Biomedical subjects

R Di Leonardo

Publications and source records attributed to R Di Leonardo.

7 recordsLinked to original sources

PubMed2006

Multipoint holographic optical velocimetry in microfluidic systems.

We show how holographic optical trapping can be used for the multipoint measurement of fluid flow in microscopic geometries. An array of microprobes can be simultaneously trapped and used to map out the fluid flow in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are displaced by the flow of the surrounding fluid and then retrapped. The particles' displacements are monitored by digital video microscopy and directly converted into velocity field values. This technique enables the measurement of a two-dimensional flow field at points arbitrarily distributed in a three-dimensional volume. The validity of the technique is demonstrated for the case of the flow around a spinning sphere and the flow at the outlet of a microchannel.

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PubMed2005

Hard-sphere-like dynamics in a non-hard-sphere liquid.

The collective dynamics of liquid gallium close to the melting point has been studied using inelastic x-ray scattering to probe length scales smaller than the size of the first coordination shell. Although the structural properties of this partially covalent liquid strongly deviate from a simple hard-sphere model, the dynamics, as reflected in the quasielastic scattering, are beautifully described within the framework of the extended heat mode approximation of Enskog's kinetic theory, analytically derived for a hard-sphere system. The present work demonstrates, therefore, the applicability of Enskog's theory beyond simple liquids.

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PubMed2005

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 nonequilibrium regimes as soon as the structural relaxation time tau(s) approaches the inverse shear rate gamma (-1) . In the shear-dominated regime (at high gamma values) the structural relaxation time is found to be strongly sensitive to the shear rate ( tau(s) approximately gamma (-alpha) , with alpha approximately 1 ) while aging proceeds at a very slow rate. The effect of the 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 time scales: 1/tau-->1/tau+A gamma .

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PubMed2004

High frequency dynamics in a monatomic glass.

The high frequency dynamics of glassy selenium has been studied by inelastic x-ray scattering at beam line BL35XU (SPring-8). The high quality of the data allows one to pinpoint the existence of a dispersing acoustic mode for wave vectors (Q) of 1.5<Q<12.5 nm(-1), helping to clarify a previ-ous contradiction between experimental and numerical results. The sound velocity shows a positive dispersion, exceeding the hydrodynamic value by approximately 10% at Q<3.5 nm(-1). The Q2 dependence of the sound attenuation Gamma(Q), reported for other glasses, is found to be the low-Q limit of a more general Gamma(Q) proportional, variant Omega(Q)(2) law, which applies also to the higher Q region, where Omega(Q) proportional, variant Q no longer holds.

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PubMed2003

Nonequilibrium thermodynamic description of the coupling between structural and entropic modes in supercooled liquids.

The density response of supercooled glycerol to an impulsive stimulated thermal grating (q=0.63 microm(-1)) has been studied in the temperature range (T=200-340 K) where the structure rearrangement (alpha relaxation) and the thermal diffusion occur on the same time scale. A strong interaction between the two modes occurs giving rise to a dip in the T dependence of the apparent thermal conductivity and a flattening of the apparent alpha-relaxation time upon cooling. A nonequilibrium thermodynamic model for the long time response has been developed. The model is capable to reproduce the experimental data and to explain the observed phenomenology.

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PubMed2001

Topological description of the aging dynamics in simple glasses.

We numerically investigate the aging dynamics of a monatomic Lennard-Jones glass, focusing on the topology of the potential energy landscape which, to this aim, has been partitioned in basins of attraction of stationary points (saddles and minima). The analysis of the stationary points visited during the aging dynamics shows the existence of two distinct regimes: (i) at short times the system visits basins of saddles whose energies and orders decrease with t; (ii) at long times the system mainly lies in basins pertaining to minima of slowly decreasing energy. The long time dynamics can be represented by a simple random walk on a network of minima with a jump probability proportional to the inverse of the waiting time.

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PubMed2000

Off-equilibrium effective temperature in monatomic Lennard-Jones glass.

The off-equilibrium dynamics of a monatomic Lennard-Jones glass is numerically investigated after sudden isothermal density jumps (crunch) from well equilibrated liquid configurations towards the glassy state. The generalized fluctuation-dissipation relation has been studied and the temperature dependence of the violation factor m is found in agreement with the one step replica symmetry breaking scenario, i.e., at low temperature m(T) is found proportional to T up to an off-equilibrium effective temperature T(eff), where m(T(eff)) = 1. We report T(eff) as a function of the density and compare it with the glass transition temperature T(g) as determined by equilibrium calculations.

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