Testing the permeability and corrosion resistance of micro-mechanically interlocked joints

Micro-mechanical interlocking (MMI) can be applied to create new and interesting composite materials. We have employed laser structuring to achieve MMI between stainless steel and plastic with extremely high joint strength. However, the water permeability and corrosion resistance of the joint must be examined. For many industrially relevant applications it is important to keep water away from certain parts and to prevent the sample from corroding. A thorough study of the permeability of the interconnected samples at different temperatures and after employing different laser-structuring techniques is conducted. The permeability seems to be consistent with the Hagen–Poiseuille equation independent of the laser structuring technique and is orders of magnitudes larger than the diffusion rate through the plastic. Two different types of corrosion tests have been undertaken, and we show that care must be taken in order not to degrade the corrosion resistance of the sample to an unacceptable level.     

Distribution and accessibility of cyclodextrins covalently bound onto silica gel

Silica gel coated with a layer of β-cyclodextrins is a well investigated system. Earlier literature studies suggest that the coating changes the mesoporous structure of the silica gel and that the coating obtained might be heterogeneous. From surface area measurements it was found that the average pore diameter and accessible pore volume was reduced after grafting with 3-glycidyloxypropyltrimethoxysilane and β-cyclodextrin, but the change was reversible by simple pyrolysis of the organic compounds. To investigate the amount of accessible β-cyclodextrin on the silica particles, a Langmuir adsorption study was used. From the Langmuir binding isotherm it was found that the 8 Anilinonaphtalene-1-sulfonic acid ammonium (1,8 ANS) guest had a much larger stability constant with surface bound β-cyclodextrin compared to that found for free β-cyclodextrin. The accessible amount of β-cyclodextrin was found to be approximately 12% lower than the total amount of β-cyclodextrin indicating that some β-cyclodextrin is inaccessible for complex formation on the silica particles. The distribution of the cyclodextrin on the particle was investigated using the enhanced fluorescence of 1,8 ANS when complexed to β-cyclodextrin using confocal laser scanning microscopy. The confocal images of the particle show that the β-cyclodextrin was positioned on the outer layer of the particle, explained by the difficulty of diffusion of the β-cyclodextrin into the small pores. It was observed that smaller particles (>20 μm) has a more homogeneous distribution of β-CD compared to larger particles where a heterogeneous distribution was observed.     

Energy conservation in molecular dynamics simulations of classical systems

Classical Newtonian dynamics is analytic and the energy of an isolated system is conserved. The energy of such a system, obtained by the discrete "Verlet" algorithm commonly used in molecular dynamics simulations, fluctuates but is conserved in the mean. This is explained by the existence of a "shadow Hamiltonian" H [S. Toxvaerd, Phys. Rev. E 50, 2271 (1994)], i.e., a Hamiltonian close to the original H with the property that the discrete positions of the Verlet algorithm for H lie on the analytic trajectories of H. The shadow Hamiltonian can be obtained from H by an asymptotic expansion in the time step length. Here we use the first non-trivial term in this expansion to obtain an improved estimate of the discrete values of the energy. The investigation is performed for a representative system with Lennard-Jones pair interactions. The simulations show that inclusion of this term reduces the standard deviation of the energy fluctuations by a factor of 100 for typical values of the time step length. Simulations further show that the energy is conserved for at least one hundred million time steps provided the potential and its first four derivatives are continuous at the cutoff. Finally, we show analytically as well as numerically that energy conservation is not sensitive to round-off errors.     

The instantaneous shear modulus in the shoving model

We point out that the instantaneous shear modulus G(∞) of the shoving model for the non-Arrhenius temperature dependence of viscous liquids' relaxation time is the experimentally accessible high-frequency plateau modulus, not the idealized instantaneous affine shear modulus that cannot be measured. Data for a large selection of metallic glasses are compared to three different versions of the shoving model. The original shear-modulus based version shows a slight correlation to the Poisson ratio, which is eliminated by the energy-landscape formulation of the model in which the bulk modulus plays a minor role.     

Equilibrium strategy and population-size effects in lowest unique bid auctions

In lowest unique bid auctions, N players bid for an item. The winner is whoever places the lowest bid, provided that it is also unique. We use a grand canonical approach to derive an analytical expression for the equilibrium distribution of strategies. We then study the properties of the solution as a function of the mean number of players, and compare them with a large data set of internet auctions. The theory agrees with the data with striking accuracy for small population-size N, while for larger N a qualitatively different distribution is observed. We interpret this result as the emergence of two different regimes, one in which adaptation is feasible and one in which it is not. Our results question the actual possibility of a large population to adapt and find the optimal strategy when participating in a collective game.     

Repulsive reference potential reproducing the dynamics of a liquid with attractions

A well-known result of liquid state theory is that the structure of dense fluids is mainly determined by their repulsive forces. The Weeks-Chandler-Andersen potential, which cuts intermolecular potentials at their minima, is therefore often used as a reference. However, this cannot reproduce the viscous dynamics of the Kob-Andersen binary Lennard-Jones liquid [Berthier and Tarjus, Phys. Rev. Lett. 103, 170601 (2009)]. This paper shows that repulsive inverse-power-law potentials provide a reference for this liquid that reproduces its structure, dynamics, and isochoric heat capacity.     

Steady‐State and Pseudo‐Steady‐State Photocrystallographic Studies on Linkage Isomers of [Ni(Et4dien)(η2‐O,ON)(η1‐NO2)]: Identification of a New Linkage Isomer

At temperatures below 150 K, the photoactivated metastable endo‐nitrito linkage isomer [Ni(Et₄dien)(η²‐O,ON)(η¹‐ONO)] (Et₄dien=N,N,N′,N′‐tetraethyldiethylenetriamine) can be generated with 100 % conversion from the ground state nitro‐(η¹‐NO₂) isomer on irradiation with 500 nm light, in the single crystal by steady‐state photocrystallographic techniques. Kinetic studies show the system is no longer metastable above 150 K, decaying back to the ground state nitro‐(η¹‐NO₂) arrangement over several hours at 150 K. Variable‐temperature kinetic measurements in the range of 150–160 K show that the rate of endo‐nitrito decay is highly dependent on temperature, and an activation energy of E_act=+48.6(4) kJ mol^?1 is calculated for the decay process. Pseudo‐steady‐state experiments, where the crystal is continually pumped by the light source for the duration of the X‐ray experiment, show the production of a previously unobserved, exo‐nitrito‐(η¹‐ONO) linkage isomer only at temperatures close to the metastable limit (ca. 140–190 K). This exo isomer is considered to be a transient excited‐state species, as it is only observed in data collected by pseudo‐steady‐state methods.     

Local Transient Unfolding of Native State PAI‐1 Associated with Serpin Metastability

The metastability of the native fold makes serpin (serine protease inhibitor) proteins prone to pathological conformational change, often by insertion of an extra β‐strand into the central β‐sheet A. How this insertion is made possible is a hitherto unresolved question. By the use of advanced hydrogen/deuterium‐exchange mass spectrometry (HDX‐MS) it is shown that the serpin plasminogen activator inhibitor 1 (PAI‐1) transiently unfolds under native condition, on a second‐to‐minute time scale. The unfolding regions comprise β‐strand 5A as well as the underlying hydrophobic core, including β‐strand 6B and parts of helices A, B, and C. Based thereon, a mechanism is proposed by which PAI‐1 makes transitions through progressively more unfolded states along the reaction coordinate to the inactive, so‐called latent form. Our results highlight the profound utility of HDX‐MS in detecting sparsely populated, transiently unfolded protein states.