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

Colloidal attraction induced by a temperature gradient

Colloidal crystals are of extreme importance for applied research, such as photonic crystals technology, and for fundamental studies in statistical mechanics. Long range attractive interactions, such as capillary forces, can drive the spontaneous assembly of such mesoscopic ordered structures. However long range attractive forces are very rare in the colloidal realm. Here we report a novel strong and long ranged attraction induced by a thermal gradient in the presence of a wall. Switching on and off the thermal gradient we can rapidly and reversibly form stable hexagonal 2D crystals. We show that the observed attraction is hydrodynamic in nature and arises from thermal induced slip flow on particle surfaces. We used optical tweezers to directly measure the force law and compare it to an analytic prediction based on Stokes flow driven by Marangoni forces.Comment: 4 pages, 4 figure

Collapse and Fragmentation Models of Prolate Molecular Cloud Cores. I. Initial Uniform Rotation

Studies of the distribution of young stars in well-known regions of star formation indicate the existence of a characteristic length scale (~0.04 pc), separating the regime of self-similar clustering from that of binary and multiple systems. The evidence that this length scale is comparable to the size of typical molecular cloud clumps, along with the observed high frequency of companions to pre-main-sequence stars, suggest that stars may ultimately form through fragmentation of collapsing molecular cloud cores. Here we use a new hydrodynamic code to investigate the gravitational collapse and fragmentation of protostellar condensations, starting from moderately centrally condensed (Gaussian), prolate configurations with axial ratios of 2:1 and 4:1 and varying thermal energy (α). All the models start with uniform rotation and ratios of the rotational to the gravitational energy β ≈ 0.036. The results indicate that these condensations collapse all the way to form a narrow cylindrical core that subsequently fragments into two or more clumps, even if they are initially close to virial equilibrium (α + β ≈ -->½). The 2:1 clouds formed triple systems for α 0.36 and a binary system for α ≈ 0.27, while the 4:1 clouds all formed binary systems independently of α. The mass and separation of the binary fragments increase with increasing the cloud elongation. The widest binaries formed from clouds with α ≈ 0.36, and starting from this value, the binary separation decreases with either increasing or decreasing α. In all cases, fragmentation did not result in a net loss of the a/m ratio (specific spin angular momentum per unit mass), as expected from stellar observations. The fragments that formed possess low values of α (~0.06) and are appreciably elongated, and so they could subfragment before becoming true first protostellar cores

Quasi-saddles as relevant points of the potential energy surface in the dynamics of supercooled liquids

The supercooled dynamics of a Lennard-Jones model liquid is numerically investigated studying relevant points of the potential energy surface, i.e. the minima of the square gradient of total potential energy $V$. The main findings are: ({\it i}) the number of negative curvatures $n$ of these sampled points appears to extrapolate to zero at the mode coupling critical temperature $T_c$; ({\it ii}) the temperature behavior of $n(T)$ has a close relationship with the temperature behavior of the diffusivity; ({\it iii}) the potential energy landscape shows an high regularity in the distances among the relevant points and in their energy location. Finally we discuss a model of the landscape, previously introduced by Madan and Keyes [J. Chem. Phys. {\bf 98}, 3342 (1993)], able to reproduce the previous findings.Comment: To be published in J. Chem. Phy

Standard mechanical testing is inadequate for the mechanical characterisation of shape-memory alloys: Source of errors and a new corrective approach

Thanks to its unique behaviour characterised by a superelastic response, Nitinol has now become the material of preference in a number of critical applications, especially in the area of medical implants. However, the reversible phase transformation producing its exceptional comportment is also responsible for a number of phenomena that make its mechanical characterisation particularly complex, by hindering the assumptions at the very basis of common uniaxial tensile testing. This necessarily reduces the level of safety and design optimization of current applications, which rely on incorrect mechanical parameters. In this study, the spurious effects introduced by the unconventional material behaviour during uniaxial tensile testing are analysed by means of digital image correlation (DIC), identifying the onset of undesirable material inhomogeneities and bending moments that are dependent on the test setup and strongly limit the reliability of standard characterisation. Hence, a more accurate and systematic testing approach, exploiting the ability of DIC to analyse the local mechanical response at specific regions of the test specimen, is presented and discussed

Holographic tracking and sizing of optically trapped microprobes in diamond anvil cells

We demonstrate that Digital Holographic Microscopy can be used for accurate 3D tracking and sizing of a colloidal probe trapped in a diamond anvil cell (DAC). Polystyrene beads were optically trapped in water up to Gigapascal pressures while simultaneously recording in-line holograms at 1 KHz frame rate. Using Lorenz-Mie scattering theory to fit interference patterns, we detected a 10% shrinking in the bead’s radius due to the high applied pressure. Accurate bead sizing is crucial for obtaining reliable viscosity measurements and provides a convenient optical tool for the determination of the bulk modulus of probe material. Our technique may provide a new method for pressure measurements inside a DAC