Anisotropic Local Stress and Particle Hopping in a Deeply Supercooled Liquid

The origin of the microscopic motions that lead to stress relaxation in deeply supercooled liquid remains unclear. We show that in such a liquid the stress relaxation is locally anisotropic which can serve as the driving force for the hopping of the system on its free energy surface. However, not all hopping are equally effective in relaxing the local stress, suggesting that diffusion can decouple from viscosity even at local level. On the other hand, orientational relaxation is found to be always coupled to stress relaxation.Comment: 4 pages, 3 figure

Computer Simulation Study of the Phase Behavior and Structural Relaxation in a Gel-Former Modeled by Three Body Interactions

We report a computer simulation study of a model gel-former obtained by modifying the three-body interactions of the Stillinger-Weber potential for silicon. This modification reduces the average coordination number and consequently shifts the liquid-gas phase coexistence curve to low densities, thus facilitating the formation of gels without phase separation. At low temperatures and densities, the structure of the system is characterized by the presence of long linear chains interconnected by a small number of three coordinated junctions at random locations. At small wave-vectors the static structure factor shows a non-monotonic dependence on temperature, a behavior which is due to the competition between the percolation transition of the particles and the stiffening of the formed chains. We compare in detail the relaxation dynamics of the system as obtained from molecular dynamics with the one obtained from Monte Carlo dynamics. We find that the bond correlation function displays stretched exponential behavior at moderately low temperatures and densities, but exponential relaxation at low temperatures. The bond lifetime shows an Arrhenius behavior, independent of the microscopic dynamics. For the molecular dynamics at low temperatures, the mean squared displacement and the (coherent and incoherent) intermediate scattering function display at intermediate times a dynamics with ballistic character and we show that this leads to compressed exponential relaxation. For the Monte Carlo dynamics we find always an exponential or stretched exponential relaxation. Thus we conclude that the compressed exponential relaxation observed in experiments is due to the out-of-equilibrium dynamics

Energy landscape of a Lennard-Jones liquid: Statistics of stationary points

Molecular dynamics simulations are used to generate an ensemble of saddles of the potential energy of a Lennard-Jones liquid. Classifying all extrema by their potential energy u and number of unstable directions k, a well defined relation k(u) is revealed. The degree of instability of typical stationary points vanishes at a threshold potential energy, which lies above the energy of the lowest glassy minima of the system. The energies of the inherent states, as obtained by the Stillinger-Weber method, approach the threshold energy at a temperature close to the mode-coupling transition temperature Tc.Comment: 4 RevTeX pages, 6 eps figures. Revised versio

A facile liquid foam based synthesis of nickel nanoparticles and their subsequent conversion to Ni<SUB>core</SUB>Ag<SUB>shell</SUB> particles: structural characterization and investigation of magnetic properties

A facile route for the synthesis of nickel nanoparticles in stable aqueous foams is reported. The Ni nanoparticles were roughly 12-15 nm in size and were stable as aqueous suspensions or powders when oleic acid was used as a capping agent. These Ni nanoparticles were subsequently coated with a silver shell in view of the extra stability and the enhanced manipulative ability afforded by the silver nanocoating. This was accomplished by a simple transmetallation reaction wherein the nanoparticle surface nickel atoms act as localized reducing agents for the silver ions in solution. As the silver shell is formed through the surface reaction a reduction in the average size of the Nicore occurs. After the core-shell structure formation, the Nicore has an average diameter of 10-20 nm while the Agshell has a thickness of 2-4 nm. The pristine oleic acid coated Ni and NicoreAgshell nanoparticles were probed for their magnetic characteristics by a vibrating sample magnetometer. The nascent, oleic acid coated Ni nanoparticles display a low superparamagnetic blocking temperature, TB, of 20 K. The field dependent magnetic behaviour above and below TB displays the standard features corresponding to superparamagnetism, as expected for very small Ni crystallites suggesting again that each 12 nm particle is polycrystalline. The magnetic contribution in the NicoreAgshell system comes from only the Ni core and predictably, the blocking temperature of this system is below 12 K due to the smaller size of the Ni core