Influence of an adsorbing polymer in the aging dynamics of Laponite clay suspensions

Clay-polymer dispersions in aqueous solutions have attracted a great interest in recent years due to their industrial applications and intriguing physical properties. Aqueous solutions of bare Laponite particles are known to age spontaneously from an ergodic state to a non ergodic state in a time varying from hours to months depending on Laponite concentration. When a polymer species like Polyethylene Oxide (PEO) is added to the solution, it weakly adsorbs on clay particle surfaces modifying the effective interaction potential between Laponite particles. A dynamic light scattering study, varying polymer concentration at fixed polymer molecular weight (Mw=200.000 g/mol), has been performed in order to understand the effect of polymer on the aging dynamics of the system. The results obtained show that arresting phenomena between clay particles are hindered if PEO is added and consequently the aging dynamics slows down with increasing PEO concentration. This process is possibly due to the progressive coverage of the clay surface by polymers that grow with increasing PEO concentration and may lead to steric stabilization.Comment: 13 pages, 6 figures, manuscript accepted for publication on Philosophical Magazin

Potential energy topology and relaxation processes in a model glass

We use computer simulation to investigate the topology of the potential energy $V(\{{\bf R}\})$ and to search for doublewell potential's (DWP) in a model glass . By a sequence of Newtonian and dissipative dynamics we find different minima of $V(\{{\bf R}\})$ and the energy profile along the least action paths joining them. At variance with previous suggestions, we find that the parameters describing the DWP's are correlated among each others. Moreover, the trajectory of the system in the 3$N$-d configurational phase space follows a quasi-1-d manifold. The motion parallel to the path is characterized by jumps between minima, and is nearly uncorrelated from the orthogonal, harmonic, dynamics.Comment: 4 pages, RevTex, 4 PostScript figure

Heat capacity of liquids: A hydrodynamic approach

We study autocorrelation functions of energy, heat and entropy densities obtained by molecular dynamics simulations of supercritical Ar and compare them with the predictions of the hydrodynamic theory. It is shown that the predicted by the hydrodynamic theory single-exponential shape of the entropy density autocorrelation functions is perfectly reproduced for small wave numbers by the molecular dynamics simulations and permits the calculation of the wavenumber-dependent specific heat at constant pressure. The estimated wavenumber-dependent specific heats at constant volume and pressure, $C_{v}(k)$ and $C_{p}(k)$, are shown to be in the long-wavelength limit in good agreement with the macroscopic experimental values of $C_{v}$ and $C_{p}$ for the studied thermodynamic points of supercritical Ar.Comment: 8 pages, 5 figure

Orientational and induced contributions to the depolarized Rayleigh spectra of liquid and supercooled ortho-terphenyl

The depolarized light scattering spectra of the glass forming liquid ortho-terphenyl have been calculated in the low frequency region using molecular dynamics simulation. Realistic system's configurations are produced by using a recent flexible molecular model and combined with two limiting polarizability schemes, both of them using the dipole-induced-dipole contributions at first and second order. The calculated Raman spectral shape are in good agreement with the experimental results in a large temperature range. The analysis of the different contributions to the Raman spectra emphasizes that the orientational and the collision-induced (translational) terms lie on the same time-scale and are of comparable intensity. Moreover, the cross terms are always found to be an important contribution to the scattering intensity.Comment: RevTeX4, 7 pages, 8 eps figure

Laser propulsion of nanobullets by adiabatic compression of surface plasmon polaritons

Laser propulsion and guide of nanosized objects is fundamental for a wide number of applications. These applications are often limited by the fact that the optical forces acting on nanoparticles are almost negligible even in the favorable case of metallic particles and hence large laser powers are needed to accelerate and guide nanosize devices in practical applications. Furthermore, metallic nanoparticles exhibit strong absorption bands and scattering and this makes more difficult controlling nanopropulsion. Thus, finding some mechanism enhancing the optomechanical interaction at the nanoscale controlled by laser is specifically challenging and pivotal. Here, we demonstrate a novel physical effect where the well-known adiabatic localization of the enhanced plasmonic surface field on the apex of metallic nanocones produces a significant optical pressure employable as a propulsive mechanism. The proposed method gives the possibility to develop new photonics devices to accelerate metallic nanobullets over long distances for a variety of applications

Aging under Shear: Structural Relaxation of a Non-Newtonian Fluid

The influence of an applied shear field on the dynamics of an aging colloidal suspension has been investigated by the dynamic light scattering determination of the density autocorrelation function. Though a stationary state is never observed, the slow dynamics crosses between two different non-equilibrium regimes as soon as the structural relaxation time approaches the inverse shear rate. In the shear dominated regime (at high shear rate values) the structural relaxation time is found to be strongly sensitive to shear rate while aging proceeds at a very slow rate. The effect of shear on the detailed shape of the density autocorrelation function is quantitatively described assuming that the structural relaxation process arises from the heterogeneous superposition of many relaxing units each one independently coupled to shear with a parallel composition rule for timescales.Comment: 5 pages, 5 figure