Flapping states of an el astically anchored wing in a uniform flow

Linear stability analysis of an elastically anchored wing in a uniform flow is investigated both analytically and numerically. The analytical formulation explicitly takes into account the effect of the wake on the wing by means of Theodorsen's theory. Three different parameters non-trivially rule the observed dynamics: mass density ratio between wing and fluid, spring elastic constant and distance between the wing center of mass and the spring anchor point on the wing. We found relationships between these parameters which rule the transition between stable equilibrium and fluttering. The shape of the resulting marginal curve has been successfully verified by high Reynolds number direct numerical simulations. Our findings are of interest in applications related to energy harvesting by fluid-structure interaction, a problem which has recently attracted a great deal of attention. The main aim in that context is to identify the optimal physical/geometrical system configuration leading to large sustained motion, which is the source of energy we aim to extract.Comment: 10 pages, 11 figures, submitted to J. Fluid. Mec

Impact-driven effects in thin-film growth: steering and transient mobility at the Ag(110) surface

Low-energy atomic impacts on the Ag(110) surface are investigated by molecular dynamics simulations based on reliable many-body semiempirical potentials. Trajectory deflections (steering) caused by the atom-surface interaction are observed, together with impact-following, transient-mobility effects. Such processes are quantitatively analysed and their dependence on the initial kinetic energy and on the impinging direction is discussed. A clear influence of the surface anisotropy on both steering and transient mobility effects is revealed by our simulations for the simple isolated-atom case and in the submonolayer-growth regime. For the latter case, we illustrate how steering and transient mobility affect the film morphology at the nanoscale.Comment: 7 pages, 9 figure

Microscopic View of Nucleation on Surfaces

Nucleation has been studied on atomic scale for Ag deposition on a Pt(111) surface at low temperature (50-120 K) by means of variable-temperature scanning tunneling microscopy. From island density versus coverage data, the transition from the initial steps of nucleation to growth and coalescence has been studied as a function of temperature. The data show that dimers are stable nuclei for T less-than-or-equal-to 110 K, which facilitates quantitative comparison with rate equations from nucleation theory. The migration barrier E(m) = 157 +/- 10 meV is determined for Ag adatoms on Pt(111) and compared with theoretical values

Strain Relief at Hexagonal-Close-Packed Interfaces

The mechanism of strain relief for compressively stressed Ag layers epitaxially grown on Pt(111) is studied by scanning tunneling microscopy. The strain in the compressed commensurate (1X1) Ag monolayer on Pt(111) is relieved in the bilayer by the formation of two weakly incommensurate structures: the metastable striped incommensurate phase, which converts into a trigonal network of domain walls upon annealing. This network is a general case for isotropic strain relief realized by the crossing of domain walls. The introduction of a phase shift of one set of domain walls allows the system to account for the energy difference of fee and hcp stacking. The stability of unidirectional via isotropic strain relief and the respective structures of domain walls is generally discussed for surface reconstructions and metal epitaxy of hexagonal-close-packed surfaces

The 2/3 power law dependence of capillary force on normal load in nanoscopic friction

During the sliding of an atomic force microscope (AFM) tip on a rough hydrophilic surface, water capillary bridges form between the tip and the asperities of the sample surface. These water bridges give rise to capillary and friction forces. We show that the capillary force increases with the normal load following a 2/3 power law. We trace back this behavior to the load induced change of the tip-surface contact area which determines the number of asperities where the bridges can form. An analytical relationship is derived which fully explains the observed interplay between humidity, velocity, and normal load in nanoscopic friction

Measuring surface diffusion from nucleation island densities

We present a critical view of the analysis of experimental island densities acquired as a function of temperature in terms of barriers and prefactors for tracer diffusion at surfaces. We investigate the achievable precision for methods ranging from simple application of scaling laws, via integration of mean-held rate equations within various approximations for the capture rates, to kinetic Monte Carlo simulations that account for the various island shapes realized on square and hexagonal lattices. The discussion of theoretical models will be accompanied by variable temperature STM data for the nucleation of Ag on a Pt(lll) surface. We introduce experimental methods to test for dimer diffusion and dissociation, as well as for transient mobility of monomers. Density scaling is analyzed in the presence of post-deposition mobility and easy adatom attachment to islands and other monomers. From extended kinetic Monte Carlo simulations,we establish density scaling for the various island shapes on square and hexagonal lattices for coverages up to percolation, which is particularly relevant for methods working in reciprocal space. [S0163-1829(99)01231-X]

LDPE-based blends and films stabilized with nonreleasing polymeric antioxidants for safer food packaging

Several novel random copolymers of ethylene and 1-olefin counits bearing a highly efficient phenolic antioxidant moiety placed at different distances from the polymerizable double bond were prepared in the presence of a metallocene catalyst. These copolymers were melt-blended with an antioxidant-free LDPE in an internal batch mixer to obtain innovative materials containing nonreleasing polymeric antioxidants suitable for safer food packaging applications. Blends and films, obtained by compression molding, were tested for their thermal and thermo-oxidative stability by thermogravimetric analysis both in dynamic and isothermal conditions. Films containing the macromolecular antioxidants showed a longer induction time before O2 uptake starts and, consequently, a higher degradation temperature than neat LDPE or LDPE containing a low molecular weight commercial additive. Aging tests demonstrated that the new polymeric antioxidants also exert a valid protection against photo-oxidation. Eventually, migration tests demonstrated the absence of any trace of products containing the antioxidant moiety when the films were kept in contact with a food simulant