreated LS. Thus, the polymer yields obtained with pre-incubated LS were lower than those obtained with unpretreated LS. (C) 2002 Elsevier Science Inc. All rights reserved
Using the atomic force microscope to study the interaction between two solid supported lipid bilayers and the influence of synapsin I
No full textAbstractTo measure the interaction between two lipid bilayers with an atomic force microscope one solid supported bilayer was formed on a planar surface by spontaneous vesicle fusion. To spontaneously adsorb lipid bilayers also on the atomic force microscope tip, the tips were first coated with gold and a monolayer of mercapto undecanol. Calculations indicate that long-chain hydroxyl terminated alkyl thiols tend to enhance spontaneous vesicle fusion because of an increased van der Waals attraction as compared to short-chain thiols. Interactions measured between dioleoylphosphatidylcholine, dioleoylphosphatidylserine, and dioleoyloxypropyl trimethylammonium chloride showed the electrostatic double-layer force plus a shorter-range repulsion which decayed exponentially with a decay length of 0.7nm for dioleoylphosphatidylcholine, 1.2nm for dioleoylphosphatidylserine, and 0.8nm for dioleoyloxypropyl trimethylammonium chloride. The salt concentration drastically changed the interaction between dioleoyloxypropyl trimethylammonium chloride bilayers. As an example for the influence of proteins on bilayer-bilayer interaction, the influence of the synaptic vesicle-associated, phospholipid binding protein synapsin I was studied. Synapsin I increased membrane stability so that the bilayers could not be penetrated with the tip
The forces at work in collodial self-assembly: A review on fundamental interactions between collodial particels
No full textColloidal particles with well-defined sizes can self-assemble into ordered, crystalline structures under non-equilibrium conditions. This phenomenon originates from the various forces acting upon them. In this article, we provide an overview on the forces at work in a colloidal system, in particular, the roles of these forces at various stages in colloidal self-assembly. Van der Waals, electrostatic, hydrodynamic, and capillary forces, as well as Brownian motions, are extensively discussed, whereas other types of interactions are briefly introduced and summarized
Sintering—Pressure- and Temperature-Dependent Contact Models
No full textSintering granular materials involves the application of pressure and temperature to make the particulate material a permanent solid. In order to better understand this complex process, the pressure-, temperature-, and time-dependent contact behaviour of micron-sized particles has been studied in close collaboration by the groups of Luding, Staedler and Kappl within the DFG SPP PiKo. This chapter summarises the modelling advances made during the project, with direct links given to the experimental results. Two aspects have been studied: (a) the dependence of the elastic as well as frictional contact forces and torques on an applied normal pressure; and (b) the formation and evolution of adhesive bonds between particles during heat-sintering. Both contact models have been experimentally calibrated and validated, using advanced techniques such as nanoindentation and AFM. As materials, borosilicate particles were used to study the pressure-dependency, while polystyrene particles were chosen due to their low glass transition temperature to study the temperature-dependency near the transition. Combining both aspects provides a multi-purpose contact model that allows the simulations of a wide range of sinter and agglomeration processes for a variety of practically relevant materials
Facing the both sides of a coin by merging biological and nonclassical crystallisation models: is a liquid amorphous precursor involved in the formation of the prismatic layer of the Mediterranean Fan Mussel Pinna nobilis?
No full textKorrosionsschutzmechanismen bei TiMgN-Hartstoffschichten auf Stahl
No full textDie Präsentation behandelt ein abgeschlossenes DFG Projekt welches die Fragestellung um das Korrosionsverhalten von TiMgN-Hartstoffschichten beleuchtet. Am Beginn des Vorhabens und in dessen weiterem Verlauf standen zahlreiche Ergebnisse die belegen, dass durch den Einbau von Mg in TiN-Hartstoffschichten das Korrosionsverhalten der beschichteten Stahlproben verbessert wird. Es gilt vorab festzuhalten, dass durch die TiMgN-Schicht eine großflächige Abschirmung des Stahls vor dem umgebenden Medium gegeben ist. Jedoch liegt bei nicht auszuschließenden Defekten bis zum Stahl, bedingt durch die Herstellung und/oder dem späteren Gebrauch, eine Gefährdung durch lokale Korrosion vor. Bei Kontakt mit wässriger, chloridhaltiger Lösung benetzt die Oberfläche und verbindet die TiMgN-Schicht über die Defekte mit dem dort freiliegenden Stahlsubstrat über den Elektrolyten. Zur Wirkungsweise des Korrosionsschutzes durch Mg in TiMgN konnten zum Ende des Vorhabens drei grundsätzliche Mechanismen identifiziert werden
IFacing the both sides of a coin by merging biological and nonclassical crystallisation models: is a liquid amorphous precursor involved in the formation of the prismatic layer of the Mediterranean Fan Mussel Pinna nobilis?
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