Stick slip contact mechanics between dissimilar materials: effect of charging and large friction

Measurements of the contact radius as a function of applied force between a mica surface and a silica surface (mica/silica) in air are reported. The load/unload results show that the contact radius generally increases with applied force. Because of the presence of charging due to contact electrification, both a short-range van der Waals adhesion force and longer-range electrostatic adhesive interaction contribute to the measured force. The results indicate that approximately 20% of the pull-off force is due to van der Waals forces. The contact radius versus applied force results can be fit to Johnson-Kendall-Roberts (JKR) theory by considering that only the short-range van der Waals forces contribute to the work of adhesion and subtracting a constant longer-range electrostatic force. Also, an additional and unexpected step function is superimposed on the contact radius versus applied force curve. Thus, the contact diameter increases in a stepped dependence with increasing force. The stepped contact behavior is seen only for increasing force and is not observed when symmetric mica/mica or silica/silica contacts are measured. In humid conditions, the contact diameter of the mica/silica contact increases monotonically with applied force. Friction forces between the surfaces are also measured and the shear stress of a mica/silica interface is 100 times greater than the shear stress of a mica/mica interface. This large shear stress retards the increase in contact area as the force is increased and leads to the observed stepped contact mechanics behavior.     

Adhesion and friction force coupling of gecko setal arrays: implications for structured adhesive surfaces

The extraordinary climbing ability of geckos is partially attributed to the fine structure of their toe pads, which contain arrays consisting of thousands of micrometer-sized stalks (setae) that are in turn terminated by millions of fingerlike pads (spatulae) having nanoscale dimensions. Using a surface forces apparatus (SFA), we have investigated the dynamic sliding characteristics of setal arrays subjected to various loading, unloading, and shearing conditions at different angles. Setal arrays were glued onto silica substrates and, once installed into the SFA, brought toward a polymeric substrate surface and then sheared. Lateral shearing of the arrays was initiated along both the "gripping" and "releasing" directions of the setae on the foot pads. We find that the anisotropic microstructure of the setal arrays gives rise to quite different adhesive and tribological properties when sliding along these two directions, depending also on the angle that the setae subtend with respect to the surface. Thus, dragging the setal arrays along the gripping direction leads to strong adhesion and friction forces (as required during contact and attachment), whereas when shearing along the releasing direction, both forces fall to almost zero (as desired during rapid detachment). The results and analysis provide new insights into the biomechanics of adhesion and friction forces in animals, the coupling between these two forces, and the specialized structures that allow them to optimize these forces along different directions during movement. Our results also have practical implications and criteria for designing reversible and responsive adhesives and articulated robotic mechanisms.     

3D force and displacement sensor for SFA and AFM measurements

A new device has been designed, and a prototype built and tested, that can simultaneously measure the displacements and/or the components of a force in three orthogonal directions. The "3D sensor" consists of four or eight strain gauges attached to the four arms of a single cross-shaped force-measuring cantilever spring. Finite element modeling (FEM) was performed to optimize the design configuration to give desired sensitivity of force, displacement, stiffness, and resonant frequency in each direction (x, y, and z) which were tested on a "mesoscale" device and found to agree with the predicted values to within 4-10%. The device can be fitted into a surface forces apparatus (SFA), and a future smaller "microscale" microfabricated version can be fitted into an atomic force microscope (AFM) for simultaneous measurements of the normal and lateral (friction) forces between a tip (or colloidal bead probe) and a surface, and the topography of the surface. Results of the FEM analysis are presented, and approximate equations derived using linear elasticity theory are given for the sensitivity in each direction. Initial calibrations and measurements of thin film rheology (lubrication forces) using the "mesoscale" prototype show the device to function as expected.     

The basis suppression method

The Basis Suppression algorithm is a simplex-based procedure which allows the efficient extension of current special structure algorithms to problems of special structure except for a single complicating side variable. A basis free of the complicating variable is maintained in this algorithm. Various properties of the algorithm are presented, including a proof of convergence. Computational effectiveness is discussed and has been verified by using the procedure to solve the maximal concurrent flow problem.     

Synthesis of Poly(dialkylphosphazenes) from N-Silylphosphinimines

Many N-silylphosphinimines Me₃SiN?P(X) RR′ undergo facile thermal decomposition with elimination of substituted silanes Me₃SiX and formation of cyclic or polymeric phosphazenes (RR′PN). The process which is a new, general synthesis of phosphazenes has been used to prepare poly(dimethylphosphazene), (Me₂PN), in nearly quantitative yield from Me₃SiN?P(OCH₂CF₃)Me₂. Convenient synthetic routes to the necessary silicon-nitrogen-phosphorus precursors are described and the results of their decomposition reactions are reported.     

Gasanalyse

Analytical and Bioanalytical Chemistry -     

A sensitive and specific liquid chromatography/tandem mass spectrometry method for quantification of nevirapine and its five metabolites and their pharmacokinetics in baboons

A highly sensitive and specific LC‐MS/MS assay was developed and validated to quantify nevirapine (NVP) and its five metabolites [2‐, 3‐, 8‐, 12‐hydroxyl NVP (OHNVP) and 4‐carboxyl NVP (CANVP)] simultaneously in baboon serum and the assay was used to characterize their pharmacokinetic studies of an oral‐dose escalation study in baboon. The lower limit of quantification (LLOQ) for NVP and its four hydroxyl nevirapine metabolites was 1.0 ng/mL and for 4‐CANVP was 5.0 ng/mL. The between‐run and within‐run precisions and accuracies at four quality control concentrations (1, 5, 50 and 500 ng/mL) were evaluated in baboon serum with less than 14% variation and 93–114% accuracies (n = 6), except for the LLOQ for 2‐OHNVP, which had an accuracy of 115.8% for between‐run validation. The pharmacokinetics of NVP and its five metabolites in non‐pregnant baboons by a single‐dose escalation study were also profiled. The major metabolites detected were 4‐CANVP and 12‐OHNVP. 3‐OHNVP and 2‐OHNVP were the minor metabolites with only a trace amount of 2‐OHNVP detected in some pharmacokinetic samples. No 8‐OHNVP was observed in all of the pharmacokinetic samples. In addition, the fragmentation for the four hydroxyl metabolite isomers is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Gasanalyse

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