Potential of mean force by thermodynamic integration: molecular-dynamics simulation of decomplexation

“Umbrella sampling” has been incorporated in the thermodynamic integration method to obtain a potential of mean force by slow growth molecular-dynamics simulations. The method was tested for liquid argon, for which good agreement was obtained with a standard potential of mean force, as derived from the radial pair-correlation function. For a sodium chloride ion-pair in aqueous solution the calculations showed resonable agreement with a literature result. The method was also applied to the decomplexation of 18-crown-6 and a potassium cation in aqueous solution

Modelling of the diffusion of carbon dioxide in polyimide matrices by computer simulation

Computer aided molecular modelling is used to visualize the motion of CO2 gas molecules inside a polyimide polymer matrix. The polymers simulated are two 6FDA-bases polyimides, 6FDA-4PDA and 6FDA-44ODA. These polymers have also been synthesized in our laboratory, and thus the simulated properties could directly be compared with “real-world” data. The simulation experiments have been performed using the GROMOS1 package. The polymer boxes were created using the soft-core method, with short (11 segments) chains. This results in highly relaxed and totally amorphous polyimide matrices. The motion of randomly placed CO2 molecules in the boxes during molecular dynamics runs was followed, revealing three types of motion: jumping, continuous- and trapped motion. The calculated diffusivities are unrealistic, but possible shortcomings in our model are given

Size selection in diving Tufted Ducks Aythya fuligula explained by differential handling of small and large mussels Dreissena polymorpha

We studied prey size selection of Tufted Ducks feeding on fresh-water mussels under semi-natural conditions. In experiments with non-diving birds, we found that Tufted Ducks use two techniques to handle mussels. Mussels less than 16 mm in length are strained from a waterflow generated in the bill (suction-feeding), while larger mussels are picked up and handled singly. Suction-feeding on small mussels proved to be more profitable. In the non-diving situation, the ducks preferentially took the smallest mussels on offer. It is suggested that smaller mussels incur lower costs of crushing mussel shells in the gizzard. Ducks diving for mussels preferred suction-feeding on all mussels up to about 16 mm in order to minimize the costs of diving. The selectivity for small mussels increased with diving depth (1-5 m), probably because ducks diving deeper spend more time at the bottom collecting small mussels, before a larger mussel is taken to the surface and ingested afterwards. We conclude that the two handling techniques we described for Tufted Duck open up an extensive feeding niche for benthic feeding diving ducks

Size selection in diving Tufted Ducks Aythya fuligula explained by differential handling of small and large mussels Dreissena polymorpha

We studied prey size selection of Tufted Ducks feeding on fresh-water mussels under semi-natural conditions. In experiments with non-diving birds, we found that Tufted Ducks use two techniques to handle mussels. Mussels less than 16 mm in length are strained from a waterflow generated in the bill (suction-feeding), while larger mussels are picked up and handled singly. Suction-feeding on small mussels proved to be more profitable. In the non-diving situation, the ducks preferentially took the smallest mussels on offer. It is suggested that smaller mussels incur lower costs of crushing mussel shells in the gizzard. Ducks diving for mussels preferred suction-feeding on all mussels up to about 16 mm in order to minimize the costs of diving. The selectivity for small mussels increased with diving depth (1-5 m), probably because ducks diving deeper spend more time at the bottom collecting small mussels, before a larger mussel is taken to the surface and ingested afterwards. We conclude that the two handling techniques we described for Tufted Duck open up an extensive feeding niche for benthic feeding diving ducks

Spiral surface growth without desorption

Spiral surface growth is well understood in the limit where the step motion is controlled by the local supersaturation of adatoms near the spiral ridge. In epitaxial thin-film growth, however, spirals can form in a step-flow regime where desorption of adatoms is negligible and the ridge dynamics is governed by the non-local diffusion field of adatoms on the whole surface. We investigate this limit numerically using a phase-field formulation of the Burton-Cabrera-Frank model, as well as analytically. Quantitative predictions, which differ strikingly from those of the local limit, are made for the selected step spacing as a function of the deposition flux, as well as for the dependence of the relaxation time to steady-state growth on the screw dislocation density.Comment: 9 pages, 3 figures, RevTe

Molecular-dynamics simulations of interfaces between water and crystalline urea

Molecular-dynamics simulations of several water-crystalline urea interfaces have been performed. The structure and dynamics of water close to the urea crystal surface are discussed in terms of density profiles, positional and orientational distribution functions, and diffusion coefficients. The water structure close to the interface is strongly determined by the structure of the crystal surface: the (001) and (111) interfaces reveal strong adsorption of water while the (110) and () interfaces do so to a lesser extent. Assuming that the growth rate of a specific crystal face decreases with increasing solvent adsorption, the appearance of only (111) on the urea growth form is predicted. We argue that on the other hand the dominance of (110) over (001) cannot be explained using a simple layer growth model

Proton irradiation induced GaAs solar cell performance degradation simulations using a physics-based model

In this study a recently developed physics-based model to describe the performance degradation of GaAs solar cells upon electron irradiation is applied to analyze the effects of proton irradiation. For this purpose GaAs solar cells with significantly different architectures are subjected to a range of proton irradiation fluences up to 5×1012 H+/cm2. The resulting J−V and EQE characteristics of the cells are measured and compared with the simulations from the model. The model requires individual degradation constants for the SRH lifetimes and the surface recombination velocities as an input. In this study these constants were obtained from the recently determined associated constants for electron irradiation using the particles non-ionizing energy loss (NIEL) values for conversion. The good fit between the simulated and experimentally obtained results demonstrate that this is a valid approach. Moreover, it suggests that the physics based model allows for a good prediction of GaAs cell performance under particle irradiation of any kind independent of the particular cell architecture as long as the layer thicknesses and doping levels are known. In addition the applied proton irradiation levels in this study were not found to induce additional Cu-related degradation in the investigated thin-film cells, indicating that the use of copper foil as a convenient carrier and rear contact does not require reconsideration for thin-film cells intended for space applications